# Ursula 0.7 - full documentation
> Open-source Distributed Durable Streams over HTTP, backed by S3.
These docs are for Ursula 0.7.x (0.7.6). The latest docs are at https://ursula.tonbo.io/llms.txt.
Source: https://ursula.tonbo.io
---
# Ursula
> [!NOTE]
> Ursula is built by
```bash
# create a bucket and stream
curl -X PUT http://127.0.0.1:4437/demo
curl -X PUT http://127.0.0.1:4437/demo/hello
# append
curl -X POST http://127.0.0.1:4437/demo/hello \
-H 'Content-Type: application/octet-stream' \
--data-binary 'hello world'
# catch-up read
curl 'http://127.0.0.1:4437/demo/hello?offset=-1'
# live tail
curl 'http://127.0.0.1:4437/demo/hello?offset=-1&live=sse'
```
```python
import requests
base = "http://127.0.0.1:4437"
requests.put(f"{base}/demo")
requests.put(f"{base}/demo/hello")
requests.post(
f"{base}/demo/hello",
headers={"Content-Type": "application/octet-stream"},
data=b"hello world",
)
# catch-up read
resp = requests.get(f"{base}/demo/hello", params={"offset": -1})
print(resp.content)
# live tail with SSE
with requests.get(
f"{base}/demo/hello",
params={"offset": -1, "live": "sse"},
stream=True,
) as r:
for line in r.iter_lines():
if line:
print(line.decode())
```
```ts
const base = "http://127.0.0.1:4437";
await fetch(`${base}/demo`, { method: "PUT" });
await fetch(`${base}/demo/hello`, { method: "PUT" });
await fetch(`${base}/demo/hello`, {
method: "POST",
headers: { "Content-Type": "application/octet-stream" },
body: "hello world",
});
// catch-up read
const data = await (await fetch(`${base}/demo/hello?offset=-1`)).text();
// live tail with native EventSource
const es = new EventSource(`${base}/demo/hello?offset=-1&live=sse`);
es.addEventListener("data", (e) => console.log(e.data));
```
## From a browser
The examples above assume same-origin access. A page served from a different origin than the gateway needs the gateway to allow that origin — see [cross-origin reads](/docs/0.7/security#cross-origin-reads-opt-in):
```bash
ursula gateway ... --cors-allowed-origin https://app.example.com
```
Two things to know once it is on:
- **Read your continuation headers.** The gateway sends `Access-Control-Expose-Headers: *`, so `Stream-Next-Offset` and friends are readable. Without exposure a browser can read one page and never advance, which looks like the stream ending. If a `fetch` succeeds but `response.headers.get("stream-next-offset")` is `null`, the origin is not allowing your origin.
- **`EventSource` cannot send `Authorization`.** It has no header API, so it only works against `public_read` streams. For a private live tail, use `fetch` and read the body:
```ts
const response = await fetch(`${base}/${bucket}/hello?offset=-1&live=sse`, {
headers: { Authorization: `Bearer ${token}` },
});
const reader = response.body!.pipeThrough(new TextDecoderStream()).getReader();
```
A subscription ends when its credential expires, and says so: the final frame is `event: credential-expired`. Treat it as a reconnect signal rather than end of stream — refresh the token and re-read from the last `Stream-Next-Offset` you saw. Any other termination deserves the same handling.
## Notes for client implementers
- After every read and append, the server returns `Stream-Next-Offset`. Track it. Use it as the `offset` query parameter on the next read. Don't construct offsets manually. They're opaque.
- For binary streams over SSE, `data` events carry raw base64 text and the response includes `Stream-Sse-Data-Encoding: base64`. Decode the data event first, then interpret the bytes using `Stream-Data-Content-Type`. See [Binary SSE](/docs/0.7/concepts/binary-sse).
- For exactly-once writes, send `Producer-Id`, `Producer-Epoch`, `Producer-Seq` headers and retry on network errors. The server deduplicates. See [Exactly-once writes](/docs/0.7/concepts/exactly-once-writes).
- For conditional writes, use `Stream-Seq`: the server accepts an append only when its value is bytewise greater than the last one accepted on the stream. Several writers can extend one log safely by sending the number of events they read. See [Conditional writes](/docs/0.7/concepts/conditional-writes).
---
# Deploy a Cluster
Production Ursula is a static-membership Raft cluster: three voting nodes across availability zones, a durable Raft log per node, and a shared S3 bucket for the cold tier.
The recommended way to run it is **Kubernetes via the Helm chart**. Bring a Kubernetes cluster with a StorageClass for the voters' volumes and an S3 bucket for the cold tier. The chart installs Ursula into it. If you are not running Kubernetes, see [bare metal](#without-kubernetes-bare-metal--vms) at the end.
## Helm
The chart and images are published to GHCR on every release. One command starts a three-voter cluster:
```bash
helm install ursula oci://ghcr.io/tonbo-io/charts/ursula --version 0.7.6
```
The chart defaults to the `ghcr.io/tonbo-io/ursula` image pinned to the chart's `appVersion`. The default install runs three voters and 64 Raft groups, with durable per-pod Raft logs on PVCs, a headless peer Service, a client Service, and a quorum-protecting PodDisruptionBudget.
For production, add shared S3 cold storage and workload identity:
```yaml
s3:
bucket: my-ursula-bucket
region: us-east-1
prefix: ursula-prod
coldStorage:
enabled: true
serviceAccount:
annotations:
eks.amazonaws.com/role-arn: arn:aws:iam::123456789012:role/ursula-s3
```
With cold storage enabled the chart also turns on cold-chunk compaction and lets Ursula pick S3 for Raft snapshots (`snapshotStore.backend: auto`). Set `coldStorage.compaction.enabled=false` or `snapshotStore.backend=inline` to opt out.
For MinIO or another S3-compatible backend, set `s3.endpoint`. Prefer workload identity over static S3 credentials. To use a registry mirror, set `global.image.repository`, `global.image.tag`, and optionally `global.imagePullSecrets`.
`server.replicaCount` controls the voter set for a **fresh** cluster and supports `1`, `3`, and `5`. Production should use `3`, or `5` only when tolerating two simultaneous voter failures justifies the larger write quorum. Changing it on an initialized cluster is not safe Raft voter reconfiguration. Safe scaling needs the future operator workflow.
### On AWS (EKS)
The chart's [`examples/production-eks.yaml`](https://github.com/tonbo-io/ursula/blob/main/charts/ursula/examples/production-eks.yaml) is a three-zone starting point: three voters on `gp3` volumes, S3 cold storage and snapshots, three gateways, and a two-replica indexer pool with its own workload identity. It needs:
- an EKS cluster with nodes in three availability zones,
- the EBS CSI driver and a StorageClass named `gp3`,
- an S3 bucket,
- IAM roles for the Ursula and indexer service accounts (IRSA or EKS Pod Identity), each limited to its own S3 prefix.
The [chart README](https://github.com/tonbo-io/ursula/tree/main/charts/ursula#prerequisites) lists the details. Replace the bucket and the role ARNs in the example, then install it:
```bash
helm install ursula oci://ghcr.io/tonbo-io/charts/ursula --version 0.7.6 \
--namespace ursula --create-namespace -f production-eks.yaml
helm test ursula --namespace ursula
```
## Verify
```bash
helm test ursula
```
Gate on this test, not on `helm install --wait`. Helm does not wait for a StatefulSet with the `OnDelete` update strategy, so `--wait` can report the release deployed before any voter is ready. The test mounts the chart-generated cluster manifest and runs `ursulactl wait-ready`. It requires the expected group inventory and leaders, a complete uniform voter set in each maintenance report, no uncertain maintenance fence, and a 200 response from every node's `/__ursula/ready` probe. To query a node directly:
```bash
kubectl port-forward svc/ursula 4437:4437
curl http://127.0.0.1:4437/__ursula/metrics
```
## Production notes
- Run three voters across three availability zones, each with its own zonal persistent volume for the Raft log. Never use ephemeral voter data in production. The chart always gives each voter pod a `raft-data` volume claim.
- Shared S3 is required for any multi-node cluster: replicas must be able to read chunks flushed by any leader.
- Put stateless gateway replicas behind authenticated TLS ingress and keep voter and peer Services private. Ursula itself has no TLS or auth (see [Security](/docs/0.7/security)).
- Kubernetes rolling updates do not transfer Raft leaders on their own. For an operationally safe restart, wrap the platform's restart in [ursulactl's maintenance verbs](/docs/0.7/cli): `ursulactl drain` the node, let Kubernetes restart the pod, then `ursulactl wait --node N` and `undrain` before moving to the next one. See [Operations](/docs/0.7/operations) for day-2 work.
- The StatefulSet readiness probe calls `GET /__ursula/ready` and removes a pod
from service during WAL disk pressure. Cluster-wide Raft readiness still
comes from `helm test` or `ursulactl wait-ready`. The mutating admin surface
is bound to pod loopback (`127.0.0.1:4438`) and is reached with
`kubectl port-forward`.
- `helm uninstall` leaves the graceful-rollout state ConfigMap (`-rollout-state`, for example `ursula-rollout-state`) behind. Delete it before you install again under the same name, so the rollout hook does not resume a rollout of the old release.
- Clusters that serve the [SQLite VFS](/docs/0.7/examples/sqlite-vfs) need per-group admission caps of at least 64 MiB (`standard` or `large` preset) so a 32 MiB commit can be admitted.
## Optional: event-time indexer (experimental)
`ursula-indexer` is an optional worker pool that builds queryable event-time indexes from `application/json` and `application/x-ndjson` streams, kept outside the voter processes so query and compaction work never touches commit latency. It is experimental: its API and S3 format may change in a minor release, which then needs a new or emptied `indexer.s3.prefix` and re-registration.
```yaml
indexer:
enabled: true
replicaCount: 2
s3:
prefix: indexes
serviceAccount:
annotations:
eks.amazonaws.com/role-arn: arn:aws:iam::123456789012:role/ursula-index
```
Register streams dynamically with `PUT /v1/indexes/{id}` on the internal indexer Service; see [Browser Telemetry](/docs/0.7/examples/browser-telemetry#event-time-queries-experimental). No Helm upgrade is needed. The Service is an internal ClusterIP. If remote applications need read access, front it with an authenticated, path-aware proxy and never expose the registration or administration routes.
## Without Kubernetes (bare metal / VMs)
The same static-membership model works by hand. Every node runs the same config file, and only the node ID differs:
```toml
[server]
listen = "0.0.0.0:4437"
[raft]
group_count = 256
init_membership_per_group = true
[raft.wal]
path = "/var/lib/ursula"
[storage.cold]
backend = "s3"
root = "ursula-prod"
[storage.cold.s3]
bucket = "my-ursula-bucket"
region = "us-east-1"
[[raft.peers]]
node_id = 1
url = "http://10.0.0.1:4437"
[[raft.peers]]
node_id = 2
url = "http://10.0.0.2:4437"
[[raft.peers]]
node_id = 3
url = "http://10.0.0.3:4437"
```
Start each node with its own ID:
```bash
ursula --config /etc/ursula/ursula.toml --node-id 1 # node 2 and 3 likewise
```
`init_membership_per_group = true` bootstraps each group on the first start of a fresh cluster. Keep it on every node across restarts: a replica that ever held its group never initializes it again, and an empty replica initializes a group only when every configured voter reports it empty and no leader (see [Rejoining a wiped node](/docs/0.7/operations#rejoining-a-wiped-node)). Verify the cluster with a one-file manifest:
```bash
cat > cluster-manifest.json <<'JSON'
{
"nodes": [
{"id": 1, "http_url": "http://10.0.0.1:4437", "host": "10.0.0.1"},
{"id": 2, "http_url": "http://10.0.0.2:4437", "host": "10.0.0.2"},
{"id": 3, "http_url": "http://10.0.0.3:4437", "host": "10.0.0.3"}
]
}
JSON
ursulactl wait-ready --config cluster-manifest.json --expected-groups 256
ursulactl status --config cluster-manifest.json
```
Every config key (peers, listeners, WAL, cold-tier tuning) is documented in [Configuration](/docs/0.7/configuration).
---
# Configuration
Everything configurable in Ursula lives on this page. Ursula is configured by a TOML config file, an optional resource preset, and a small set of CLI overrides, merged in this order:
```text
built-in defaults < preset defaults < config file < CLI overrides
```
The normal startup command is:
```bash
ursula --config /etc/ursula/ursula.toml --node-id 1
```
With no config file, `ursula` falls back to the `default` preset, which starts a single-node in-memory runtime without Raft, and nothing survives a restart. That's what [Install](/docs/0.7/install) uses. Every other preset, and a config file loaded without `--preset default`, runs Raft on the disk WAL described under [Raft](#raft).
## CLI flags
| Flag | Default | Notes |
| ---- | ------- | ----- |
| `--config FILE` | searches `./ursula.toml`, `/etc/ursula/ursula.toml`, then user config paths | TOML config file |
| `--preset PRESET` | `default` when no config file is found, otherwise none | Resource preset: `default`, `dev`, `tiny`, `small`, `standard`, or `large` |
| `--node-id ID` | config file or preset | Overrides `raft.node_id` |
CLI `--node-id` intentionally overrides any `raft.node_id` value in the config file so the same file can be mounted on every node. Helm derives it from the StatefulSet ordinal.
## Resource presets
`--preset` supplies a base configuration before the config file is merged, and config-file values win. Pick by node size:
| Preset | Intended use | Main defaults |
| ------ | ------------ | ------------- |
| `default`, `dev` | Local development | Single-node in-memory runtime without Raft |
| `tiny` | Memory-bound tests or very small nodes | 64 groups, 64 MiB cold cache, 4 MiB cold flushes, 2 cold writes, 8 MiB hot/admission caps, 64 MiB HTTP in-flight body budget |
| `small` | Small nodes or cost-sensitive tests | 128 groups, 64 MiB cold cache, 4 MiB cold flushes, 2 cold writes, 16 MiB hot/admission caps, 64 MiB HTTP in-flight body budget |
| `standard` | Production baseline | 256 groups, 256 MiB cold cache, 8 MiB cold flushes, 4 cold writes, 64 MiB hot/admission caps, 256 MiB HTTP in-flight body budget |
| `large` | Larger nodes with more cache and write headroom | 512 groups, 512 MiB cold cache, 16 MiB cold flushes, 8 cold writes, 128 MiB hot/admission caps, 512 MiB HTTP in-flight body budget |
## Server and runtime
| Config key | Default | Purpose |
| ---------- | ------- | ------- |
| `server.listen` | `127.0.0.1:4437` | Client/API listener |
| `server.cluster_listen` | none | Optional separate Raft/cluster listener |
| `runtime.cpu_affinity` | automatic | On Linux, `true` forces pinning and `false` disables it; omitted pins only when cpuset size equals core_count |
| `runtime.core_count` | `available_parallelism` | Owner worker threads; Linux automatically pins only when the allowed cpuset matches this count. Journal writers and the main Tokio runtime use additional threads |
| `runtime.live_read_max_waiters_per_core` | `65536` | SSE waiter cap per core. `0` disables the limit |
| `observability.tokio_console` | `false` | Enables Tokio console when the binary is built with the feature |
Without `server.cluster_listen`, `server.listen` serves both client/API and Raft/cluster routes, and each `[[raft.peers]].url` should point at that address. With `server.cluster_listen` set, `server.listen` is client/API-only and peer URLs must point at the cluster listener instead. Note that the same peer URL is currently also used for HTTP leader redirects. A client that talks to a node directly follows it, so it must be reachable by such clients as well as by peers. `ursula gateway` follows a redirect to an upstream directly when its `--upstream` equals the peer URL, and searches its upstreams for the leader otherwise.
## Raft
| Config key | Default | Purpose |
| ---------- | ------- | ------- |
| `raft.node_id` | none | This node's stable ID, usually overridden per host with `--node-id` |
| `raft.append_transport_budget_bytes` | `134217728` | Node-wide charged append bytes per direction, shared across cores and peers; excludes tonic/socket buffers |
| `raft.group_count` | `core_count × 16` | Total Raft groups. Higher values improve stream-to-group hash spread |
| `raft.wal.path` | none | Directory of the Raft WAL. Required when `raft.peers` is set. Raft logs live under `PATH/raft-log`, and removing the directory wipes state cleanly. Without it, a single node runs its WAL in a temporary directory that is removed on clean shutdown |
| `raft.wal.fsync` | `never` | When journal appends reach stable storage. `never` acknowledges ordinary data appends from the page cache. Membership batches are always fsynced before acknowledgment. `always` acknowledges a batch after `fsync`. Votes, each group's log state and the node's run state are always written with `fsync` |
| `raft.wal.segment_size` | `64MiB` | Size at which each core's journal starts a new segment. Purge deletes whole segments, so this is also how much disk space comes back at a time. At least `4KiB` |
| `raft.wal.cache_size` | `256MiB` | Recent Raft entries kept in memory across the node, split evenly between its groups. Older entries, such as a lagging follower's catch-up, are read from disk |
| `raft.wal.min_available_size` | `512MiB` | Reject writes and mark `/__ursula/ready` unavailable below this much free WAL-filesystem space. `0` disables the guard |
| `raft.wal.resume_available_size` | `1GiB` | Free space required before disk-pressure admission clears. Must exceed the minimum |
| `raft.peers` | empty | Static gRPC peer list, with one `[[raft.peers]]` entry per voter, including this node |
| `raft.init_membership_per_group` | `false` | Per-group membership bootstrap. One voter per group initializes it once every configured voter reports the group empty and no leader. A replica that ever held its group never initializes it again, so keep it `true` across restarts. The Helm chart sets `true` |
The Raft WAL is always the per-core disk journal. A node with `raft.peers` set
refuses to start without `raft.wal.path`. A single node without a path creates a
fresh `ursula-wal-*` directory under the system temporary directory, logs its
location at startup, and removes it on a clean shutdown (`SIGTERM` or Ctrl-C).
A failed start, or a shutdown that cannot close the WAL, leaves the directory in
place. Its streams survive no restart. Set `raft.wal.path` to keep them.
With `fsync = "never"`, a process crash loses nothing because the page cache
survives it, and a graceful shutdown `fsync`s every journal before it exits. A
host crash or power loss can drop the unsynced tail of that replica's journal.
The replica restarts from the frames that still verify and reports that it is
recovering. Each of its groups stays behind a recovery gate: it does not
campaign or vote until it has applied a fresh barrier from the group's leader,
so it never helps elect a leader that lacks a write it acknowledged. While at
most a minority of a group's voters lost their tail, the other voters hold
every acknowledged write and the group keeps serving. If a majority lost it at
once, the group has no leader and refuses writes until an operator accepts the
loss (see [Operations](/docs/0.7/operations#recovering-after-a-host-crash)).
With `fsync = "always"`, a host crash loses no acknowledged write. Every batch
waits for an `fsync`, which costs write latency. A host crash can still leave a
complete invalid frame after the last `fsync` in a journal's newest segment.
Recovery cannot tell it from media corruption, so it gates that core's groups as
above. A group gated on a majority of its voters, or a single-voter group, then
waits for an operator although nothing was lost (see
[Operations](/docs/0.7/operations#recovering-after-a-host-crash)).
## Cold storage
Ursula keeps recent data in an in-memory hot ring on every replica and flushes older segments and snapshot blobs to a cold backend. Multi-node clusters should use S3 or an S3-compatible object store so every replica can read chunks flushed by any leader. All replicas must point at the same shared bucket.
```toml
[storage.cold]
backend = "s3"
root = "ursula-prod-20260518"
[storage.cold.s3]
bucket = "my-ursula-bucket"
region = "us-east-1"
# endpoint = "http://127.0.0.1:9000"
# access_key_id = "AKIA..."
# secret_access_key = "..."
# session_token = "..."
# server_side_encryption = "aes256" # default; "aws-kms" or "none"
# kms_key_id = "arn:aws:kms:..." # customer managed key for "aws-kms"
```
When `access_key_id` and `secret_access_key` are omitted, Ursula uses the standard AWS SDK credential chain (instance profile, environment, profile, and so on). Prefer that over static keys. Set `endpoint` for S3-compatible stores such as MinIO, R2, or TOS.
Every cold-tier object write requests server-side encryption by default (`server_side_encryption = "aes256"`, i.e. SSE-S3) — free on AWS S3, and Raft snapshot uploads inherit the same setting. `aws-kms` switches to SSE-KMS, using the AWS managed key unless `kms_key_id` names a customer managed key. MinIO honors SSE headers only when a KMS/KES is configured; for a MinIO deployment without one, set `server_side_encryption = "none"` explicitly or writes fail with an SSE error.
| Config key | Default | Purpose |
| ---------- | ------- | ------- |
| `storage.cold.backend` | `none` | `none`, `memory`, or `s3` |
| `storage.cold.root` | none | Prefix prepended to every cold key. Use a date-stamped value for benchmark runs so cleanup can't touch production data |
| `storage.cold.s3.bucket` | none | Required when `backend = "s3"` |
| `storage.cold.s3.server_side_encryption` | `aes256` | SSE mode for every object write: `aes256` (SSE-S3), `aws-kms`, or `none` |
| `storage.cold.s3.kms_key_id` | none | Customer managed KMS key; only valid with `aws-kms` |
| `storage.cold.s3.probe_timeout` | `5s` | Deadline of the snapshot-store health probe, retries included. A probe whose first attempt fails fast still has time for one retry, while an attempt that hangs runs into this deadline. Must be non-zero |
| `storage.cold.s3.unhealthy_ticks` | `3` | Bad health ticks, with no good tick between them, before the node sheds its leaderships. Must be non-zero |
| `storage.cold.s3.heal_ticks` | `2` | Good health ticks in a row before a node that shed may lead again. Must be non-zero |
| `storage.cold.flush_interval` | `1s` | Background flush worker tick interval |
| `storage.cold.flush_size` | `8MiB` | Target bytes flushed per group per tick |
| `storage.cold.flush_max_concurrency` | `4` | Parallel cold writes in flight |
| `storage.cold.max_hot_size_per_group` | `64MiB` | Backpressure ceiling: when a group's hot bytes exceed this, new writes get HTTP `503` until the flush catches up. Explicit `0` disables the cap |
| `storage.cold.flush_max_hot_age` | `5m` | A stream's hot tail older than this is flushed whole even when its group is below the flush threshold. `0` disables |
| `storage.cold.compaction_enabled` | `true` | Background compaction: merges small chunks that flushes recorded as compaction debt (no bucket listing) and rewrites packed slices once a stream holds 64 of them |
| `storage.snapshot.backend` | `auto` | `auto` (S3 snapshots when `storage.cold.backend = "s3"`, inline otherwise), `inline`, or `s3` |
| `raft.snapshot_log_budget` | `1GiB` | Node Raft-log budget of the snapshot driver. A group snapshots once its log since the last snapshot reaches the larger of this budget over twice the group count (at most `16MiB`) and twice its last snapshot's size. Above three quarters of the budget a pressure pass snapshots the groups that hold the most log first, one at a time, until the node would fall to half the budget. Past the budget it runs more builds at once, in even steps up to `raft.snapshot_build_max_concurrency` before twice the budget. With the default of 2 that is two builds past the budget. With 4 it is two past the budget, three past 1.33 times it and four past 1.67 times it. Over twice the budget the node refuses writes with a body until its log is below 1.5 times the budget |
| `raft.snapshot_build_max_concurrency` | `2` | Snapshot builds the node runs at once. Each build holds its group's captured state (the hot window, at most `storage.cold.max_hot_size_per_group`, and the streams' metadata) and, while it uploads, its compressed snapshot, about as large again, and it sends its snapshot-store requests one after another. The snapshot driver starts a build only while fewer run than the node's log calls for: one below `raft.snapshot_log_budget` and more past it. Builds it started past the budget finish after the log falls below it. A snapshot an operator triggers while every build slot is taken is deferred. With the driver off (`storage.snapshot.drive_interval = 0`), OpenRaft's own policy starts the builds, up to this many at once, where the default of 1 before 0.7.4 ran one. Set `1` to keep one build at a time, as before 0.7.4. Raise it, to `4` for example, when the node keeps reaching twice the budget with every build in use (`raft_snapshot_builds_in_flight_max` at this value) and its memory holds that many builds of its largest groups. The server refuses to start with `0` or more than `16` |
| `raft.snapshot_pressure_max_groups_per_tick` | `16` | Groups one snapshot driver pass starts while the node's log is below `raft.snapshot_log_budget`. Past the budget a pass starts this many for each build it keeps in flight |
Hot bytes here are payload bytes: the hot window keeps no per-message bookkeeping, so admission and flush thresholds charge exactly the unflushed payload. Flush defaults are conservative, so tune them under load. A typical benchmark profile drops the interval to `200ms`, raises concurrency to `32`, and bumps the per-group ceiling. Advanced deployments can split the flush threshold and batch size with `storage.cold.flush_min_hot_size` and `storage.cold.flush_max_size`, but the single `storage.cold.flush_size` knob is the normal path. Once a group's accumulated hot payload reaches the threshold, Ursula packs eligible slices from that group into one immutable object up to the configured maximum; single-stream slices of 1 MiB or more keep the direct-object layout, and smaller ones are packed even when they are alone.
With S3 snapshots, a health gate checks the store every `storage.snapshot.drive_interval` (`5s` by default). Setting `drive_interval = "0s"` turns off the snapshot driver and this gate with it. Each tick probes the store and reads the node's cold-flush writes. A tick is bad when the probe fails or misses its deadline, or when a write failed within the flush window and no write has succeeded since. The window is as long as the S3 client can take to give up on a write: `storage.cold.s3.timeout` for each of its `max_retries` plus one attempts, the backoff between them, and the flush worker's pause between passes. That is 55 seconds with the defaults. A tick in which writes both failed and succeeded neither ends a bad streak nor adds to it. One failed probe is not enough on its own, and once a later write succeeds a failed write weighs only on its own tick. A store that refuses writes but answers the probe is still caught. After `storage.cold.s3.unhealthy_ticks` bad ticks with no good tick between them the node stops campaigning and hands its leaderships to its peers. With the defaults that is 10 to 20 seconds into an outage when the probes fail, about 10 to 30 seconds when only writes fail (sooner when the store refuses them outright), and about a minute when every write attempt hangs until its timeout. While a tick is bad the node also holds routine snapshots back, though a log-pressure pass still runs. After `storage.cold.s3.heal_ticks` good ticks in a row it may lead again. For a node shed for failed writes, the good ticks start with the first write that succeeds after the last failed one. A node that has handed off every leadership flushes nothing, so its good ticks start only once its last failed write leaves the window, about a minute after that write with the defaults.
A shedding node hands off at most four leaderships per tick, the groups with the most unflushed bytes first, and only to peers that may campaign. A peer that is draining, waiting at its recovery gate, or shedding for its own snapshot store, cluster egress or WAL disk is passed over. The gate asks each peer for its leadership-shed state and waits `governance.leadership_balance.peer_timeout` (`500ms` by default, must be non-zero) for the answer, the same timeout the leadership balancer uses. A peer that does not answer in time is treated as unable to campaign. A handoff is done once another node leads the group, and one that Raft abandons is handed off again on a later tick. A node with N leaderships therefore evacuates in about `ceil(N / 4)` ticks, for example 22 ticks or 110 seconds for 86 leaderships at the default interval. The cap is per gate. The cold-health gate (`governance.cold_health`, every `2s` by default) drains the same way and independently, so a node that sheds for both moves up to four leaderships per 2 seconds plus four per 5 seconds. The leadership balancer (`governance.leadership_balance`) also keeps moving leaderships off a node that holds more than its fair share, up to `max_per_tick` per balancer tick. The cluster-egress gate (`governance.cluster_probe`) still hands every leadership off at once, because a node that cannot reach a quorum cannot replicate, and it plans again on every tick for the ones still pending. Its questions to the peers travel the path it judges impaired, so a lost answer says little about a peer. It passes over a peer that answered that it may not campaign. A peer that did not answer may take a leadership's first handoff, and the Raft handoff check decides. A leadership whose handoff did not move goes only to a peer that answered that it may campaign, so a peer that never answers costs each leadership at most one stalled handoff per shed. On a tick when a peer answered that it may not campaign, the gate passes over the peers that did not answer too, because the shed may be cluster-wide.
Raft snapshotting does not force a partial cold flush. Payload below the cold threshold stays in the bounded hot tail and is included in the next group snapshot, so a short snapshot interval does not turn small tails into small cold objects.
Helm and the EC2 scripts map their deployment inputs onto these same keys, so this table applies to every deployment method.
## Complete examples
Disk-backed single node (streams survive a restart):
```toml
[server]
listen = "127.0.0.1:4437"
[raft]
node_id = 1
group_count = 16
[raft.wal]
path = "./data"
```
```bash
ursula --config ./ursula.toml
```
Three-node durable cluster (same file on each host, override the node ID per host):
```toml
[server]
listen = "0.0.0.0:4437"
[runtime]
core_count = 16
[raft]
node_id = 1
group_count = 256
init_membership_per_group = true
[raft.wal]
path = "/var/lib/ursula"
[storage.cold]
backend = "s3"
root = "ursula-prod-20260518"
[storage.cold.s3]
bucket = "my-ursula-bucket"
region = "us-east-1"
[[raft.peers]]
node_id = 1
url = "http://10.0.0.1:4437"
[[raft.peers]]
node_id = 2
url = "http://10.0.0.2:4437"
[[raft.peers]]
node_id = 3
url = "http://10.0.0.3:4437"
```
```bash
ursula --config /etc/ursula/ursula.toml --node-id 1
```
See [Deploy a Cluster](/docs/0.7/deploy-cluster) for the full deployment walk-through.
---
# Security
> [!WARNING]
> Ursula does not terminate TLS, authenticate clients, or restrict admin endpoints. Treat the listening port as fully trusted. Run it on a private network behind a reverse proxy that owns TLS termination and request authentication.
The current `v0.x` security model is deliberately narrow. Ursula is built to slot behind your existing edge layer, not to be one.
## What Ursula does
- **Quorum-acknowledged writes.** An append is acknowledged only after a majority of voters has replicated it.
- **Per-group backpressure.** When a group's hot ring exceeds `storage.cold.max_hot_size_per_group`, appends to that group return `503` with `Retry-After` until cold flush catches up. Per-group, not global or per-client.
- **Stream-level isolation.** Streams hash to disjoint Raft groups and disjoint owner cores. A hot stream on one group cannot starve writes on a different group on a different core.
## What Ursula does not do
Handle the following outside Ursula:
- **TLS / HTTPS.** The public listener serves plain HTTP. No built-in `rustls`.
- **Inter-node encryption.** Peer gRPC (Raft heartbeats, append-entries, snapshots, and leader-read checks) runs over h2c. Peers must share a private network. Non-leader HTTP writes return a `307` redirect to the current group leader rather than being forwarded over gRPC.
- **API authentication on nodes.** Ursula nodes themselves accept any caller with network reach. Bearer-token validation is available as an opt-in feature of the gateway (see below); node listeners must stay on a private network either way.
- **Authorization / multi-tenancy on nodes.** Nodes enforce no per-user, per-bucket, or per-scope ACLs. The gateway's opt-in access control provides a bucket-level tenant boundary; anything finer stays upstream.
- **Admin endpoint isolation.** `/__ursula/metrics`, `/__ursula/flush-cold/*`, `/__ursula/raft/*`, and the public stream endpoints share the same listener with no auth gate.
- **Per-client rate limiting.** A single noisy client can saturate a core's mailbox or a group's hot ring.
- **Health/readiness endpoints.** `GET /__ursula/ready` is unauthenticated on
the client plane and reports only readiness plus WAL free-space guard state.
`/__ursula/metrics` contains substantially more operational detail; keep the
node listener private.
- **At-rest encryption beyond the cold tier.** Cold-tier S3 writes (including Raft snapshots) request SSE-S3 by default (`storage.cold.s3.server_side_encryption`, switchable to `aws-kms` or `none`). The hot ring is in memory; WAL and Raft log directories live on disk in plaintext — use full-disk encryption at the host level. Per-tenant KMS keys and client-side encryption are out of scope.
CORS is permissive (`Access-Control-Allow-Origin: *`). Restrict at the proxy for browser traffic.
Tenant offboarding has a first-class erasure path: the admin-plane [bucket purge endpoint](/docs/0.7/operations#tenant-offboarding-bucket-purge) removes a tenant's streams, bucket, and cold objects idempotently, leaving other tenants untouched. It deliberately retains aggregate monotonic usage counters so asynchronous accounting cannot miss committed work; see the operations note for the remaining identifier-erasure limitation.
## Gateway access control (opt-in)
A shared or internet-facing deployment can enable OAuth resource-server checks on `ursula gateway`. The feature is off by default; without the flags the gateway keeps its original trusted pass-through behavior.
```bash
ursula gateway \
--upstream http://ursula-0:4437 \
--auth-issuer https://issuer.example \
--auth-audience https://streams.example \
--auth-policy /etc/ursula/policy.toml
```
- **Authentication.** `Bearer` credentials are validated as RFC 9068 JWT access tokens: the header must declare `typ: at+jwt` (OIDC ID tokens are rejected), the signature must verify against the issuer's JWKS, and `iss`, `aud`, `sub`, `client_id`, `iat`, `exp`, and `jti` must all be present and valid. The JWKS location comes from `--auth-jwks-url` or RFC 8414 metadata discovery; keys are cached by `kid` and refetched on rotation.
- **Tenant boundary.** The bucket is the top-level namespace and logical tenant boundary. The policy file declares each bucket's owners (issuer-qualified subjects) and whether anonymous reads are allowed:
```toml
[[bucket]]
id = "tenant-a"
public_read = true
owners = [{ issuer = "https://issuer.example", subject = "user-1" }]
```
- **Concealment.** Unknown buckets, private buckets probed by strangers, and write attempts without ownership all answer the same `404` a missing resource would, so a private stream's existence is not observable.
- **Internal paths.** Every `/__ursula/*` path answers `404`, the gateway's own metrics at `/__ursula/gateway/metrics` included. The counters hold no tenant data, but they show traffic volume. `--auth-public-metrics` serves them to every caller, with or without a token, as a gateway without access control does. The gateway then does not check a presented token on that path, so an invalid or expired one gets the counters too. Set the flag only where untrusted clients cannot reach the gateway, or where a proxy in front of it denies `/__ursula/*`. Gateways before 0.7.4 served the metrics to every caller, with or without access control.
- **Credential termination.** The gateway strips `Authorization` before forwarding; upstream nodes never see end-user credentials and must remain on a private network.
- **Anonymous public reads.** `public_read` grants exactly the read-only actions (read, head, tail, snapshot read) to unauthenticated callers — never writes, deletes, or bucket administration.
- **Subscriptions do not outlive their credential.** A live tail is one long-lived request, so a single admission check at connection time would turn a short-lived token into an unbounded read. The gateway ends an SSE subscription at the credential's `exp` with a final `event: credential-expired` frame, so a client can tell expiry from end-of-stream and re-authenticate. Anonymous reads on `public_read` buckets have no credential and so no deadline. Note that this bounds duration, not revocation: a token revoked before its `exp` keeps an open subscription until then.
An access-controlled gateway can additionally enable per-tenant admission limits and usage accounting:
```bash
ursula gateway ... \
--quota-policy /etc/ursula/quotas.toml \
--usage-log /var/log/ursula/usage.jsonl
```
- **Quotas** (`--quota-policy`): per-bucket request rate (429 with `Retry-After`), concurrent live-read connections, and request body size. Limits are gateway-process-local; a horizontally scaled deployment multiplies effective limits by replica count. Ursula's own `503` backpressure semantics are unchanged. Neither Ursula nor this gateway enforces per-bucket stream-count or retained-bytes limits; enforce them in your control plane from the aggregated `/__ursula/usage` counters.
- **Usage** (`--usage-log`): per-tenant request, ingress, and egress byte counters aggregated by `(bucket, principal, action class)` and appended as sequence-numbered JSONL batches on `--usage-flush-secs` intervals. `--usage-chunk-bytes` adds a `chunks` counter alongside them: each append is counted as `ceil(bytes / unit)`, never below one, summed per request. It exists because that sum cannot be recovered afterwards — two appends of 5 KiB and 25 KiB and two of 10 KiB and 20 KiB agree on both request count and byte total while owing four units and three — so a deployment that charges per write unit has to be handed the sum rather than the ingredients. The unit size is a pricing choice and Ursula does not pick one; 10 KB and 25 KB are both in use. A batched append costs proportionally less than the same records sent individually, which is the intended incentive: batching is cheaper to serve. A failing sink delays reporting (batches queue and merge) but never blocks requests or drops counts. Egress is counted from actually streamed bytes, including SSE bodies. Committed-truth counters (append bytes surviving retries, retained bytes) come from Ursula's replicated state and are a separate, complementary ledger.
### Cross-origin reads (opt-in)
`public_read` is only nominally public until the origin answers CORS: without it, browser JavaScript cannot read a public stream cross-origin. Allowed origins are deployment policy, so nothing is sent unless you list them.
```bash
ursula gateway ... \
--cors-allowed-origin https://app.example.com \
--cors-allowed-origin https://studio.example.com
```
Pass `*` instead to allow any origin.
Three properties are deliberate:
- **Credentials are never allowed.** Ursula authenticates from an `Authorization` header the caller sets explicitly, which CORS does not treat as credentials, and no cookies are involved. So `*` grants no ambient access — a cross-origin page must still present its own bearer token.
- **`Access-Control-Expose-Headers: *`.** A read carries its continuation in response headers (`stream-next-offset`, `stream-cursor`), and a browser cannot see those without exposure — an unexposed client can read one page and never advance. The wildcard is only honoured while credentials stay disallowed, which is the second reason they are.
- **Preflight never consults the resource.** `OPTIONS` arrives without `Authorization`, so a per-bucket answer would tell an unauthenticated caller whether a private bucket exists. The preflight reply is identical for every path and is produced before authorization runs, which keeps concealment intact.
> [!NOTE]
> `EventSource` cannot set request headers, so a browser cannot open an SSE tail on a **private** stream with it. Use `fetch` with a `ReadableStream` and an `Authorization` header. `EventSource` is fine for `public_read` streams.
## Recommended deployment
```
Untrusted internet
│
v
┌─────────────────────┐
│ Reverse proxy │ TLS, authn, per-client
│ (nginx / Envoy / …) │ rate limiting, CORS
└──────────┬──────────┘
│ plain HTTP, private network
┌────────────┼────────────┐
v v v
┌────────┐ ┌────────┐ ┌────────┐
│ Ursula │ │ Ursula │ │ Ursula │
│ node │ │ node │ │ node │
└────────┘ └────────┘ └────────┘
↕ gRPC h2c on private network
(Raft replication)
```
### Checklist
- **Bind to the private interface.** Set `server.listen = "10.0.0.X:4437"` or use a security group / firewall so the listener is unreachable from public networks.
- **Terminate TLS at the proxy.** Ursula stays plain HTTP on the internal side.
- **Authenticate at the proxy.** Validate the caller (OAuth2, mTLS, signed requests) and reject unauthenticated traffic before it reaches Ursula.
- **Block admin paths from public traffic.** Deny `/__ursula/*` on the public listener and allow it only on an internal or ops network.
- **Use IAM roles for S3.** Omit static `storage.cold.s3.access_key_id` / `storage.cold.s3.secret_access_key` values and let the AWS SDK credential chain discover credentials.
- **Encrypt data volumes.** Apply full-disk encryption to `raft.wal.path`.
- **Keep peer traffic private.** Never route gRPC peer traffic across the public internet.
## Reporting vulnerabilities
Open a GitHub Security Advisory on [tonbo-io/ursula](https://github.com/tonbo-io/ursula/security/advisories) rather than a public issue.
---
# Resumable AI Stream
> Stream LLM tokens through a durable timeline so clients can refresh, reconnect, and resume without losing a token.
Instead of piping a model's output straight into one HTTP response, the generating worker appends tokens to a stream, and clients replay and tail that stream. A client that reconnects mid-generation replays the answer so far and continues from the same position, without losing or duplicating tokens. The upstream [`chat-aisdk` example](https://github.com/durable-streams/durable-streams/tree/main/examples) uses the same pattern.
## One stream per run
Create a text stream per generation, with a TTL so finished runs clean themselves up:
```bash
curl -X PUT http://127.0.0.1:4437/agent/run-42 \
-H 'Content-Type: text/plain' \
-H 'Stream-TTL: 86400'
```
## Producer: append tokens, close at the end
The worker that talks to the model appends each chunk. Producer headers make retries safe: a chunk re-sent after a network error is deduplicated instead of appended twice:
```ts
const base = "http://127.0.0.1:4437";
let seq = 0;
for await (const chunk of modelResponse) {
await fetch(`${base}/agent/run-42`, {
method: "POST",
headers: {
"Content-Type": "text/plain",
"Producer-Id": "worker-1",
"Producer-Epoch": "1",
"Producer-Seq": String(seq++),
},
body: chunk.text,
});
}
// Seal the stream: readers get an EOF signal, further appends return 409.
await fetch(`${base}/agent/run-42`, {
method: "POST",
headers: { "Stream-Closed": "true" },
});
```
If the worker crashes and restarts, bump `Producer-Epoch` and start `Producer-Seq` back at `0` (see [Exactly-once writes](/docs/0.7/concepts/exactly-once-writes)).
## Consumer: replay, then tail
`offset=-1` replays everything appended so far, then stays live for new tokens. Reopening the page mid-generation therefore shows the partial answer immediately and continues streaming:
```ts
const es = new EventSource(`${base}/agent/run-42?offset=-1&live=sse`);
es.addEventListener("data", (e) => {
answerEl.textContent += e.data;
});
es.addEventListener("control", (e) => {
if (JSON.parse(e.data).streamClosed) es.close(); // generation finished
});
```
Or watch it from a terminal:
```bash
curl -N 'http://127.0.0.1:4437/agent/run-42?offset=-1&live=sse'
```
The stream is durable and readable by any number of clients, so the same transcript can be tailed from a second device, read after the fact, or consumed by another process. The producer does not need to know about its readers.
## Next
- [Chat Room](/docs/0.7/examples/chat-room): the same pattern with JSON records and multiple writers
- [Read stream](/docs/0.7/api/read): catch-up, long-poll, and SSE read modes
- [Exactly-once writes](/docs/0.7/concepts/exactly-once-writes): producer epoch and sequence rules
---
# Chat Room
> One JSON stream per room: append messages, replay history from an offset, and tail new messages live.
A chat room is a JSON timeline: every chat message is one JSON message in the stream, stored in commit order, and every participant replays history and tails the same stream. This mirrors the chat examples in the [upstream Durable Streams repo](https://github.com/durable-streams/durable-streams/tree/main/examples).
## One stream per room
```bash
curl -X PUT http://127.0.0.1:4437/chat/room-7 \
-H 'Content-Type: application/json'
```
Each appended JSON object is stored as one line of NDJSON. Positions in the stream are offsets: opaque tokens that Ursula returns in `Stream-Next-Offset`. Clients store and pass back the offsets Ursula returned and never compute one themselves.
## Send a message
```bash
curl -X POST http://127.0.0.1:4437/chat/room-7 \
-H 'Content-Type: application/json' \
--data '{"user":"ada","text":"hello","sent_at":"2026-07-23T09:00:00Z"}'
```
Each append returns the room's new tail in `Stream-Next-Offset`. Appending a top-level JSON array stores one message per element, which is useful for flushing an offline outbox in one request. To make send retries safe against double-posting, add the [exactly-once producer headers](/docs/0.7/concepts/exactly-once-writes) with one `Producer-Id` per device.
JSON messages are validated on append and stored exactly as the sender wrote them, minus insignificant whitespace, so member order and number spelling survive.
## Replay history
Read the room from its start, or from an offset the client stored earlier:
```bash
curl -i 'http://127.0.0.1:4437/chat/room-7?offset=-1'
```
```json
{"user":"ada","text":"hello","sent_at":"2026-07-23T09:00:00Z"}
{"user":"lin","text":"hey ada","sent_at":"2026-07-23T09:00:07Z"}
```
The body is NDJSON, one message per line. A response returns at most 8 MiB and can end mid-line: buffer an incomplete trailing line and continue from the response's `Stream-Next-Offset` until a response carries `Stream-Up-To-Date: true`.
Loading only the last N messages needs a server-side paging primitive that does not exist yet; until it ships, replay from the start or from a stored offset, or keep a [snapshot](/docs/0.7/concepts/snapshots) of the recent history.
## Tail new messages live
`offset=now` skips history and streams only messages committed from now on:
```ts
const base = "http://127.0.0.1:4437";
const es = new EventSource(`${base}/chat/room-7?offset=now&live=sse`);
let pending = "";
es.addEventListener("data", (e) => {
pending += e.data + "\n";
const lines = pending.split("\n");
pending = lines.pop() ?? "";
for (const line of lines) {
if (line) renderMessage(JSON.parse(line));
}
});
```
A data event may hold several messages, one per line; SSE `data:` lines are transport framing, not message boundaries. Control events publish `streamNextOffset`. Store it and reconnect with `offset=` to resume without missing or repeating a message.
## Next
- [Resumable AI Stream](/docs/0.7/examples/resumable-ai-stream): the single-writer variant for streaming model output
- [Offsets](/docs/0.7/concepts/offsets): how resumption works
- [Browser Telemetry](/docs/0.7/examples/browser-telemetry): an advanced example with event-time indexing with offset locators
---
# Browser Telemetry
> Collect replayable browser events in a JSON stream and query them by client event time.
Browser telemetry is a natural fit for an `application/json` stream: the browser appends ordinary JSON over HTTP, Ursula stores each event as one message in commit order, and consumers replay or tail events from an offset.
The runnable example is in [`examples/browser-telemetry`](https://github.com/tonbo-io/ursula/tree/main/examples/browser-telemetry).
## Data model
Keep one timestamp: the time captured by the browser. It remains application data and may arrive out of order after offline buffering or retries.
```json
{
"captured_at": "2026-07-18T10:30:00.100Z",
"type": "network_response",
"status": 500
}
```
Ursula does not reorder the stream by `captured_at` and does not invent a second protocol timestamp. Commit order and offsets are the stream's only coordinates: an out-of-order `captured_at` never changes the committed order, the stored bytes or any offset. Event time is therefore not a stream read parameter (there is no `?captured_at=` read); the experimental indexer below answers time-range queries with offset locators.
## Browser collector
Point plain `fetch` at a same-origin route that proxies to the Ursula stream. The gateway owns browser authentication and CORS policy.
```js
const events = [{
captured_at: new Date().toISOString(),
type: "navigation",
duration_ms: performance.getEntriesByType("navigation")[0]?.duration,
}];
const response = await fetch("/telemetry/events", {
method: "POST",
headers: { "Content-Type": "application/json" },
body: JSON.stringify(events),
keepalive: true,
});
console.log(response.headers.get("Stream-Next-Offset"));
```
One array append stores one message per element. A successful response returns the stream's new tail in `Stream-Next-Offset`.
## Replay and live tail
Read events from the start of the stream:
```bash
curl -i 'http://127.0.0.1:4437/telemetry/events?offset=-1'
```
The body is NDJSON, one event per line. A response returns at most 8 MiB and can end mid-line: buffer any incomplete trailing line and continue from the response's `Stream-Next-Offset`; `Stream-Up-To-Date: true` means the read reached the tail. Offsets are opaque: store and pass back the values Ursula returns, and compare them only with each other. For a live native `EventSource`, add `live=sse`.
## Event-time queries (experimental)
Event time is an ordinary secondary index, not another Durable Streams coordinate. The optional `ursula-indexer` worker pool is **experimental** in this release: its API and storage format may change in a minor release. It:
1. reads each registered stream with ordinary offset reads;
2. extracts an event time from every message, skipping and counting messages that have none;
3. writes immutable sorted Parquet parts to S3 and advances a manifest with an ETag compare-and-swap;
4. returns locators — an opaque `offset` plus the message's `len` — that callers resolve back through Ursula.
The source stream remains canonical, and the index can be rebuilt from retained history. Equal timestamps are ordered by offset.
For Kubernetes, deploy one shared worker pool with its own S3 prefix and workload identity:
```yaml
s3:
bucket: my-telemetry-index
region: us-east-1
indexer:
enabled: true
replicaCount: 2
s3:
prefix: production/telemetry-indexes
serviceAccount:
annotations:
eks.amazonaws.com/role-arn: arn:aws:iam::123456789012:role/ursula-telemetry-index
```
When an application creates a browser stream, register it without changing the Helm release:
```bash
curl -X PUT http://ursula-indexer:4493/v1/indexes/browser-session-42 \
-H 'Content-Type: application/json' \
-d '{"stream_url":"http://ursula-gateway:4437/sessions/browser-session-42",
"extract":{"time":["/captured_at"]}}'
```
`extract` names the event time with JSON pointers: `time` is a fallback list, `end` optionally names when an event ended, `each` selects several elements of one message (with `*` matching every member or element), and `unit` is `auto`, `rfc3339`, `s`, `ms`, `us` or `ns`. The older `{"timestamp_field":"captured_at"}` form still works. Registration accepts `application/json` and `application/x-ndjson` streams and starts at the oldest retained event, or at the tail with `"start":"tail"`.
Query a time window:
```http
GET http://ursula-indexer:4493/v1/indexes/browser-session-42/events?from=2026-07-18T10:00:00Z&until=2026-07-18T11:00:00Z&limit=100
```
```json
{
"source": {"stream_url": "http://ursula-gateway:4437/sessions/browser-session-42", "incarnation": "1784370000000"},
"coverage": {"from": "00000000000000000000", "floor": "00000000000000000000",
"through": "00000000000000052114", "durable": "00000000000000052114",
"complete": true, "trimmed_bytes": 0},
"skipped": {"missing": 0, "invalid": 0, "unparseable": 0, "oversize": 0},
"entries": [
{"t_ms": 1784370600100, "t_end_ms": 1784370600100, "offset": "00000000000000004211", "len": 97},
{"t_ms": 1784370600100, "t_end_ms": 1784370600100, "offset": "00000000000000006630", "len": 88}
],
"next": "8000019f74c698a400000000000019e6"
}
```
Fetch an event with its locator. A read returns at most 8 MiB, so keep reading from each response's `Stream-Next-Offset` until `len` bytes have arrived; never compute an offset yourself. Locators are valid for `source.incarnation`; if the stream's `Stream-Incarnation` (HEAD) differs, the stream was recreated and the locators are stale:
```js
async function fetchEvent(streamUrl, { offset, len }) {
const chunks = [];
let received = 0;
while (received < len) {
const response = await fetch(`${streamUrl}?offset=${offset}&max_bytes=${len - received}`);
if (!response.ok) throw new Error(`event is no longer readable: ${response.status}`);
const chunk = new Uint8Array(await response.arrayBuffer());
if (chunk.length === 0) throw new Error("event is no longer readable");
chunks.push(chunk);
received += chunk.length;
offset = response.headers.get("Stream-Next-Offset");
}
const bytes = new Uint8Array(received);
let at = 0;
for (const chunk of chunks) {
bytes.set(chunk, at);
at += chunk.length;
}
return JSON.parse(new TextDecoder().decode(bytes));
}
```
Fetch the next page with the first response's `next` and `coverage.through`, so events indexed in the meantime do not appear halfway through one paginated answer:
```http
GET /v1/indexes/browser-session-42/events?from=2026-07-18T10:00:00Z&until=2026-07-18T11:00:00Z&limit=100&after=8000019f74c698a400000000000019e6&through=00000000000000052114
```
`match=overlap` returns events whose `[t_ms, t_end_ms]` span intersects the window instead of events that start inside it.
`GET /v1/indexes/{id}/status` reports the same `source`, `coverage` and `skipped` fields. Bad data never stops indexing: messages without a usable time are counted under `skipped`. When retention trims events before they were indexed, the index follows the stream's retained position and reports `"complete": false`. When the stream is deleted and created again, the index restarts on the new stream and `/status` reports `restarted_from_incarnation`. Indexing stops only when two workers disagree about the same events; the existing index stays queryable, and `POST /v1/indexes/{id}/status/resume` clears the block after repair.
S3 stores immutable content-addressed Parquet parts and versioned manifests. Parts are split into UTC-day event-time partitions, and bounded size-tiered compaction merges a fixed number of parts within one day. Manifest-aware garbage collection keeps a configurable number of recent generations and an in-flight grace window. The local cache is bounded and disposable, so a replacement pod recovers from S3 without a persistent volume.
## OpenTelemetry (OTLP-JSON)
The same indexer serves OpenTelemetry data exported as OTLP-JSON. Create one `application/json` stream per service, signal and day, and point the stock collector's `otlphttp` exporter at it:
```yaml
exporters:
otlphttp/ursula:
traces_endpoint: http://ursula-gateway:4437/otel/traces-checkout-20261003
encoding: json
compression: none
processors:
batch:
send_batch_max_size: 2048
```
Ursula does not decompress request bodies, so keep `compression: none`, and keep batches well under the 32 MiB request limit. Each export request is stored as one message. Register the stream with an extractor that walks every span of a batch:
```bash
curl -X PUT http://ursula-indexer:4493/v1/indexes/traces-checkout-20261003 \
-H 'Content-Type: application/json' \
-d '{"stream_url":"http://ursula-gateway:4437/otel/traces-checkout-20261003",
"extract":{"each":"/resourceSpans/*/scopeSpans/*/spans/*",
"time":["/startTimeUnixNano"],"end":["/endTimeUnixNano"],"unit":"ns"}}'
```
Each batch becomes one entry spanning its earliest span start to its latest span end; query it with `match=overlap`. For logs, use `"each":"/resourceLogs/*/scopeLogs/*/logRecords/*"` and `"time":["/timeUnixNano","/observedTimeUnixNano"]`: OTLP leaves an unset `timeUnixNano` at `0`, which counts as missing, so the observed time is used instead.
---
# SQLite on Ursula
`@tonbo/sqlite-ursula` gives a Node.js application a SQLite database that an Ursula stream replicates.
`open` needs `@tonbo/sqlite-ursula` 0.7.5 or later. With 0.7.4 and earlier, [attach the file yourself](#attach-a-file-yourself).
Start a single node in a terminal of its own. It keeps nothing across restarts.
```bash
docker run --rm -p 4437:4437 ghcr.io/tonbo-io/ursula:0.7.6
```
Install the package, which needs Node.js 22.19 or later, and run:
```bash
npm install @tonbo/sqlite-ursula
```
```ts
import { open } from "@tonbo/sqlite-ursula";
const db = open("http://127.0.0.1:4437/demo/notes");
db.exec("CREATE TABLE IF NOT EXISTS notes(id INTEGER PRIMARY KEY, body TEXT)");
db.prepare("INSERT INTO notes(body) VALUES (?)").run("hello"); // returns once the stream holds it
console.log(db.prepare("SELECT id, body FROM notes").all());
db.close();
```
`open` loads the `sqlite-ursula-vfs` extension into `node:sqlite`, catches a local copy of the database up from the stream, claims the stream for this process and returns the database. Every commit is appended to the stream before it reaches the local file, so any host can rebuild the database from the stream: the same code on another host opens the same database, and fences this one. The database is a `node:sqlite` `DatabaseSync`, so libraries that take one take it. The local copy is a cache, kept in a file named after the stream URL (see [Local files](#local-files)).
The local copy outlives the node. After you restart the node, its stream is gone, and `open` refuses to put an empty stream over a local copy that holds data (`not_found`). Delete `demo-notes-*` in `~/Library/Caches/sqlite-ursula` (`~/.cache/sqlite-ursula` on Linux), or open another stream.
## Errors
From 0.7.5, a failure of the extension throws an `UrsulaError`. Its `action` says what to do, its `kind` names what failed, and its `cause` is the error the failure surfaced as, usually SQLite's `disk I/O error` or `SQL logic error`. Every other error, such as a constraint violation, a syntax error or a write to a read-only database, is `node:sqlite`'s own, unchanged. `open` throws these errors, and so do the database's `exec`, `prepare` and `applyChangeset`, its statements' `run`, `get`, `all` and `iterate`, and `attach` and `status`.
| `action` | What it means for you |
| --- | --- |
| `retry` | Nothing reached the stream, and the database stays usable. Run it again later on the same database: the statement, or the whole transaction if it was one, since SQLite rolled the transaction back. No new `open` is needed. When `open` itself failed, call `open` again later. A layer in front of the stream that refuses writes, such as a quota service, or an endpoint that cannot be reached, fails this way. A commit that went out and got no answer is `reattach` instead. |
| `reattach` | The local file may no longer match the stream, for example after a commit whose outcome is unknown. Close the database and call `open` again: the new attach replays the stream, and the database then holds what the stream holds. The database's own `close()` and `open()` methods attach nothing. |
| `stop` | Another owner took the stream over. Stop using the database, and never reopen it in a loop: every `open` takes the stream back from the other owner, which only a deliberate takeover does. Exit, but only where nothing starts this process on the stream again (see [Rules](#rules)). |
| `fix` | The request cannot succeed as made: a stream URL the server refuses, a missing stream, a database the process has open already, a directory that refuses the local files, a transaction too large for one append. The message says what to change. |
```ts
import { UrsulaError } from "@tonbo/sqlite-ursula";
try {
db.prepare("INSERT INTO notes(body) VALUES (?)").run("hello");
} catch (error) {
if (!(error instanceof UrsulaError)) throw error; // SQLite's own error, such as a constraint violation
if (error.action === "stop") process.exit(1); // another owner took over, see Rules for what may restart us
throw error; // retry: try again later. reattach: close and open again. fix: change the setup.
}
```
Branch on `action`. The classes group the failures by what happened:
| Class | `action` | `kind` |
| --- | --- | --- |
| `UrsulaWriteRefusedError` | `retry` | `unauthorized`, `forbidden`, `rate_limited`, `not_applied`, `unreachable`, `refused`, `unavailable` |
| `UrsulaPaymentRequiredError`, a subclass | `retry` | `payment_required` |
| `UrsulaWriteRefusedError` | `fix` | `too_large`: the same transaction never fits in one append, so split it |
| `UrsulaReattachError` | `reattach` | `outcome_unknown`, `local_damage`, `local_state`, `internal` |
| `UrsulaFencedError` | `stop` | `fenced` |
| `UrsulaReplicationError` | `fix` | `unsupported`, `stream_state`, `local_setup`, `invalid_url` (a commit that met a redirect), any kind whose action is `fix`: a commit poisoned the file for a cause to fix before you open it again |
| `UrsulaSetupError` | `fix` | `invalid_argument`, `invalid_url`, `invalid_name`, `not_found`, `in_use`, `unreplicated_file`, `other_stream`, `unsupported`, `stream_state`, `not_attached`, `local_setup` |
`UrsulaWriteRefusedError` means that nothing reached the stream. For a refused commit its `kind` and `reason` are `status(file)`'s `refused_kind` and `refused_reason` (see [Writes refused before they reach the stream](#writes-refused-before-they-reach-the-stream)). For a failed attach its `kind` is the extension's, and its message says what refused it. Only `attach` itself gets a `402`'s own code, so `status` of a path whose attach a `402` refused throws `UrsulaWriteRefusedError` of kind `refused`, not `UrsulaPaymentRequiredError`. `UrsulaFencedError` and `UrsulaReattachError` are subclasses of `UrsulaReplicationError`, the class of a commit the stream did not take, whose `fenced` is `true` for a fence. A fenced database's reads fail too, with a message that starts with `read refused`. The kinds are those of the extension, which the [design doc](https://github.com/tonbo-io/ursula/blob/main/docs/architecture/sqlite-vfs.md#errors) lists, except that where the extension's kind of a refused commit is `refused`, the error's `kind` names what refused it.
The message of a failed attach is the extension's, `ursula_attach: : `, which says what failed and how to fix it. The message of a failed statement names the failure and what to do, such as `commit refused with 402 Payment Required in front of the stream: quota exhausted. Nothing of it reached the stream, and the database stays usable: run it again later, the whole transaction if it was one.` When `cause` is SQLite's error, its `code`, `errcode` and `errstr` are copied to the `UrsulaError`, so code that reads them keeps working.
SQLite rolls a transaction back when the extension fails one of its statements or its COMMIT. A transaction helper, yours or a library's such as drizzle's, then runs a ROLLBACK, which would fail with "cannot rollback - no transaction is active" and replace the `UrsulaError`. The database keeps the `UrsulaError` instead: such a ROLLBACK as a prepared statement succeeds, since nothing is left to roll back, and through `exec`, or as a ROLLBACK TO or RELEASE of a savepoint, it throws the `UrsulaError` again. A library that wraps errors, as drizzle does, keeps the `UrsulaError` as its error's `cause`.
Classification covers the file `open` attached. Other paths throw `node:sqlite`'s errors as they are: a tag store, a session, `backup()`, and a second replicated file used through SQL `ATTACH DATABASE`. A connection you open yourself (see [Attach a file yourself](#attach-a-file-yourself)) does too, and so does one made through `db.constructor`. A failed commit or read then names nothing by itself: `status(file).error` of the file that failed says what failed (`kind`) and what to do (`action`) while writes fail (`db.location()` is the file `open` attached).
## Rules
- One owner per stream. `open` (and `attach`) claims the stream for the process with a new producer epoch, which fences every earlier owner, on this host or another. The fenced owner's next commit fails with `UrsulaFencedError`, and from then on so do its reads.
- To move a database to another host or process, open it there. On the same host, a second process opens the same local file named after the URL, so it fails with `in_use` until the old one exits: stop the old process first, or give the new one its own `dir` or `path`, which rebuilds the database from the stream and fences the old one. The new owner's claim takes the stream over, and the old owner must stop, as the `stop` action says. Opening it again takes the stream back, so only a deliberate takeover does that, and nothing may start the old owner on the stream again while the new one runs. A scheduler that restarts an exited process in place does exactly that: the restarted process's `open` takes the stream back, and the two take turns fencing each other. Under Kubernetes, which restarts an exited container in place, stop the old pod before the new one opens the stream (the `Recreate` strategy, or a StatefulSet with one replica), or keep the fenced process running and unready until it is terminated. A rolling deploy that starts the new process before it stops the old one hands the stream over only when the old process stays down.
- With `open`, open a stream once per process and share the database. Opening it again while the database is open fails with `in_use`, and opening it at another `path` takes the stream over from the first database. Close it first to open the stream again, as the `reattach` action does.
- One process owns a database file at a time (a lock file next to it enforces this on the host). Another process that opens the same file fails with `in_use` until the first exits. Use as many connections inside that process as you like (see [Attach a file yourself](#attach-a-file-yourself)).
- WAL mode only, 4 KiB pages (SQLite's default). Attach leaves the file in WAL mode, so you set nothing: do not change `journal_mode` or the page size. `locking_mode=EXCLUSIVE` is not supported.
- Treat the local file as a cache: do not copy it elsewhere and attach it to another stream. A commit is durable once Ursula acknowledges it. Commits do not fsync the local files (only the db file, once per WAL wrap), so `PRAGMA synchronous` does not matter.
- After a reboot, a power loss, or on another host, attach discards the local files and rebuilds them from the latest snapshot and the stream's tail (a crashed process on the same boot resumes from its local files). A volume restored from a disk snapshot without a reboot is checked: attach uses it only if its WAL still holds what the sidecar `-ursula` records, and rebuilds otherwise. Keep the file on a local disk, not a network filesystem, and do not copy it with `cp` or `rsync` while the process runs. Delete the file to force a rebuild.
- A commit whose append gets no answer is retried with the same producer sequence for up to 30 s (`URSULA_VFS_RETRY_MS`), then fails, and the file must be attached again.
- From 0.7.3 the extension keeps a connection for 90 s after its last request (`URSULA_VFS_IDLE_MS`, in milliseconds, 0 turns reuse off), so commits up to that far apart reuse it instead of opening a new TCP and TLS connection. A request that gets no answer (its connection was reset or closed, or it timed out) is sent again on a new connection: a commit with the same producer sequence, and a read within `URSULA_VFS_RETRY_MS`. A refused connection fails a read at once. On macOS a reset can show as `Invalid argument (os error 22)`: from 0.7.4 that counts as no answer too, so a read that meets it is sent again, as on Linux. A NAT, a load balancer or a firewall on the way that drops idle connections sooner than 90 s without telling either end makes the first request after such a pause wait out its timeout (10 s) before it is sent again. Behind such a device, set `URSULA_VFS_IDLE_MS` below its idle limit.
- From 0.7.3 the extension never follows a redirect. A `307` (a gateway before 0.7.3 passing on a redirect to the stream's leader, which moved, or a node that the URL names and that does not lead the stream's group) is sent again to the same URL within `URSULA_VFS_RETRY_MS`, like a `503`. Any other redirect fails the request at once with the redirect's `Location` in the error: a commit then fails, and the file must be attached again. A URL that names one node of a multi-node cluster works only while that node leads the stream's group. A URL that a proxy redirects (from `http://` to `https://`, say, usually with a `308`) fails, so use the address the redirect points to.
- A transaction must fit one request: up to 32 MiB of changed pages after compression by default (the server's request body limit), set by `URSULA_VFS_MAX_FRAME_BYTES` in bytes. That is about 8,000 pages of incompressible data, such as embeddings, media or encrypted bytes, and about three times as many of typical text. A larger transaction is never sent. It fails alone with `SQLITE_IOERR_AUTH` and `refused_kind` `too_large` (an `UrsulaWriteRefusedError` whose `action` is `fix`), and the file stays usable. Split bulk imports into smaller transactions. Statements that rewrite a whole table or index are one transaction too: `CREATE INDEX` on a large table, `REINDEX`, an `ALTER TABLE` that rebuilds the table, and `VACUUM`, which rewrites every page. On a database whose compressed size is above the limit, `VACUUM` fails the same way. Delete rows before it, so that what it writes compresses below the limit, or copy the rows into a new database in transactions below the limit.
- Behind a gateway or proxy with a lower body limit, set `URSULA_VFS_MAX_FRAME_BYTES` at or below it. A larger commit sent there usually meets a closed connection rather than a readable `413` ([#540](https://github.com/tonbo-io/ursula/issues/540)), and then poisons the file once `URSULA_VFS_RETRY_MS` runs out. `URSULA_VFS_MAX_FRAME_BYTES` takes 64 KiB to 32 MiB: a value outside is taken to the nearer bound, and one that is not a number of bytes is ignored, both logged once as `setting_adjusted`.
- A file that has been attached (it has a sidecar `-ursula`) opens in a new process only after its attach there succeeds. From 0.7.5 a file whose attach failed in the process (the server is unreachable, or refused the stream's name, for example) opens only after an attach of it succeeds, with or without a sidecar. Opening it otherwise fails, so no commit can bypass the stream: SQLite reports "unable to open database file", and the log and `status(file)` name the failed attach's reason. An attach refused because of the file itself before it changed anything (`in_use`, `unreplicated_file`, `other_stream`) is no failed attach: the file opens as before. A failed attach may already have changed the local files, as any attach does: discarded the ones it did not trust, or folded their WAL into the file. One that fails before it wrote anything into a new file removes the sidecar it created, which never touches the file, its `-wal` or `-shm`. A file it replayed pages into keeps its sidecar: the next attach resumes it when the sidecar names the file, and rebuilds it otherwise.
- From 0.7.5 attach refuses (`in_use`), changing nothing, while the process has the file, its `-wal`, `-shm` or `-journal` open outside the extension, read-only or not: a connection opened before the extension was loaded, or one opened through another VFS, such as `?vfs=unix`. Its commits would never reach the stream. While such a connection is open, SQLite also keeps the files of closed connections open. Close every such connection, then attach. The check reads the process's open files where it can: on Linux with `/proc` mounted, and on macOS. Without that, on other platforms, or when the file's directory cannot be resolved, attach does not check, and it never finds a file opened through another hard link or mount of it. A database that was never attached and holds data is refused, never replicated over, and opens as before: copy its rows into a newly attached file, in the same process if you like (with `ATTACH`, for example), or move it and its `-wal` aside to attach the path.
- Never write an attached file outside the extension. A write after the attach, through a connection that bypasses the extension or from a process without it, is not detected, as in 0.7.4: it never reaches the stream, and the owner's later commits and snapshots carry pages built on it, so copies rebuilt from the stream can diverge from the file or be malformed.
- A stream deleted and recreated at the same path is a new stream: attach never resumes the local file on it, and rebuilds it from the new stream, whatever its length. When the stream of a file that holds data is missing (deleted by mistake, expired, or not carried over by a fresh install), attach refuses and keeps the file, which may be the only copy left. Delete the file to start over from an empty stream, or attach a new file under another stream URL. Neither the file nor a copy of it attaches under another URL. To salvage its data, copy the file and its `-wal` to a new name before deleting anything. The copy has no sidecar, so it opens as a plain SQLite file: read its rows from there into a newly attached database. An owner of the deleted stream is normally fenced at its next commit, but stop every owner before deleting a stream you will recreate: a snapshot it is publishing, or an attach racing the recreate, can still put its data into the new stream (see the design doc).
- Within a minor (0.7.x), extension versions take over each other's streams, so upgrade owners one at a time: the new owner's attach fences the old one. Across minors, follow the release notes. A minor that changes the extension's `Producer-Id` cannot fence owners of the other version, so both would commit and could corrupt the database: stop every owner of a stream before attaching it with the new version (and the reverse on a downgrade). A frame or snapshot of another minor's format stops attach with an error that names it.
- An owner idle for 7 days loses its producer state on the server. Its next commit takes the stream back only if nobody else wrote meanwhile.
- Every commit carries a `Stream-Seq` above all earlier ones. Do not append to the stream yourself: a write with a higher `Stream-Seq` fences the owner at its next commit, and one without breaks every rebuild that replays it, until the owner publishes a snapshot past it.
- `URSULA_VFS_SNAPSHOT_MIN_BYTES` sets the smallest log between snapshots (default 8 MiB).
## How it works
`sqlite-ursula-vfs` is a SQLite loadable extension that replicates a database file to an Ursula stream. Your application keeps using SQLite unchanged, for example `node:sqlite` or Pi Durable's official `SqliteStorage`: every commit is appended to the stream before it reaches the local file, and any host can rebuild the database from the stream. The extension lives in [`clients/sqlite-vfs`](https://github.com/tonbo-io/ursula/tree/main/clients/sqlite-vfs), and the TypeScript helpers in [`clients/sqlite-ursula`](https://github.com/tonbo-io/ursula/tree/main/clients/sqlite-ursula), published to npm as `@tonbo/sqlite-ursula` with the extension prebuilt. The package has no runtime dependencies, and its Pi Durable adapter is a separate entry, [`@tonbo/sqlite-ursula/pi`](#pi-durable).
- One database is one binary stream (`application/octet-stream`). A commit is one append carrying the transaction's final page images, zstd-compressed.
- The append happens at SQLite's commit point. `COMMIT` returns only after Ursula acknowledged it. If the append is rejected, SQLite rolls the transaction back and nothing of it stays local.
- Attaching a file claims the stream with a new producer epoch. Every earlier owner is fenced: its next commit fails, and from then on so do its reads.
- The owner publishes a [snapshot](/docs/0.7/concepts/snapshots) of the database whenever the log since the last one outgrows the database (8 MiB at least), reads it back, and then advances the stream's retention to the snapshot before it. A new host starts from the latest snapshot and replays only the tail.
## Local files
Requires 0.7.5 or later of `@tonbo/sqlite-ursula`.
`open(streamUrl, options)` takes `node:sqlite`'s own options (`timeout`, `readBigInts` and the rest), which it checks before it attaches anything, so a wrong one claims nothing. It takes three of its own:
- `path`: the local file to keep the database in, a plain file path. A relative one is made absolute when `open` runs, so a later `process.chdir` changes nothing. `:memory:`, `""` and `file:` URIs are refused (`invalid_argument`) before anything is attached: SQLite would open a temporary database or read a URI, not the attached file.
- `dir`: the directory of a file named after the stream URL, used when `path` is not given. `open` creates it (mode `0700`) when it is missing. By default it is `sqlite-ursula` in your cache directory: `$XDG_CACHE_HOME` when that is set to an absolute path, `~/Library/Caches` on macOS and `~/.cache` elsewhere, in the home directory that `HOME` names, or the user database when `HOME` is not an absolute path. Without either, `open` needs `dir` or `path`.
- `readOnly`: attach the file [read-only](#read-only-attach) and open it read-only. Without `path`, on a file of its own (see below).
The stream URL names the stream alone. A URL with a query or a fragment is refused (`invalid_url`) before any request is sent, since the extension adds its own query to the stream URL.
The file named after the URL is `-.db`, and `--ro.db` for a read-only open:
- `` is the first 32 hex digits (128 bits) of the SHA-256 of the stream the URL names: its scheme and host (lowercased), its port when it is not the scheme's default, and its path, with `.` and `..` segments resolved, every segment percent-decoded and encoded again one fixed way, and no trailing `/`, as the extension drops it. Userinfo plays no part. So every spelling of one stream URL maps to the same file, whatever the Node.js version, and two different stream URLs never map to the same one, short of a SHA-256 collision on 128 bits.
- `` only makes the name readable: the last two segments of the URL's path (normally the bucket and stream ids), percent-decoded, joined by `-`, lowercased, with every run of characters other than `a-z`, `0-9`, `_` and `-` replaced by `_`, as is a leading `-`, cut to 64 characters, and `stream` when the path has no segment.
- `-ro` keeps a read-only open on a file of its own: the first of `-ro1` to `-ro8` that no other connection or process holds. So a reader on the owner's host does not hold up the owner, and two readers on one host both open. Each such file is a copy of the database. A process holds every file it attached until it exits, closed or not, so a ninth reader of a stream on a host, or a reader after a process that opened eight, fails with `in_use` and a message that says so: give it its own `dir` or `path`.
`http://127.0.0.1:4437/demo/notes` maps to `demo-notes-.db`. The name is at most 104 characters of `a-z`, `0-9`, `_`, `-` and `.db`, and never starts with `-` or `.`, so it is a safe file name for any bucket and stream id, on a case-sensitive file system or not. Next to it are SQLite's `-wal` and `-shm` and the extension's sidecar `-ursula` and lock `-ursula.lock`.
The extension records in the sidecar the stream URL a file was attached with, and refuses a later attach of the file under another spelling of the stream as the cache of another stream (`other_stream`). So `open` attaches a file named after the URL with the URL's one spelling (the path as the hash reads it, the userinfo kept), and every spelling of the stream opens it. With `path`, the URL goes to the extension as you give it: spell it one way. Two URLs that reach one stream through different hosts, such as a gateway's and a node's, or `localhost` and `127.0.0.1`, map to two files: opening the second takes the stream over from the first.
The local file is a cache of the stream, so its directory does not need a backup. After a reboot, or on another host, attach rebuilds the file from the latest snapshot and the stream's tail. A process that restarts on the same boot resumes from the file. Keep the directory on a local disk, and in a container point `dir` at a volume that outlives the container if a restart should not rebuild the database.
When a stream goes missing while its local file holds data, `open` refuses (`not_found`) and keeps the file, which may be the only copy left (see [Rules](#rules)). Do not open the stream under another URL meanwhile, through another host or a gateway, say: that file is another one, so the attach creates an empty stream, and the next open of the first URL then discards the first file as the cache of a stream that was deleted and recreated. Copy the rows out of the file first.
## Install
```bash
npm install @tonbo/sqlite-ursula
```
The package needs Node.js 22.19 or later. It ships the extension prebuilt for Linux x64 and arm64 with glibc 2.28 or later (Debian 11, Ubuntu 20.04, Amazon Linux 2023 and newer), Linux x64 with musl (Alpine), and macOS 11 or later on Apple silicon. npm installs only the build for your platform.
On other platforms, build the extension and point `SQLITE_URSULA_VFS` at it. The variable also overrides the prebuilt extension.
```bash
cd clients/sqlite-vfs
cargo build --release # target/release/libsqlite_ursula_vfs.so (.dylib on macOS)
export SQLITE_URSULA_VFS=$PWD/target/release/libsqlite_ursula_vfs.so
```
## Attach a file yourself
`open` is `attach` followed by `new DatabaseSync(path)`. Make the two steps yourself to open more than one connection to a database, or to hand the file to code that opens it. With 0.7.4 and earlier, which have no `open`, this is the way. The extension calls whatever SQLite loads it, here `node:sqlite`. Load it once per process: it registers the `ursula` VFS as SQLite's default. `open`, `attach` and `openUrsulaPiStorage` load it if you have not.
```ts
import { attach, loadUrsulaVfs, status } from "@tonbo/sqlite-ursula";
import { DatabaseSync } from "node:sqlite";
loadUrsulaVfs(); // the prebuilt extension, or the one SQLITE_URSULA_VFS names
// Catch the file up (from the latest snapshot if it is missing or behind), claim the stream,
// and attach. No connection to the file may be open yet.
attach("/data/app.db", "https://ursula.example.com/my-bucket/app-db");
const db = new DatabaseSync("/data/app.db"); // in WAL mode already
db.exec("CREATE TABLE IF NOT EXISTS notes(id INTEGER PRIMARY KEY, body TEXT)");
db.prepare("INSERT INTO notes(body) VALUES (?)").run("replicated"); // returns after Ursula has it
console.log(status("/data/app.db"));
// { offset, epoch, poisoned, fenced, reason, read_only, writes_refused, refused_kind, refused_reason, snapshot, ..., error (from 0.7.5) }
```
A connection you open this way throws SQLite's errors as they are. From 0.7.5, `status(file).error` names a failure (see [Errors](#errors)). Before, `poisoned`, `fenced`, `reason`, `writes_refused` and `refused_kind` name it. SQLite rolls the transaction back on such an error, so run a ROLLBACK after it only while `db.isTransaction` is `true`: otherwise its "cannot rollback" replaces the error. From 0.7.5 `attach` throws an `UrsulaError`, as `open` does. Before, it threw `UrsulaWriteRefusedError` for a refused attach and SQLite's error otherwise.
From SQL, the same functions are `ursula_attach(path, url)` (`ursula_attach(path, url, 'read_only')` for a [read-only attach](#read-only-attach)), `ursula_status(path)`, `ursula_stats(path)` and `ursula_set_token(token)`, on the connection that loaded the extension (`loadUrsulaVfs()` returns it). They can be called from top-level SQL only, not from a trigger or a view. Offsets (`attach`'s result, and `offset`, `snapshot`, `retained`, `local` and `installed`) are the server's [opaque offset](/docs/0.7/concepts/offsets) strings: compare two of the same stream as strings, never as numbers. `"-1"` means none (no snapshot, no retention, a file rebuilt from nothing).
## What the cluster needs
- Room for one transaction per append: up to 32 MiB of compressed page images.
- For databases whose snapshots exceed 1 MiB, a cold backend, so snapshot bodies are stored in the cold tier (up to 1 GiB). Without it, snapshots are kept inline, up to 32 MiB.
- Snapshots above 32 MiB must go through a gateway whose `--max-request-body-bytes` is raised, or straight to a single-node server. With `--body-budget-bytes` unset, that gateway's body budget grows to hold one body at the limit beside its 32 MiB small-body headroom, An explicit budget must be at least that. Give the gateway about twice the budget in memory.
- A stream URL that reaches the stream's leader by itself: a gateway, or a single-node server (see [Rules](#rules)).
## Versions
Within a minor release (0.7.x), the formats the extension writes to the stream and to the local files do not change. Any 0.7.x extension can attach a stream that another 0.7.x extension wrote, resume from the local files that extension left on the host, and fence the stream's owner, so you can upgrade owners one at a time. The API of the extension and of `@tonbo/sqlite-ursula` can still change in a patch release, as it did in 0.7.3, so upgrade the package together with the extension. A new minor release may also change the formats. Either way, the [release notes](/docs/0.7/release-notes) say how to upgrade (see also [Rules](#rules)). The design doc lists the [known limits](https://github.com/tonbo-io/ursula/blob/main/docs/architecture/sqlite-vfs.md#7-limits).
## TLS and credentials
`https://` stream URLs work with the certificate authorities bundled with the extension (Mozilla's). For a private CA, point `URSULA_VFS_CA_FILE` at a PEM bundle: the extension then trusts only the CAs in it.
Behind `ursula gateway` with access control (see [Security](/docs/0.7/security)), every request needs a bearer token. Name a file that holds it, and the extension reads it again whenever it changes (a Kubernetes projected token, or a file a sidecar refreshes):
```bash
export URSULA_VFS_TOKEN_FILE=/var/run/secrets/ursula/token
```
Or set it from the application. It takes precedence over the file until you set `null`:
```ts
import { setToken } from "@tonbo/sqlite-ursula";
setToken(accessToken);
```
The token is process-wide. A request whose token is refused (`401`) is sent again, with the token read again, within the retry budget (`URSULA_VFS_RETRY_MS`). A commit whose token is still refused then fails alone: the file is not poisoned, and the next commit is sent as usual (see [Writes refused before they reach the stream](#writes-refused-before-they-reach-the-stream)). A `403` from the gateway's authorizer, which the gateway marks as applying nothing from 0.7.3 on, fails a commit alone at once. Any other `403` fails the commit, and the file must be attached again. Neither marks the owner fenced. The gateway's built-in access policy answers `404` for a private bucket to a client without a token, or with a token the policy does not allow, so attach then reports the stream as missing. From 0.7.3 the gateway marks that `404` as applying nothing, so a commit it denies fails alone, with `refused_kind` `not_applied` and `refused_reason` `NotFound`: the credentials or the policy deny the write, not load.
`setToken` cannot rescue a commit that is already retrying. `node:sqlite` is synchronous, so the thread that runs the commit cannot call `setToken` until the commit returns. A commit whose set token has expired waits out the retry budget (30 s by default, and reads of the file in the process wait with it), then fails alone. Call `setToken` with a fresh token and commit again. To avoid the wait, call `setToken` before the current token expires, or use a token file, which is read again on every retry.
When the token file holds no token as it is read (empty, half-written, or missing while it is replaced), the extension keeps the last token it read from it, and logs `token_file_unreadable` once until the file holds a token again.
Over `http://` the token crosses the network in clear. Use `https://` for any endpoint on another host. The extension logs `token_in_clear` once when it sends a token over `http://` to a host that is not a loopback address.
## Writes refused with 402
Requires 0.7.1 or later of `@tonbo/sqlite-ursula` and of the extension.
A layer in front of the stream, such as an authorizer or a quota or billing service, may refuse writes with `402 Payment Required`. Ursula itself never does. The extension takes a `402` to mean that nothing of the write reached the stream, so it fails that transaction alone:
- The commit fails with `SQLITE_IOERR_AUTH` (`errcode` 7178 in `node:sqlite`), and SQLite rolls the transaction back. The file is not poisoned, and the owner keeps its claim on the stream.
- Reads keep working from the local file.
- Later commits are sent as usual, and succeed once the layer accepts writes again. No new attach is needed.
- `status(file)` reports `payment_required: true` until a commit succeeds or the file is poisoned, with the layer's explanation as `payment_reason`: its response body, the first 256 bytes followed by `...` when it is longer. Through `openUrsulaPiStorage`, and from 0.7.5 through `open`'s database, the commit throws `UrsulaPaymentRequiredError`, whose `reason` is the same text (and from 0.7.5 whose `action` is `retry`).
One case still poisons. When a commit's append got no answer, a `5xx`, or a `307` before the `402`, the append may already be in the stream, so the extension retries it like any append whose outcome is unknown. If the layer still refuses it when `URSULA_VFS_RETRY_MS` runs out, the commit fails and the file must be attached again.
This is safe only if the layer follows the rule in [Writes refused before they reach the stream](#writes-refused-before-they-reach-the-stream).
An attach claims the stream, which is a write. When the layer refuses it, `open` and `attach` throw `UrsulaPaymentRequiredError` (`ursula_attach` fails with `SQLITE_AUTH`, `errcode` 23), and the file opens again only after an attach succeeds. A [read-only attach](#read-only-attach) can read the database meanwhile.
While writes are refused, the owner takes no snapshot. It publishes the next one after a commit succeeds, and a refused snapshot does not count in `snapshot_failures`. A transaction refused as `too_large` holds no snapshot back.
## Writes refused before they reach the stream
Requires 0.7.3 or later of `@tonbo/sqlite-ursula` and of the extension. Ursula marks its refusals from 0.7.3 on, and the marks count only behind `ursula gateway` 0.7.3 or later (see below).
A `402` is one of several answers that refuse a write before it reaches the stream. The extension handles all of them the same way, because nothing of such a write was applied:
- A refusal (a `4xx` or `5xx`) with both the header `x-ursula-not-applied` and the header `x-ursula-not-applied-checked`. A redirect or a success proves nothing, whatever headers it carries. The first header marks the refusal, and its value names the reason. The Ursula server sets it on writes it refuses before applying them, for example a `503` with `RaftLogPressure` while its Raft log is over its limit under sustained write load, or `WalDiskPressure` while its WAL disk is full. The gateway sets it on a request it refuses before forwarding it, for example while its token issuer is down, or when its authorizer denies the write with a `403`. The second is the gateway's vouch. `ursula gateway` adds it from 0.7.3 on to every answer it returns, after it has dropped the marks it cannot vouch for, such as a server's mark on an answer it got by following a redirect that carried no mark, since the server may have sent that redirect after it proposed the write. The server never adds it.
- A `401`: the token was refused.
- A `429`: the request was rate limited.
- From 0.7.4, an attempt that failed before any byte of it went out: the stream URL's host did not resolve, or no connection to it was made (refused, timed out, or a failed TLS handshake). An attempt that went out on a connection and then failed is not one of these.
So the mark counts only behind `ursula gateway` 0.7.3 or later. With no gateway in front of the servers, or an older one, an answer with the mark alone proves nothing, and only a `401`, `402` or `429`, or attempts that never went out, fail a commit alone.
A `403` counts only with both headers. Any other `403` fails the commit, and the file must be attached again, as before 0.7.3, because a filtering proxy can answer `403` in place of the stream's answer after the stream applied the write. A `403` with a `Producer-Epoch` header is the server fencing an older owner, which poisons as before.
A `402` fails the commit at once, and so does a `4xx` with both headers, such as an authorizer's `403` or a request that is too large. The others (a `401`, a `429`, a `5xx` with both headers, an attempt that never went out) are retried for up to `URSULA_VFS_RETRY_MS`, since they often clear within seconds. When the budget runs out and every attempt was refused this way, the commit fails alone:
- The commit fails with `SQLITE_IOERR_AUTH` (`errcode` 7178 in `node:sqlite`), and SQLite rolls the transaction back. The file is not poisoned, and the owner keeps its claim on the stream.
- Reads keep working from the local file.
- Later commits are sent as usual, and succeed once the refusals stop. No new attach is needed.
- `status(file)` reports `writes_refused: true` until a commit succeeds or the file is poisoned. `refused_kind` says what refused the write: `payment_required` (a `402`), `unauthorized` (a `401`), `forbidden` (a `403`), `rate_limited` (a `429`), `too_large` (a transaction too large for one request, or a `413` with both headers, see [Rules](#rules)), `not_applied` (any other refusal with both headers) or `unreachable` (the last attempt never went out). `refused_reason` says why: the refusing layer's response body (the first 256 bytes, followed by `...` when it is longer), or the value of `x-ursula-not-applied` when the body is empty, and always for `not_applied`. For `too_large` it gives the frame's size and the limit, or the header's value, and for `unreachable` the last attempt's failure. [Troubleshooting](/docs/0.7/troubleshooting#a-commit-or-attach-is-refused-with-error-code-7178) lists the reasons and what to do about each.
- Through `openUrsulaPiStorage`, and from 0.7.5 through `open`'s database, the commit throws `UrsulaWriteRefusedError`, whose `kind` and `reason` are the same. For a `402` it is an `UrsulaPaymentRequiredError`, a subclass. From 0.7.5 its `action` is `retry`, and `fix` for `too_large`.
When any attempt of a commit went out and got no answer, a `307`, or a `5xx` without both headers, the commit may already be in the stream. Refusals after that attempt prove nothing, so the commit fails and the file must be attached again, as before. Without a gateway of 0.7.3 or later, every `503` of the server is such an answer. Through a gateway with several upstreams this can happen while a single node refuses writes, because the gateway drops the mark when it reaches that node by following a leader redirect that carries none.
An attach whose claim is refused this way makes `open` and `attach` throw `UrsulaWriteRefusedError` (`ursula_attach` fails with `SQLITE_IOERR_AUTH`, `errcode` 7178, or with `SQLITE_AUTH` for a `402`). Nothing was claimed. Attach again later, or attach [read-only](#read-only-attach) to read the database meanwhile. From SQL, call `ursula_attach` without a `FROM` clause, as in `SELECT ursula_attach(?, ?)`: 7178 is an I/O error code, and SQLite rolls back the calling connection's open transaction when a statement that reads a table fails with one.
This is safe only if every layer in front of the stream follows one rule: answer `401`, `402` or `429`, or send `x-ursula-not-applied` with `x-ursula-not-applied-checked`, only on a request that the stream did not apply. A layer that passes an append on to the stream and then refuses it, for example with a `402` from metering that runs after the stream answered, breaks the rule. So does a proxy or service mesh that sends a write upstream again, because it retries or follows a redirect, and relays the refusal of a later attempt: an earlier attempt may have been applied. Turn off retries and redirect following for writes in any proxy in front of Ursula. Some service meshes retry unless told not to. Before 1.24, Istio's default retry policy retries a `503` twice, POSTs included, and Istio 1.24 to 1.26 does the same when istiod runs with `EXCLUDE_UNSAFE_503_FROM_DEFAULT_RETRY=false`, which `compatibilityVersion=1.23` sets (see [redirects and retries](/docs/0.7/api/overview#redirects-and-retries)). Any version of Istio retries on a route whose `retryOn` includes `503`, `5xx`, `gateway-error` or `reset`. Envoy does the same when a retry policy is set. Set `retries: attempts: 0` on the routes that carry stream writes, to the gateway and from the gateway to the servers. No layer between `ursula gateway` and the servers may answer `401`, `402` or `429` at all: the gateway passes such an answer on after it follows a leader redirect, and the extension trusts these statuses without a mark. Ursula's own components never answer so there.
When the rule breaks, the stream holds the refused commit and the file does not. The next commit goes out with the refused commit's producer sequence, and the stream answers it as that commit's duplicate. From 0.7.3 the extension catches that answer: the next commit fails, and the file is poisoned with a `reason` that says a commit refused as never applied may have been applied after all. The answer does not tell which commit the stream holds, the refused one or the next one, which a gateway sends again to a new leader when the old leader redirects it after proposing the commit. Attaching the file again replays whichever it is, so check which before you run either transaction again (see [Troubleshooting](/docs/0.7/troubleshooting#the-file-is-poisoned-at-the-commit-after-a-refused-one)).
The check has gaps:
- When an attempt of that next commit got no answer, a `307`, or a `5xx` without both headers before the duplicate's answer, the extension takes the answer for that attempt's own. The next commit is then acknowledged without being appended, and the file and the stream diverge without an error. The extension logs `refused_commit_unchecked` when this happens at the producer sequence a refused commit left free. Before 0.7.3, a gateway whose `--upstream` addresses differ from the servers' peer URLs could not follow their leader redirects and passed them on, so behind it a commit often met a `307`. From 0.7.3 the gateway looks for the leader among its upstreams instead.
- When the file is attached again before its next commit, for example after a restart, the attach replays the refused commit with no error. A transaction that failed with 7178 is then in the database, and running it again applies it twice.
- After 7 days without a write the stream forgets the producer, and the extension claims the stream again before its next commit. When a false refusal comes before such a gap, or refuses that claim, the next commit reports a fence (`fenced: true`) although no other owner exists.
Ursula's gateway and server follow the rule whatever versions run together. A gateway before 0.7.3 follows a leader redirect and passes on the mark of the answer it gets, and a server can send that redirect after it proposed the write. But such a gateway never adds `x-ursula-not-applied-checked`, so the extension ignores that mark, and the commit fails and poisons the file, as before 0.7.3. So the upgrade order is a recommendation, not a requirement. The marks count from the moment the gateways run 0.7.3. Where you can, upgrade the gateways to 0.7.3 before the servers, and roll back the servers before the gateways: the gateway's own refusals then keep failing alone while the versions are mixed. A Helm upgrade or rollback that changes both at once is safe, also while the versions stay mixed until the voters are rolled. See [refusals that applied nothing](/docs/0.7/api/overview#refusals-that-applied-nothing).
## Read-only attach
Requires 0.7.1 or later of `@tonbo/sqlite-ursula` and of the extension, and 0.7.5 or later for `open`. Called from plain JavaScript, the 0.7.0 `attach` and `openUrsulaPiStorage` ignore `readOnly` and claim the stream, which fences its owner.
```ts
const db = open("https://ursula.example.com/my-bucket/app-db", { readOnly: true }); // 0.7.5 or later
// or, for a file you open yourself (0.7.1 or later)
attach("/data/app.db", "https://ursula.example.com/my-bucket/app-db", { readOnly: true });
```
A read-only attach catches the file up like any attach but claims nothing, and it never writes the stream. It does not fence the owner, and it works while a layer in front of the stream refuses writes. Use it to export a database from another host, or to read one whose owner cannot claim it. `open` with `readOnly` also opens the file read-only in `node:sqlite`, so SQLite refuses a write at its statement, and keeps its own file (see [Local files](#local-files)), so it opens next to the owner on the owner's host, and next to another reader.
- Every write fails with `SQLITE_READONLY` ("attempt to write a readonly database"), SQLite's own error, not an `UrsulaError`. A transaction that changes nothing still commits.
- A failed `COMMIT`, or a failed statement outside an explicit transaction, leaves no transaction open. Inside an explicit transaction, a statement whose changes outgrow SQLite's page cache fails before `COMMIT`, and the transaction stays open. After an error, run `ROLLBACK` if the transaction is still open (`isTransaction` in `node:sqlite`). Until then, even reads on that connection fail with the same error. Pi's storage rolls back a failed transaction itself.
- The file holds the stream as of the attach. Attach it again to catch up.
- A missing stream is an error. A read-only attach never creates one.
- `status(file)` reports `read_only: true`, and `epoch` is the highest epoch claimed up to `offset`.
- `openUrsulaPiStorage(file, url, { readOnly: true })` opens Pi's storage on a database that Pi has already written. Its reads work, and commits that change the database fail.
- To write again, close the file's connections and attach it without `readOnly`, or `open` it without `readOnly`.
## Health and alerts
`status(file)` (`ursula_status`) reports the owner's health along with its offsets:
- `commits`, `append_retries`: commits acknowledged since the attach, and append attempts beyond the first (unknown outcomes, `429`, `503`, `307`, a refused token).
- `log_bytes`, `snapshot_due_bytes`: the log since the latest snapshot, which a rebuild replays, and the size at which a snapshot is taken (the database's size, or `URSULA_VFS_SNAPSHOT_MIN_BYTES` if larger).
- `snapshot_age_ms`, `snapshot_failures`, `snapshot_error`: milliseconds since this owner last saw a snapshot published (`null` for none since the attach), and the snapshot attempts that failed in a row with the last reason. After a failure the next attempt waits 100 ms, and twice as long after each further failure, up to an hour while the body is too large and up to five minutes otherwise, give or take a fifth. A body over 1 MiB whose publish fails while the stream still answers counts as too large, and one that fails while the stream does not answer as an outage. A publish the server puts off (a `503` with `Retry-After`, or marked as applying nothing, as the server's admission does under pressure) is an outage too, and the next attempt waits at least as long as it asked. The attempt starts at once when the database has shrunk by more than a quarter since the failure (after a `VACUUM`, for example), and after an outage the first commit that succeeds brings it forward once, to within 30 s. A new attach starts without a wait. A publish of more than 1 MiB is sent twice at most, so a short outage during one counts as a failure. A retention move that fails is not counted here: it is logged as `retention_failed` and tried again after the next snapshot.
- `snapshot_bytes`: the compressed size of the latest snapshot body, published or not (`null` for none since the attach). Ursula's gateway refuses a body above its `--max-request-body-bytes` (32 MiB by default). A body a little over the limit gets a `413`, which `snapshot_error` reports with the size. One far over it usually meets a closed connection instead, reported as a failed write such as `io: Broken pipe` or `io: Connection reset by peer`, and on macOS also `io: Invalid argument` (see [#540](https://github.com/tonbo-io/ursula/issues/540)). Compare `snapshot_bytes` with the limit either way.
- `attach_ms`: how long the attach took.
- `writes_refused`, `refused_kind`, `refused_reason`: the latest write was refused before it reached the stream, what refused it and why (see [Writes refused before they reach the stream](#writes-refused-before-they-reach-the-stream)). `writes_refused` is `false` while `poisoned` is `true`, since a poisoned file needs a new attach whatever refused its writes.
- `payment_required`, `payment_reason`: `true` when the latest refusal a layer answered is a `402`, with the layer's explanation (see [Writes refused with 402](#writes-refused-with-402)). A transaction refused as `too_large` does not change them.
- `read_only`: the file is attached read-only.
- `error` (from 0.7.5): why writes fail now, `null` while they go through. Its `kind` is a stable name, its `action` what to do: `retry` (nothing reached the stream, the file is usable), `reattach` (attach the file again), `stop` (another owner holds the stream) or `fix` (change the URL, the setup, the transaction or the files). See [Errors](#errors). The design doc lists the kinds.
`instrument(file, meter)` reports the same as metrics on an OpenTelemetry `Meter`, which can be any object with `createHistogram`, `createCounter` and `createObservableGauge`. The package does not depend on OpenTelemetry. The metrics are `sqlite_ursula.commit.duration` and `sqlite_ursula.append.duration` (ms), `sqlite_ursula.append.retries`, `sqlite_ursula.stream.bytes`, `sqlite_ursula.commit.unmeasured`, and the gauges `sqlite_ursula.log.bytes`, `sqlite_ursula.snapshot.due_bytes`, `sqlite_ursula.snapshot.age`, `sqlite_ursula.snapshot.failures`, `sqlite_ursula.snapshot.bytes`, `sqlite_ursula.poisoned`, `sqlite_ursula.fenced`, `sqlite_ursula.payment_required` and `sqlite_ursula.writes_refused`. It drains `drainStats` every second, or every `intervalMs` milliseconds with `instrument(file, meter, { intervalMs })`, so do not drain the same file elsewhere. Call the function it returns to stop. Between drains the extension keeps the newest 8,192 measurements of each kind (512 KiB of commits), so a file nobody drains holds a bounded amount. `drainStats` reports what it dropped in `dropped`. `instrument` adds the dropped commits' bytes and retries to its counters, and counts the dropped commits in `sqlite_ursula.commit.unmeasured`: the duration histograms see at most 8,192 commits per drain, while `status(file).commits` counts every commit. At the default interval that is every commit up to about 8,000 commits a second. A file that commits faster needs a shorter `intervalMs`.
The extension logs one line per event on stderr, in logfmt: `sqlite-ursula-vfs level=warn event=poisoned file=... stream=... fenced=true reason="..."`. The events: `poisoned`, `payment_required` (a layer refused a write with `402`, logged when the refusals start, with what was refused and the layer's explanation), `writes_refused` (a write was refused before it reached the stream for another reason, logged when refusals of a kind start, with what was refused, the `kind` and the reason), `writes_resumed` (the first commit that succeeds after them), `append_retry` (a commit's append failed and is tried again, with the attempt, the time since the commit's first attempt and the reason), `append_gave_up` (the same for the last attempt, after which the commit fails), `refused_commit_unchecked` (a commit at the producer sequence a refused commit left free was acknowledged as the duplicate of one of its own attempts whose outcome was unknown, such as one that got no answer or a `307`, so the stream may hold the refused commit instead, see [Writes refused before they reach the stream](#writes-refused-before-they-reach-the-stream)), `snapshot_failed` (with the failures in a row, the wait before the next attempt as `retry_ms`, and what it failed on as `kind`: `size`, `outage` or `other`), `retention_failed` (a retention move after a published snapshot failed: it is tried again after the next snapshot), `open_refused`, `journal_refused`, `local_files_discarded`, `attach_retrying`, `attach_rebuilding`, `token_file_unreadable`, `token_in_clear`, `ca_file_unreadable`, `ca_file_without_certificates`, `dns_stale` (from 0.7.4: a lookup of the stream's host failed, and its addresses from the last successful lookup serve the requests, with their `age_ms` and the lookup's failure as `reason`, logged at most once per host every 5 s), `setting_adjusted` (a setting out of range or not a number, with the value used instead), `snapshot_thread_panicked`, `attach_restore_failed` (from 0.7.5: an attach that failed could not remove the sidecar it created), `snapshot_thread_not_resumed` (from 0.7.5, `level=error`: after an attach refused up front the file stays attached without snapshots until its next attach), and `panic` (`level=error`).
A `panic` event is a bug inside the extension. It names the `entry` that panicked and the panic's `message`, and Rust's own panic message precedes the line on stderr. The call failed and the process went on. After a panic in a file method such as `x_write`, the file is poisoned, so attach it again. A panic in `xOpen` or `xClose` poisons nothing: the open fails, and the close releases the file. A panic while `attach` runs fails that attach, and the file opens again after an attach succeeds.
Alerts worth setting:
- `poisoned` is 1: commits fail until the file is attached again. `reason` says why. Attach again once the cause is fixed.
- `payment_required` is 1: a layer in front of the stream refuses writes, and `payment_reason` says why. Commits fail until the layer accepts them again. Then they succeed without an attach.
- `writes_refused` stays 1: writes are refused before they reach the stream, and `refused_kind` and `refused_reason` say what and why (see [Troubleshooting](/docs/0.7/troubleshooting#a-commit-or-attach-is-refused-with-error-code-7178)). For `not_applied`, look at the Ursula cluster or the gateway that refuses them. Commits fail until the refusals stop. Then they succeed without an attach. For `too_large` only that transaction fails, so split it: other commits succeed as usual, and the next one that does clears it.
- `fenced` is 1 without a planned handoff: another process claimed the stream. Find the second owner before attaching again. After a refused commit, a refusal that was false can show as a fence too (see [Writes refused before they reach the stream](#writes-refused-before-they-reach-the-stream)).
- `log_bytes` stays above a few times `snapshot_due_bytes`, or `snapshot_failures` keeps growing: snapshots are not being published, for example a body over the gateway's `--max-request-body-bytes` or a long reader pinning WAL frames. Every rebuild then replays the whole log.
- `snapshot_bytes` approaches the gateway's `--max-request-body-bytes`: the next snapshots will be refused. With a cold backend, raise the limit (snapshots up to 1 GiB), and the gateway's body budget grows to at least the limit and 32 MiB (an explicit budget must be at least that). Give the gateway about twice the budget in memory. Otherwise make the database smaller: delete rows, then `VACUUM`, which is one transaction and must itself compress below the commit limit (see [Rules](#rules)), or copy the rows into a new database in transactions below the limit.
- The p99 of `sqlite_ursula.commit.duration` rises: look at the Ursula cluster first. A commit waits for its append.
## Pi Durable
Install Pi Durable next to the package:
```bash
npm install @tonbo/sqlite-ursula @earendil-works/pi-durable
```
`openUrsulaPiStorage`, from the `@tonbo/sqlite-ursula/pi` entry, attaches the file and opens it with Pi's own node driver. The error classes come from the root entry:
```ts
import { UrsulaFencedError } from "@tonbo/sqlite-ursula";
import { openUrsulaPiStorage } from "@tonbo/sqlite-ursula/pi";
const storage = await openUrsulaPiStorage("/data/harness-1.db", "https://ursula.example.com/pi/harness-1");
// hand `storage` to the Pi harness, and storage.close(ctx) on shutdown
```
`@tonbo/sqlite-ursula` declares `@earendil-works/pi-durable` as an optional peer dependency in the range `>=1.0.0 <1.2.0`, and npm refuses the install with `ERESOLVE` when your version is outside it. On 1.0.0 and on 1.1.0, CI runs Pi's storage conformance suite through the extension, and the tests of how `openUrsulaPiStorage` reports fenced and refused commits. Before 0.7.5, `openUrsulaPiStorage` came from `@tonbo/sqlite-ursula` itself.
The storage reports failures with the errors of [Errors](#errors), from the same mapping as `open`'s database. A commit the stream does not accept surfaces as `UrsulaReplicationError`, as before 0.7.5, and from 0.7.5 as one of its subclasses. `UrsulaFencedError` (`fenced: true`, `action` `stop`) means another host took the stream over, the stream was deleted and recreated (opening it again then rebuilds the file from the new stream, see [Rules](#rules)), or another writer appended to it with a higher `Stream-Seq`. From 0.7.5 a fenced storage's reads fail too, with the same error and a message that starts with `read refused`, since the file no longer holds what the stream does. Opening the storage again takes the stream back, so do that only for a deliberate takeover. `UrsulaReattachError` (`fenced: false`, `action` `reattach`) means the commit's outcome is unknown, or the local file failed: open the storage again. A poison whose cause must be fixed first stays a plain `UrsulaReplicationError`, whose `action` is `fix`.
A commit refused before it reached the stream surfaces as `UrsulaWriteRefusedError` instead, and as its subclass `UrsulaPaymentRequiredError` for a `402`. The storage stays usable, and its next commit succeeds once the refusals stop (see [Writes refused before they reach the stream](#writes-refused-before-they-reach-the-stream)). A failed attach throws an `UrsulaError`, as `open` and `attach` do.
A failure of the extension that Pi wraps in its own error, such as the `StorageRejected` around a read that failed while it copied a document, surfaces as the Ursula error, never as Pi's semantic rejection, after which a session would go on. From 0.7.5 the messages of these errors end with one sentence that says what to do, such as `commit not replicated: . Another owner took the stream over: stop using this database, and never reopen it in a loop.`
## Related
- [Snapshots](/docs/0.7/concepts/snapshots)
- [Exactly-once writes](/docs/0.7/concepts/exactly-once-writes)
- The design: `docs/architecture/sqlite-vfs.md`
---
# Streams
A stream is an append-only byte sequence addressed by a URL: `/{bucket}/{stream}`. Once data is written it cannot be modified or reordered. Only new data can be appended.
## Naming
Stream IDs may contain any byte that is not `/`, `\0`, or `..`. Maximum stream ID length is 122 bytes, and the combined bucket-plus-stream path is also capped at 122 bytes. The literal name `streams` is reserved and cannot be used as a stream ID. A stream ID cannot start with `$` either; that prefix is reserved for bucket-level subresources (`400`).
A workspace app might use `/workspace-a/doc-123`. An agent system might use `/agents/run-2026-05-13-abc`. Anything within the rules above is fair.
## Content type
A stream's content type is set when it is created with `PUT` and defaults to `application/octet-stream` if no `Content-Type` header is supplied. `POST` never creates a stream: an append to a stream that does not exist answers `404`. Subsequent appends must declare the same content type, or the server rejects them with `409`. The content type rides along on reads so clients can dispatch on it without inspecting payloads.
## Lifecycle
Every stream is in one of these states:
- **Open.** Accepts appends. The default on creation.
- **Closed.** Sealed by `Stream-Closed: true` on a POST. Readers receive an EOF signal. Further appends return `409`. Close is irreversible.
- **Expired.** Past its `Stream-TTL` or `Stream-Expires-At` deadline. Reads and writes return `404`. Expired streams are eventually garbage-collected.
- **Deleted.** Removed by `DELETE`. Reads return `404`.
`Stream-TTL` (seconds, relative) and `Stream-Expires-At` (RFC 3339 absolute) are mutually exclusive. Sending both yields `400`.
`Stream-TTL` is a sliding window, not a deadline fixed at creation:
- Every accepted append or close renews it; a deduplicated retry or a repeat close does not.
- Reads renew it at most once per quarter of the TTL, so a stream that is only read can expire after about 0.75×TTL of idleness following the last read. `HEAD` does not renew it.
- An SSE connection renews it when it opens, not while it waits for new data. Ursula does not close idle SSE connections (the base protocol expects servers to end them about every 60 seconds so readers reconnect), so an open subscription alone does not keep a stream with no writes alive. Each long-poll request renews like any other read.
- Renewal never moves expiry earlier, even when the node that handled the request has a slower clock than the one that last renewed it.
## Related
- [Buckets](/docs/0.7/concepts/buckets): how streams are grouped
- [Snapshots](/docs/0.7/concepts/snapshots): compacting a long stream
- [Bootstrap](/docs/0.7/concepts/bootstrap): efficient first-load for new clients
---
# Buckets
Streams are organized into buckets. A bucket is a namespace, like a folder that holds related streams under one URL prefix:
```
/{bucket}/{stream}
```
A collaborative editing app might use one bucket per workspace, with one stream per document: `/workspace-a/doc-1`, `/workspace-a/doc-2`. An agent platform might use one bucket per tenant: `/tenant-acme/run-xyz`.
## Naming
Bucket IDs match the regex `[a-z0-9_-]{4,64}`: 4–64 bytes, lowercase ASCII letters and digits plus `_` and `-`. Uppercase letters, `/`, `.`, and most punctuation are rejected with `400`.
The combined bucket-plus-stream URL path also has a 122-byte ceiling. With a 64-byte bucket name, you have 57 bytes left for the stream ID.
## Lifecycle
Buckets are implicit. A bucket exists once a stream is created in it; there is no separate create step.
- **`PUT /{bucket}`** validates the bucket ID and changes nothing: `201` for a valid ID, `400` for an invalid one. Clients that create the bucket first can keep doing so.
- There is no bucket metadata read, stream listing, or bucket delete. Track your streams in your own application.
- Removing a bucket and every stream in it is an operator action: the admin-plane [bucket purge](/docs/0.7/operations#tenant-offboarding-bucket-purge), which permanently fences the bucket ID: later stream creates in it answer `404`.
## Related
- [Streams](/docs/0.7/concepts/streams)
- [API: create bucket](/docs/0.7/api/create-bucket)
---
# Offsets
An offset is a position inside a stream. Clients use offsets to read from a specific point or resume where they left off.
## Format
Offsets are opaque tokens. Ursula returns them as 20-character strings, for example `"00000000000000000042"`, but their content carries no meaning for clients. Take each offset from a server response and pass it back unchanged. To tell which of two offsets of the same stream comes first, compare them lexicographically. Don't parse offsets or compute new ones from them.
Two special values are accepted on read requests:
- `offset=-1`: start from the beginning of the stream. After a retention advance has trimmed the beginning, it answers `410 Gone` like any trimmed offset; continue from that response's `Stream-Next-Offset`, which is the earliest offset still readable.
- `offset=now`: start from the stream's current tail. Useful for "only new data" subscriptions.
## Response headers
After every read or append, the server returns two related headers:
- **`Stream-Next-Offset`**: the offset to pass back on the next request. Every successful read and append response carries it (on SSE, in each control event's `streamNextOffset` instead of a header), except an append answered as a duplicate whose receipt has left the receipt window ([exactly-once writes](/docs/0.7/concepts/exactly-once-writes#how-dedup-works)). Error responses carry it only where the [read](/docs/0.7/api/read) and [append](/docs/0.7/api/append) status tables say so.
- **`Stream-Cursor`**: set on live (long-poll, SSE) responses. A counter for caches, not a position: echo it as `?cursor=` on the next live request. Each response's cursor is past the one the request sent, so successive long-poll URLs differ and a cache can't serve the same empty response in a loop.
Resume from `Stream-Next-Offset` (in SSE, the latest control event's `streamNextOffset`) in every read mode. `Stream-Cursor` carries no stream identity, so it doesn't reveal that a stream was recreated; see [Recreated streams](#recreated-streams).
## Stability across snapshots
Offsets are stable across snapshot publishes. Publishing a snapshot at an offset doesn't renumber later offsets or delete earlier data. A separate retention advance trims the data before its offset. A read at a trimmed offset returns `410 Gone`, and its `Stream-Next-Offset` header is the earliest offset still readable, which `HEAD` also reports as `Stream-Retained-Offset`.
## Recreated streams
An offset doesn't identify which stream it came from. If a stream is deleted and recreated at the same URL, an offset saved from the old stream may be accepted by the new one and return the new stream's data. `HEAD` returns a `Stream-Incarnation` token that changes when a stream is deleted and recreated. A client that keeps offsets across a possible recreate should store the incarnation with them and compare it for equality before resuming. A restore from backup keeps the incarnation but can rewind the stream to the backup's tail, so offsets read after that backup was taken are not valid on the restored stream.
## Related
- [Read modes](/docs/0.7/concepts/read-modes)
- [Snapshots](/docs/0.7/concepts/snapshots)
- [Bootstrap](/docs/0.7/concepts/bootstrap): recovery when offsets you remember have been trimmed
---
# Read Modes
Streams support three read modes, all via `GET`:
- **Catch-up**: `GET /b/s?offset=-1` returns all data from the given offset immediately. Use this to sync historical data.
- **Long-poll**: `GET /b/s?offset=...&live=long-poll` returns immediately if data is available, otherwise holds the connection until new data arrives or a timeout. Good for simple polling loops.
- **SSE**: `GET /b/s?offset=...&live=sse` opens a persistent Server-Sent Events connection. The server pushes new data as it arrives. This is the recommended mode for real-time frontends (`EventSource` in the browser works out of the box).
---
# Exactly-Once Writes
Network retries can produce duplicate appends. Ursula provides server-side deduplication so your application doesn't need its own bookkeeping. Three headers form the identity:
- **`Producer-Id`**: a stable client identifier (UUID, hostname, etc.).
- **`Producer-Epoch`**: bumped on producer restart. Must be ≥ the last epoch the server saw from this producer.
- **`Producer-Seq`**: a per-epoch sequence number. Starts at `0` for a new epoch and must increase by exactly `1` per append.
Both epoch and seq are capped at `2^53 − 1` so they survive a round-trip through JSON.
## How dedup works
The server records an exact receipt for every accepted sequence in the current epoch from each `Producer-Id` per stream, within a per-stream receipt window of 1,024 items, and always keeps each producer's newest receipt. On an append:
- **Delayed duplicate** (same epoch and any seq already accepted): silently deduplicated. The response carries that sequence's original byte and record ranges, even when newer sequences have committed, as long as its receipt is in the window. The newest sequence always gets its ranges. An older duplicate whose receipt left the window is still deduplicated (never written twice) and answered `204` with `Producer-Seq` but without range headers.
- **Next in sequence** (`seq = last_seq + 1`): accepted.
- **Out of order** (seq skips ahead): rejected with `409`. The response carries `Producer-Expected-Seq`, the sequence the server wanted, and `Producer-Received-Seq`.
- **Epoch regression** (epoch less than the last accepted one): rejected with `403 Forbidden`. The response's `Producer-Epoch` is the current epoch. A producer that gets `403` has been replaced by a newer instance and should stop writing.
## Restart and epoch hygiene
When a producer crashes and restarts, **bump `Producer-Epoch`**. Otherwise the server still expects the next contiguous seq within the old epoch, and the producer (which has lost its in-memory seq counter) will collide. A new epoch starts the seq counter back at `0`.
## Per-stream state
Dedup state is scoped per stream. The same `Producer-Id` writing to two different streams maintains two independent epoch/seq counters, so you do not have to elect a single global writer per producer.
The server retains exact receipts for the current epoch per `(producer_id, stream)` pair, bounded by the stream's receipt window. Older epochs and their receipts are dropped once a higher epoch is observed. A producer that has not written to a stream for 7 days is forgotten: its next append must start at sequence `0` (a later sequence gets `409`), so bump the epoch or restart the sequence after long pauses. A stream holds at most 4,096 producers: a new producer beyond that replaces the least recently seen producer idle for at least an hour, and when none has been idle that long its append is rejected with `429` and a `producer_limit` body.
## When you don't need this
For append-only workloads where each write is independent (event logging, audit trails), exactly-once headers are optional. Just POST without them. Add them when retries are real and double-applies would be visible.
## Related
- [Conditional writes](/docs/0.7/concepts/conditional-writes): coordinate multiple writers with `Stream-Seq`, and combine it with producer headers to publish committed facts
- [API: append](/docs/0.7/api/append)
---
# Conditional Writes
Writers that share a stream sometimes need to say "append this only if the stream is still what I read." Ursula does this with the base protocol's `Stream-Seq` header, which is a compare-and-set on a value the writers choose. There are no locks and no cross-stream transactions.
## How `Stream-Seq` is checked
- Each stream stores the last accepted `Stream-Seq`. An append that carries `Stream-Seq` is accepted only when its value is strictly greater than the stored one, and the value is then stored.
- The comparison is bytewise, not numeric: `"10"` sorts before `"9"`. Use fixed-width values, such as a counter zero-padded to 20 digits.
- Values must be printable ASCII. A header with any other byte is ignored: the append is not checked and nothing is stored.
- An equal or smaller value answers `409 Conflict`, and nothing from the request is committed. The `409` carries `Stream-Next-Offset` set to the stream's current tail.
- The scope is the stream. A value is checked only against earlier values on the same stream, whoever sent them.
- An append without `Stream-Seq` is not checked and leaves the stored value unchanged.
- A `PUT` that creates the stream stores its `Stream-Seq` as the starting value without checking it. A `PUT` that finds the stream already there ignores the header.
- Producer deduplication runs first. A retry of an append that was already accepted from the same `Producer-Id`, `Producer-Epoch` and `Producer-Seq` is answered as a duplicate, not `409`, even though its `Stream-Seq` is no longer greater than the stored value. While its receipt is in the stream's receipt window the answer carries its original `Stream-Next-Offset`; a duplicate whose receipt was evicted carries none (see [Exactly-once writes](/docs/0.7/concepts/exactly-once-writes#how-dedup-works)). An append rejected with `409` uses up no `Producer-Seq`.
- A `Stream-Seq` `409` carries neither `Stream-Closed` nor `Producer-Expected-Seq`. A closed stream answers `409` with `Stream-Closed: true`, and an out-of-order `Producer-Seq` answers `409` with `Producer-Expected-Seq`.
- A `Content-Type` that differs from the stream's also answers `409`, with the same headers as a `Stream-Seq` `409`; only the plain-text body differs. Types are compared after normalization, which keeps parameters, so `application/json; charset=utf-8` is not `application/json`. The recipes below send exactly the content type `HEAD` reports. Then a `409` without `Stream-Closed` or `Producer-Expected-Seq` can only come from the `Stream-Seq` check, and they treat every other `409` as an error.
- A value is at most 256 bytes.
## Prefix-count compare-and-set
This pattern lets several writers extend one log while each commit is validated against the whole log before it.
1. **Every writer, every append.** Every append to the stream, by every writer, carries a prefix-count `Stream-Seq`: an append without it is not checked and leaves the stored value unchanged, so a writer that read only the events before it could still win. Create the stream with a `PUT` without `Stream-Seq`, so no value is stored and the first commit's `0` is accepted; a `PUT` that seeds a value can make the first commit answer `409` on every retry.
2. **Commits.** Each append is one commit of one or more events. On an `application/json` stream, `POST` the commit as a JSON array: the elements are committed together, one message per event. Never append an empty commit.
3. **Read to the tail.** Read with `consistency=leader`, continuing from `Stream-Next-Offset`, until a response carries `Stream-Up-To-Date: true`. Count the events you read; call it `n`. `n` counts every event since the stream was created. Once retention has trimmed the start of the stream, those events can't be read (a read from `offset=-1` answers `410`), so keep the event count in every snapshot you publish and count on from it, or don't advance retention on a prefix-count stream.
4. **Append.** Validate the commit against what you read, then append it with `Stream-Seq` set to `n`, zero-padded to a fixed width. On a new stream, the first commit sends `0`.
5. **On a `Stream-Seq` `409`.** Another writer committed first. Read on from where you stopped until `Stream-Up-To-Date: true`, count the `k` new events, validate again, and retry with `n + k`.
```bash
# The writer read 3 events, ending with Stream-Up-To-Date: true.
curl -i -X POST http://127.0.0.1:4437/demo/run-42 \
-H 'Content-Type: application/json' \
-H 'Stream-Seq: 00000000000000000003' \
--data-binary '[{"type":"step","n":4},{"type":"step","n":5}]'
```
Why it works: every commit holds at least one event, so the stored value is always the event count before the latest commit, and it grows with every commit. A count beats it only when it includes the latest commit, so a writer whose append is accepted has seen the whole log. When two writers send the same count, one wins and the other gets `409`.
The read must end at the tail. A read shortened by `max_bytes` can stop inside a commit, and a count taken there can beat the stored value without the writer having seen the rest of that commit.
A value above the true count, from a counting bug or a seeding `PUT`, is stored like any other. Every honest writer's count is then too small, and each of its retries answers `409`.
Do not send the count after your own commit (the last position your events take). That is not a compare-and-set: with 3 events in the log, writer A appends 1 event with `4`, and writer B, which also saw only 3, appends 2 events with `5`. Both are accepted, and B's events were validated against a stale log.
If the response to an append is lost, you can't tell whether it committed. Put an operation ID in each commit, and before retrying, read the new events and look for it.
## Publishing committed facts (outbox)
When the facts are decided somewhere else, such as a database transaction or a [SQLite VFS](/docs/0.7/examples/sqlite-vfs) database, publish them to a stream after they commit:
1. **Number the commits** in the source of truth, and publish only facts that have committed, in commit order.
2. **Send both guards** on each append: `Producer-Id`, `Producer-Epoch` and `Producer-Seq` for retries, and `Stream-Seq` set to the commit's number, zero-padded.
3. **On takeover**, the new owner uses a higher `Producer-Epoch` and starts `Producer-Seq` again at `0`. If it republishes a fact the old owner already published, the `Stream-Seq` check answers `409`, which here means "already published": go on to the next fact with the same `Producer-Seq`. Only a `Stream-Seq` `409` means that. A `409` with `Stream-Closed` or `Producer-Expected-Seq`, or any `409` while your `Content-Type` differs from the one `HEAD` reports, is an error, and skipping on it would lose facts.
4. **Old owners.** Once the new owner's first append is accepted, appends with the old epoch answer `403`. Before that, an old owner can still publish only facts that committed, and `Stream-Seq` accepts each of those at most once and in commit order.
5. **Gaps.** If a fact can no longer be published, for example because the source pruned it, append a snapshot or reset marker that tells readers to resynchronize. A fact skipped once can never be appended later, because its number is no longer greater than the stored value.
## Guarding against a recreated stream (`Stream-Incarnation`)
`Stream-Seq` and the producer headers are state of one stream. A stream deleted and recreated at the same URL (by a `DELETE` and a `PUT`, or by TTL expiry and a later `PUT`) starts without either, so they can't tell a client that its offsets, producer state or cached data belong to a stream that no longer exists. `Stream-Incarnation` does:
- Every successful response that addresses a stream, except a `DELETE`, carries `Stream-Incarnation`, an opaque token of the stream's incarnation. Keep it with whatever you derived from the stream.
- Send it back as a request header on any later request: appends, reads (including long-poll and SSE), `HEAD`, `DELETE`, a `PUT` of an existing stream, bootstrap, snapshot publish and read, retention. The server compares it for equality with the stream's current incarnation, atomically with the request (a write when it is applied), and answers `412 Precondition Failed` when the stream is another incarnation; a refused write applies nothing. The `412` carries the current token.
- A request that would answer `404` (or, for a read, `410` or `416`) without the header still does. A `PUT` with the header for a missing stream answers `412` and creates nothing.
- Without the header nothing changes.
`412` never mixes with the `409`s above: a `Stream-Seq` or producer conflict is about the same stream; a `412` means the stream you knew is gone. Start over from the new stream: discard derived state, read from its start or bootstrap, and keep the new token.
- **Prefix-count `Stream-Seq`**: several writers extend one log, and each commit must be validated against everything before it.
- **Commit-number `Stream-Seq`**: one logical publisher, possibly failing over between owners, must not publish a fact twice or out of order.
- **Producer headers** ([exactly-once writes](/docs/0.7/concepts/exactly-once-writes)): retries of the same append must not be written twice.
- **`Stream-Incarnation`**: any client that keeps offsets, producer state or a cache across requests, so it never acts on a stream recreated at the same URL. Combine it with the others.
- **Neither**: append-only workloads where every write is independent, such as event logging. Just `POST`.
Ursula has no precondition on the stream's tail offset (`If-Match`).
## Related
- [API: append](/docs/0.7/api/append)
- [Exactly-once writes](/docs/0.7/concepts/exactly-once-writes)
- [Durability and consistency](/docs/0.7/concepts/durability-and-consistency): what `consistency=leader` guarantees
---
# Snapshots
As a stream grows, replaying it from `offset=-1` becomes expensive. Snapshots solve this by storing a compacted representation of all data up to a given offset.
```
PUT /{bucket}/{stream}/snapshot/{offset} publish a snapshot
PUT /{bucket}/{stream}/retention/{offset} advance retained history
GET /{bucket}/{stream}/snapshot/{offset} read a specific snapshot
HEAD /{bucket}/{stream} Stream-Snapshot-Offset names the latest snapshot
```
## Who publishes
Snapshots are an **application-level** operation. Ursula stores them but does not produce them. The writer that knows how to compute the merged state is responsible for publishing. A CRDT editor, for example, periodically computes the merged document and publishes it as the latest snapshot. An agent system might snapshot accumulated tool-call state.
Application snapshots are separate from the Raft-internal snapshots Ursula takes for replication and recovery. The two never collide.
## Size and content type
Snapshot bodies of up to **32 MiB** are kept inline in the stream's replicated state. On a cluster with a cold backend, bodies of at least `runtime.external_payload_min_size` (1 MiB by default) are stored as cold-tier objects instead, up to **1 GiB**: replicated state keeps only the offset, content type, digest and object reference, and reads and `/bootstrap` stream the body from the cold tier. A superseded cold body is deleted five minutes after the publish that replaced it, so a read that started before still completes. Through `ursulagw`, bodies above its `--max-request-body-bytes` (32 MiB by default) are refused; publish large snapshots directly to a node or raise that limit. An unset `--body-budget-bytes` then grows to hold one body at the limit and 32 MiB, and an explicit budget must be at least that. Give the gateway about twice the budget in memory. The snapshot has its own content type (separate from the stream's), stored as `snapshot_content_type` and defaulting to `application/octet-stream`. A binary CRDT document can be snapshotted under `application/octet-stream` even when the stream itself carries `application/json` deltas.
## Checkpoint and retention
Publishing a snapshot is safe on its own: it creates a replay checkpoint but leaves all stream history readable. After the application has verified the checkpoint, it may explicitly advance retention to the same or an older checkpoint boundary with `PUT /{bucket}/{stream}/retention/{offset}`. Only that operation makes earlier reads return `410 Gone` and makes old cold-tier data eligible for GC.
On a JSON stream, snapshot and retention offsets must be message boundaries: an offset just after a message's LF, such as a `Stream-Next-Offset` Ursula returned. An offset inside a message answers `400`; when Ursula cannot read the byte before the offset to check it, it publishes nothing and answers `503`. Retention is exact.
## Immutability
Snapshots are immutable at an offset. Retrying the same body and content type is idempotent and returns the same `Stream-Snapshot-Digest`; different content at the same offset returns `409`. Snapshots cannot be deleted. To replace a snapshot, publish a new one at a higher offset.
## Related
- [Bootstrap](/docs/0.7/concepts/bootstrap): fetch latest snapshot plus post-snapshot updates in one request
- [API: publish snapshot](/docs/0.7/api/publish-snapshot)
- [API: read snapshot](/docs/0.7/api/read-snapshot)
---
# Bootstrap
Bootstrap is the new-client initialization endpoint. A single request returns:
1. The latest snapshot (if any)
2. The updates after the snapshot (one part per record on a JSON stream, one part in all on any other stream), up to the stream's current tail or up to an honest stopping point
```
GET /{bucket}/{stream}/bootstrap
```
The response is `multipart/mixed`:
- **First part.** The snapshot. If no snapshot exists, this part is present but empty.
- **Subsequent parts.** Incremental updates from the snapshot offset (or from the earliest retained offset, if no snapshot exists).
Each part has its own `Content-Type`: the snapshot part uses `snapshot_content_type`. Update parts use the stream's content type. The client does not need to know in advance whether a snapshot exists. The same parsing path handles both cases.
## Partial responses
Bootstrap only serves updates that are still in the hot tier, and at most 8 MiB of them per response. When updates after the snapshot have already moved to cold storage, or there are more than 8 MiB of them, the response stops early: `Stream-Next-Offset` is where it stopped and `Stream-Up-To-Date` is absent. In the cold case, and for a non-JSON stream above 8 MiB, the response holds only the snapshot part; a JSON stream above 8 MiB stops at a record boundary. It never skips bytes.
Clients therefore follow one rule: after applying the bootstrap response, read from `Stream-Next-Offset` with ordinary reads until `Stream-Up-To-Date: true`, then go live. A complete response simply makes that loop empty.
## When to use bootstrap vs catch-up
| You want… | Use |
| --- | --- |
| Latest state, fastest first-load | `/bootstrap` |
| Replay every byte from the beginning | `GET /{b}/{s}?offset=-1` |
| Resume from a known offset that's still retained | `GET /{b}/{s}?offset=N` |
| Resume from an offset the server has since trimmed (`410 Gone`) | `/bootstrap`, then continue from its `Stream-Next-Offset` |
## Live continuation
Bootstrap does not accept `?live=sse`. Combining the multipart body with an SSE event stream is rejected with `400`. To go live after bootstrap, finish reading the bootstrap response, then open a separate `GET /{b}/{s}?offset=&live=sse`. SSE delivers any remaining catch-up data first, so this also covers a partial response.
## Related
- [Snapshots](/docs/0.7/concepts/snapshots)
- [Read modes](/docs/0.7/concepts/read-modes)
---
# Durability and Consistency
## Durability
An append is acknowledged once a **majority of voters** has replicated it. A single node failure cannot lose an acknowledged write. With the default `raft.wal.fsync = "never"`, a replica holds the newest appends in the page cache, so a host crash of a majority of voters at once can lose the writes acknowledged after their last `fsync`. Set `always` to acknowledge only after `fsync` (see [Configuration](/docs/0.7/configuration#raft)). Acknowledged data lives in replicated hot state across the cluster, and a background worker flushes older segments to S3 on a configurable interval, after which the data inherits S3-grade durability.
In a cross-region deployment with a 5-second flush interval, per-message durability is approximately 9-10 nines. The window where acknowledged-but-unflushed data is at risk from a simultaneous multi-region failure is measured in seconds.
## Consistency
Writes are **linearizable**. Each Raft group serializes appends through its current leader, which assigns a total order.
Catch-up reads may be served by any replica that has applied the relevant stream state. Followers can return already replicated historical bytes locally because stream positions are immutable. Tail-sensitive reads still preserve protocol-visible semantics: `HEAD`, `Stream-Up-To-Date: true`, `Stream-Closed: true`, and `offset=now` are generated by the leader path unless the follower has applied a terminal closed state. Requests that need a write-side access transition, such as expiry or TTL touch, are also routed to the leader.
A follower can be behind an acknowledged append. If a catch-up cursor exceeds
its local tail, it checks that cursor through a quorum-confirmed leader read
instead of returning `416` from its stale prefix. A genuinely invalid cursor
still returns `416`; without a confirmed leader, the request redirects or
returns a retryable `503`.
A catch-up client recovering from an acknowledged append can request
`consistency=leader`. That opt-in routes the ordinary GET through the owning
Raft leader instead of treating a follower-local tail as authoritative. The
leader first confirms with a majority of voters that it still leads (Raft
ReadIndex) and waits until it has applied everything committed up to that
point, so the read is **linearizable**: it reflects every append acknowledged
before the read started, including appends acknowledged by a newer leader. A
leader cut off from the majority redirects to the new leader or answers `503`
with `Retry-After` instead of serving its stale state. The confirmation costs
one round trip to the voters; concurrent leader reads to the same Raft group
share rounds, and a read that arrives while a round is in flight waits for the
next one. On the leader, a read waits for its confirmation before it reaches
the group, so appends and other reads to the group do not queue behind it. A
leader that misses a confirmation under load retries it for up to one election
timeout before it answers `503`; a read that arrived during a round can wait
for that round and its own, so up to two. A client should retry a `503` after
`Retry-After`. It is not a live-read mode; long-poll and SSE already have
leader-owned watcher semantics. The [Read path](/docs/0.7/specs/extensions#92-leader-consistency)
section of the extensions specification states this contract normatively.
`HEAD`, `GET …/bootstrap`, snapshot reads and the registration of a long-poll
or SSE reader take the same leadership confirmation on the leader, as do
`offset=now` on a `consistency=leader` read.
A long-poll or SSE reader takes everything else it needs from that
registration: its `offset=now` position, whether the stream exists, its
content type and incarnation all come from the leader's state at or after the
confirmed point. A long-poll also answers `416` there for an offset beyond the
tail; an SSE session reports it as an `error` event from its first read. Its
first read is served only by that leader while it still leads; a node that
lost leadership in between redirects or answers `503` instead of asking a new
leader that may not have applied everything yet. A long-poll that times out
answers `204` at the requested offset.
`offset=now` without `consistency=leader` skips the confirmation and uses the
leader's applied state, so right after a leader change it may resolve to a
tail that lags the committed one.
With `consistency=leader`, `Stream-Up-To-Date: true` on a read means it reached the committed tail. On a local read it means the tail as the serving node knew it: a leader that has lost leadership without knowing it yet can mark a stale tail. A response without it is partial; keep reading from `Stream-Next-Offset`.
## Availability and durability (standard layout)
Three voting replicas across availability zones, plus two non-voting replicas in a second region.
| Property | What it means |
|---|---|
| **Write availability** | Writes continue as long as a majority of voters is healthy. The layout tolerates any single voting-AZ failure. Non-voting replicas hold extra copies but do not vote. |
| **Read availability** | Any reachable replica can serve replicated historical catch-up data locally. Reads that need fresh tail metadata, leader-owned state changes, or live watcher ownership still track write availability per group, not per cluster. |
| **Per-message durability** | ~9-10 nines with a 5-second flush interval. |
| **Annual zero-loss probability** | ~3-4 nines. Probability of no data-loss events in a year. |
---
# Binary SSE
The SSE wire format is text-only: every `data:` field must be valid UTF-8. Ursula therefore advertises the decoded data payload type with `Stream-Data-Content-Type` on every SSE read.
For `text/*` streams, data events carry UTF-8 text directly.
For `application/json` streams, data events carry newline-delimited JSON and the response includes `Stream-Data-Content-Type: application/x-ndjson`. SSE `data:` lines are transport framing, not JSON record boundaries, so clients should buffer until newline before parsing records.
For other content types (`application/octet-stream`, custom binary), Ursula base64-encodes the data event payload as raw text. It is not wrapped in a JSON envelope:
```text
event: data
data: AQIDBAUG
data: BwgJCg==
event: control
data: {"streamNextOffset":"123456_789","streamCursor":"abc"}
```
The choice is automatic, determined by the stream's content type. There is no client opt-in. What you get is what the stream's content type implied. The response advertises base64 data with `Stream-Sse-Data-Encoding: base64`. When that header is absent, the `data` event payload is already UTF-8 text with the type identified by `Stream-Data-Content-Type`.
## Decoding
For binary streams, concatenate the `data:` lines for an `event: data`, remove line breaks inserted by SSE framing, then base64-decode the resulting text. Interpret the decoded bytes according to `Stream-Data-Content-Type`.
For text and JSON streams, the `data` field is already the payload text. JSON streams use newline-delimited JSON (`application/x-ndjson`), so buffer until newline before parsing.
Browser `EventSource` works for both modes. Only the `data` handler differs.
## Related
- [Read modes](/docs/0.7/concepts/read-modes)
- [API: read](/docs/0.7/api/read)
- [Length-prefixed framing](/docs/0.7/concepts/len-prefixed-framing): when one SSE event contains multiple application records
---
# Length-Prefixed Framing
Streams are raw byte sequences. The protocol does not impose message boundaries. Every append and every read returns whatever bytes are in flight, possibly mid-record. For streams carrying many small messages (CRDT operations, agent events), Ursula recommends a simple framing convention:
```
[4-byte big-endian length][payload bytes]
```
Each application-level append writes one framed record. Each read returns a concatenation of framed records that the client parses incrementally.
## Server is frame-agnostic
The server **does not** validate or enforce frames. It stores and serves raw bytes. That means:
- A read may end mid-frame. The client must handle partial frames and resume parsing on the next read.
- An ill-formed frame (wrong length, truncated payload) is the producer's bug, not a server-rejectable error.
- The framing convention is per-application. Writers and readers of a stream must agree on it.
## When you don't need framing
- Streams using `application/json` typically use newline-delimited JSON or a single self-describing document. JSON has its own delimiters.
- Streams holding one logical blob per stream don't need framing at all.
Use framing when one stream carries many independently meaningful messages and the client wants to dispatch on each one without scanning.
## Related
- [Streams](/docs/0.7/concepts/streams)
- [Binary SSE](/docs/0.7/concepts/binary-sse): over SSE, each event is one delivery. Framing lets one event carry multiple records.
---
# API overview
Ursula exposes a small public HTTP API for durable append-only streams. Most users only need the `/{bucket}/{stream}` route family, which maps cleanly to buckets and stream IDs.
This API is Ursula's implementation of the [Durable Streams Protocol](/docs/0.7/specs/durable-stream), defined by the durable-streams community, with additional route families for compatibility and deployment needs.
## Bucket operations
| Method | Path | Description |
| -------- | ----------------------------- | ------------------------------------------------------------ |
| `PUT` | `/{bucket}` | [Create a bucket](/docs/0.7/api/create-bucket) (validates the ID; buckets are implicit) |
There is no bucket metadata read, stream listing, or bucket delete. Removing a bucket is an operator action ([bucket purge](/docs/0.7/operations#tenant-offboarding-bucket-purge)), which permanently fences the bucket ID: later stream creates in it answer `404`.
## Stream operations
| Method | Path | Description |
| -------- | ----------------------------- | ------------------------------------------------------------ |
| `PUT` | `/{bucket}/{stream}` | [Create a stream](/docs/0.7/api/create-stream) |
| `POST` | `/{bucket}/{stream}` | [Append data or close](/docs/0.7/api/append) |
| `GET` | `/{bucket}/{stream}` | [Read (catch-up, long-poll, SSE)](/docs/0.7/api/read) |
| `HEAD` | `/{bucket}/{stream}` | [Get stream metadata](/docs/0.7/api/head-stream) |
| `DELETE` | `/{bucket}/{stream}` | [Delete a stream](/docs/0.7/api/delete-stream) |
## Bootstrap and snapshots
| Method | Path | Description |
| -------- | --------------------------------------------- | ---------------------------------------------------- |
| `GET` | `/{bucket}/{stream}/bootstrap` | [Snapshot + tail replay](/docs/0.7/api/bootstrap) |
| `GET` | `/{bucket}/{stream}/snapshot/{offset}` | [Read snapshot at offset](/docs/0.7/api/read-snapshot) |
| `PUT` | `/{bucket}/{stream}/snapshot/{offset}` | [Publish a snapshot](/docs/0.7/api/publish-snapshot) |
## Common request patterns
### Create a bucket and stream
```bash
curl -X PUT http://127.0.0.1:4437/demo
curl -X PUT http://127.0.0.1:4437/demo/hello
```
### Append data
```bash
curl -X POST http://127.0.0.1:4437/demo/hello \
-H 'Content-Type: application/octet-stream' \
--data-binary 'hello world'
```
### Read from the beginning
```bash
curl 'http://127.0.0.1:4437/demo/hello?offset=-1'
```
### Subscribe with SSE
```bash
curl 'http://127.0.0.1:4437/demo/hello?offset=-1&live=sse'
```
## Refusals that applied nothing
A client may count a write attempt as never applied, which means that nothing of it can apply later, only when the response
- has the status `401`, `402` or `429`, or
- carries both `x-ursula-not-applied` and `x-ursula-not-applied-checked`. Only a gateway at 0.7.3 or newer sends the second header.
Any other response proves nothing, including a `403`, `413` or `503` without both headers. A `503` without them, or a timeout, can follow a write that committed. After such an attempt, keep retrying with the same `Producer-Id`, `Producer-Epoch` and `Producer-Seq` until an answer settles the write, so that the server deduplicates it (see [exactly-once writes](/docs/0.7/concepts/exactly-once-writes)).
`x-ursula-not-applied`, the marker, answers a request that was refused before anything of it was proposed, so nothing of it can apply later. Its value is a short token that names the refusal, such as `RaftLogPressure`. Log the token, but rely only on the presence of the header. `x-ursula-not-applied-checked: 1`, the vouch, comes only from a gateway at 0.7.3 or newer, which sets it on every response it returns after it applied the redirect rule below. Nodes never set the vouch, so a client that talks to the nodes directly ignores the marker.
The three statuses count without the headers. A layer in front of the nodes, such as the gateway's access control and tenant limits, answers them before it forwards a request. The server never answers `401` or `402`, and answers `429` only to a new producer on a stream that has no room for one. That refusal applies nothing. Within the same attempt, a replay of a write that did commit, such as the one a gateway sends when it follows a redirect, would be deduplicated instead.
The server sets the marker on every refusal of its write admission, which runs before a request reaches the stream:
| Status | Token | Cause |
| ------ | ----- | ----- |
| `503` | `WalDiskPressure` | The WAL filesystem's free space is below `raft.wal.min_available_size`, or the free-space check failed. Writes pass again once the free space reaches `raft.wal.resume_available_size`. |
| `503` | `RaftLogPressure` | The node's unsnapshotted Raft log is over twice `raft.snapshot_log_budget`. Writes with a body pass again once the log is below 1.5 times the budget. Writes without a body still pass. |
| `503` | `IngressBodyBytesLimitReached` | Write bodies in flight on the node leave too little of `server.http_inflight_body_size` for this body. |
| `413` | `RequestBodyTooLarge` | The `Content-Length` is over the route's limit. |
The `503` refusals also carry `Retry-After: 1` and name the token as the JSON `error`.
A node also sets the marker on some refusals it makes after admission, before it proposes the write to the stream's Raft group. It does so only for these writes and refusals:
| Status | Token | Writes | Cause |
| ------ | ----- | ------ | ----- |
| `307` or `503` | `NotLeader` | Appends and closes. Creates too, unless the body is staged in object storage or `storage.cold.max_hot_size_per_group` is `0`. | The node does not lead the stream's Raft group. A `307` sends the same request to the leader in `Location`. A `503` means that the node knows no leader yet, for example during an election. |
| `307` or `503` | `GroupNotHosted` | Creates, appends, closes and deletes. | The node does not host the stream's Raft group. A `307` sends the same request to a node that does. A `503` means that the node knows none. |
| `503` | `RaftUncommittedBackpressure` | Creates and appends with a body that is not staged in object storage. | The group's write bytes that are proposed but not yet applied would exceed `raft.max_uncommitted_size_per_group`. |
| `503` | `ColdBackpressure` | Creates and appends with a body that is not staged in object storage. | The group's hot bytes would exceed `storage.cold.max_hot_size_per_group`. Writes pass again once cold flush catches up. |
These `503` refusals also carry `Retry-After: 1`. A node without peers answers a `NotLeader` refusal with `500` and `Retry-After: 1` instead. With a cold store, a node stages a body of at least `runtime.external_payload_min_size` in object storage before it proposes the write. Staging proposes nothing, and after a marked refusal the node deletes the staged object.
Every other redirect and `503` from a node carries no marker, even when nothing was proposed. A node hands a delete, a create whose body is staged in object storage, and a create on a node where `storage.cold.max_hot_size_per_group` is `0` to Raft without checking leadership itself, so it cannot tell whether a redirect it answers for them came before or after the proposal. Snapshot and retention `PUT`s carry no marker either, and need none. Repeating a snapshot publish with the same body, or a retention advance to the same offset, is harmless, so retry one whose outcome is unknown.
The gateway sets the marker on every request it refuses before forwarding it, and on a request it could not send because the connection to its first upstream failed. When every redirect a request met was marked, it also marks its `503` for a node it then could not connect to, and its `503` when it stops following the redirects. It passes a node's marker through, except after a leader redirect, as described below. Its tokens are listed in the [gateway README](https://github.com/tonbo-io/ursula/blob/main/crates/ursula-gateway/README.md#refusals-that-applied-nothing). A proxy in front of the gateway can make its own refusals count too: answer them with `401`, `402` or `429`, but only on requests it did not pass on.
The marker speaks for one attempt only. A marked answer to a retry says nothing about the earlier attempts.
### Redirects and retries
A node can answer a write with a leader redirect after it proposed the write, and that redirect carries no marker. A node marks only the redirects listed above, which it sends before it proposes anything. The gateway follows Raft leader redirects itself, so its clients never get one: up to three hops in a row, none started later than 5 s after the first redirect, and through the upstreams it has not tried yet when a redirect names an address that is none of them, or one it already went to. It keeps the final answer's marker only if every redirect carried one too. So when a leader refuses writes at admission, for example with `RaftLogPressure`, a write that the gateway follows there from marked redirects still gets the marked refusal, with the vouch. When no upstream answers without redirecting within those limits, the gateway answers `503` with `Retry-After: 1`, marked `LeaderNotFound` only if every redirect was marked. Each hop sends the request again, so a plain append without a producer or `Stream-Seq` can apply more than once, once for each node that proposed it before it redirected. Use a producer for exactly-once writes. Gateways older than 0.7.3 followed one redirect and passed others on, and passed the leader's marker on after any redirect, but they never send the vouch, so a client that requires the vouch does not trust their markers. Upgrading gateways before nodes, and rolling back nodes before gateways, is still recommended, so that clients keep the benefit of the marker during a rollout. But no order, including whichever order a `helm upgrade` or `helm rollback` replaces gateways and voters in, can make a client count a committed write as never applied.
An HTTP library or proxy that follows redirects or retries a write on its own hides the earlier attempts, and the vouch covers only the one request the gateway received. Turn both off for writes before relying on the marker. Checking for a followed redirect, such as a `fetch` response whose `redirected` is `true`, is not enough, because a retry leaves no such trace.
The vouch also covers only the exchanges the gateway makes itself, so point its `--upstream` at the nodes themselves. A proxy, service-mesh sidecar or another gateway between the gateway and the nodes must not resend requests, or the gateway vouches for a marker from a later delivery. For example, a node can propose a write and then answer `503` without a marker. If a sidecar resends the request and a node refuses the copy at admission with a marked `503`, the gateway sees only that refusal and vouches for it. For the same reason, a 0.7.3 gateway must not sit in front of an older gateway, which follows redirects itself and passes a leader's marker on after any of them.
Nothing between the gateway and the nodes may answer `401`, `402` or `429` either, as a sidecar's local rate limit does. A client counts those statuses as never applied without the headers, and after a redirect without a marker the gateway returns the leader's status unchanged, although the node that redirected may have proposed the write.
So a proxy in front of the gateway must not retry writes, and nothing behind it may resend requests or answer `401`, `402` or `429`. With ingress-nginx, set `nginx.ingress.kubernetes.io/proxy-next-upstream: "off"`, as the chart's [TLS ingress example](https://github.com/tonbo-io/ursula/blob/main/charts/ursula/examples/tls-ingress.yaml) does. In a service mesh, turn off retries for the stream routes on both sides of the gateway, and local rate limits between the gateway and the nodes. Istio before 1.24, for example, resends a request up to twice by default when it gets a `503`, and any version resends requests on a route whose `retryOn` includes `503`, `5xx`, `gateway-error` or `reset`. Istio 1.24 to 1.26 restores the default `503` retry when istiod runs with `EXCLUDE_UNSAFE_503_FROM_DEFAULT_RETRY=false`, which `compatibilityVersion=1.23` sets. Set `retries: {attempts: 0}` on those routes in their `VirtualService`.
## Related concepts
- [Read modes](/docs/0.7/concepts/read-modes): catch-up vs long-poll vs SSE
- [Bootstrap](/docs/0.7/concepts/bootstrap): snapshot + tail recovery
- [Snapshots](/docs/0.7/concepts/snapshots): snapshot lifecycle
- [Exactly-once writes](/docs/0.7/concepts/exactly-once-writes): producer deduplication
- [Conditional writes](/docs/0.7/concepts/conditional-writes): `Stream-Seq` as a compare-and-set
---
# Create bucket
Bucket ID. Must match `[a-z0-9_-]{4,64}`.
## Response
| Status | Meaning |
| ------ | -------------------------------------- |
| `201` | The bucket ID is valid. Returned on every call, whether or not streams already exist in the bucket. |
| `400` | The bucket ID is invalid. |
> [!NOTE]
> `PUT /{bucket}` validates the bucket ID and changes no state. Buckets are implicit: a bucket exists once a stream is created in it, so creating the bucket first is optional. It is kept for clients that do so.
```bash Example
curl -X PUT http://127.0.0.1:4437/demo
```
---
# Create stream
Bucket ID. Must match `[a-z0-9_-]{4,64}`; an invalid ID answers `400`.
Stream ID within the bucket: 1 to 122 bytes, without `/`, `\0` or `..`, not starting with `$`, and `{bucket}/{stream}` together at most 122 bytes. `streams` is reserved. An invalid ID answers `400`.
Content type of the initial payload (e.g. `application/json`). Becomes the stream's content type.
Set to `true` to close the stream immediately after creation.
Sliding time-to-live in seconds. Appends and closes restart the window; reads renew it at most once per quarter of the TTL. See [stream lifecycle](/docs/0.7/concepts/streams#lifecycle).
Absolute expiration timestamp (RFC 3339). Mutually exclusive with `Stream-TTL`.
Seeds the stream's stored `Stream-Seq`: later appends that carry `Stream-Seq` must be bytewise greater than it. A create never checks it, and a `PUT` that finds the stream already there ignores it. See [how `Stream-Seq` is checked](/docs/0.7/concepts/conditional-writes#how-stream-seq-is-checked).
Producer identity for [exactly-once writes](/docs/0.7/concepts/exactly-once-writes).
Producer epoch (must accompany `Producer-Id`).
Producer sequence number (must accompany `Producer-Id`).
Optional precondition: the stream's incarnation token, as a previous response's `Stream-Incarnation` header returned it. The create applies only if the stream exists as that incarnation (it then answers as a create of an existing stream: `200` or `409`); a missing stream answers `412` and is not created. Without the header, the request behaves exactly as described here. A malformed value answers `400`. See [`Stream-Incarnation`](/docs/0.7/specs/extensions#24-snapshot-metadata-on-head-and-stream-incarnation).
Optional initial payload. If provided, becomes the first entry in the stream.
## Response
| Status | Meaning |
| ------ | ------------------------------------------------------------------------- |
| `201` | Stream created. |
| `200` | Stream already exists (idempotent). |
| `400` | Invalid bucket ID, invalid stream ID (including one that starts with `$`), invalid headers, or bad JSON payload. |
| `404` | The bucket was purged; its ID is permanently fenced. |
| `409` | Stream already exists with a different content type, closed state or lifetime, or `Producer-Seq` is not `0`. |
| `412` | `Stream-Incarnation` did not match: the stream is another incarnation (its token is in the response's `Stream-Incarnation`), or it does not exist. Nothing was created. |
Response headers include `Location`, `Content-Type`, `Stream-Next-Offset`, `Stream-Incarnation`, and lifetime headers (`Stream-TTL` / `Stream-Expires-At`) when set. `Stream-Closed: true` is set if the create request also closed the stream. `ETag` is set on reads only.
In an `application/json` stream, a top-level JSON array in the initial payload creates one message per element.
```bash Create empty stream
curl -X PUT http://127.0.0.1:4437/demo/hello
```
```bash Create with initial payload
curl -X PUT http://127.0.0.1:4437/demo/hello \
-H 'Content-Type: application/json' \
--data-binary '{"msg": "first entry"}'
```
```bash Create with TTL
curl -X PUT http://127.0.0.1:4437/demo/ephemeral \
-H 'Stream-TTL: 3600'
```
> [!NOTE]
> If a stream with the same ID already exists and has identical configuration, the response is `200 OK` (idempotent). If the existing stream differs in content type, closed state, TTL or expiry, the response is `409 Conflict`. A `PUT` answered `200` ignores its body and its `Stream-Seq`.
---
# Append
Bucket ID.
Stream ID within the bucket.
Must match the stream's content type (set when the stream was created with `PUT`) after normalization: parameters are trimmed, lowercased and joined with `; `. Required when the body is non-empty. A mismatch returns `409`.
Set to `true` to close the stream after this append.
A compare-and-set token. The append is accepted only when the value is bytewise greater than the last `Stream-Seq` accepted on this stream; it is then stored. An equal or smaller value answers `409` with `Stream-Next-Offset` at the tail. Without the header the append is not checked and the stored value is unchanged. Printable ASCII, at most 256 bytes. See [how `Stream-Seq` is checked](/docs/0.7/concepts/conditional-writes#how-stream-seq-is-checked).
Stable producer identity (UUID, hostname, etc.) for [exactly-once writes](/docs/0.7/concepts/exactly-once-writes). Dedup state is per-stream. At most 256 bytes.
Producer epoch. Bumped on producer restart. Must be ≥ the last epoch the server accepted for this `Producer-Id`. A new epoch resets the seq counter. Max value `2^53 − 1`.
Producer sequence number. Starts at `0` for a new epoch and must increase by exactly `1` per append. Exact `(epoch, seq)` duplicates are silently deduplicated. Max value `2^53 − 1`.
Optional precondition: the stream's incarnation token, as a previous response's `Stream-Incarnation` header returned it. The request applies only if the stream is still that incarnation, checked atomically with the write; otherwise it answers `412` and changes nothing. Without the header, the request behaves exactly as described here. A malformed value answers `400`. See [`Stream-Incarnation`](/docs/0.7/specs/extensions#24-snapshot-metadata-on-head-and-stream-incarnation).
The bytes to append. Must not be empty unless `Stream-Closed: true` is set (close-only request).
## Response
| Status | Meaning |
| ------ | ------------------------------------------------------------------ |
| `204` | Append successful (default success response, no body). |
| `200` | Append successful with body - returned when a `Producer-Id` was supplied and the append was not deduplicated, so the response carries producer ack headers. |
| `307` | In a cluster, this node does not lead or host the stream's group. `Location` is the same request on the leader, or on a node that hosts the group. When the node raised the redirect before it proposed the append, the response carries `x-ursula-not-applied` (`NotLeader` or `GroupNotHosted`). |
| `400` | Empty body without `Stream-Closed: true`, missing content type, invalid JSON or UTF-8, a JSON message nested deeper than 127 levels, a `Producer-Id` or `Stream-Seq` longer than 256 bytes, or a header removed in 0.6.0 (see the [release notes](/docs/0.7/release-notes)). |
| `403` | `Producer-Epoch` is older than the last epoch accepted for this `Producer-Id`. The response's `Producer-Epoch` is the current epoch. |
| `404` | Stream not found. |
| `409` | Content type does not match the stream, stream is already closed, `Stream-Seq` not greater than the stored value, or `Producer-Seq` out of order. |
| `412` | `Stream-Incarnation` did not match: the stream was deleted and recreated. Nothing was appended; the response's `Stream-Incarnation` is the current token. |
| `413` | Body larger than 32 MiB. |
| `503` | Write admission refused the append (the JSON `error` and `x-ursula-not-applied` name `WalDiskPressure`, `RaftLogPressure` or `IngressBodyBytesLimitReached`), or the stream's Raft group refused it with cold-write or uncommitted-bytes backpressure (`x-ursula-not-applied` names `ColdBackpressure` or `RaftUncommittedBackpressure`). A node also answers `503` when it knows no leader for the group yet (`NotLeader`) and when it knows no node that hosts the group (`GroupNotHosted`). When it found that before it proposed the append, the response carries `x-ursula-not-applied` with that name. Retry after the duration in `Retry-After`. |
A response with both `x-ursula-not-applied` and `x-ursula-not-applied-checked`, the vouch that a gateway at 0.7.3 or newer adds, proves that this attempt appended nothing, whatever the status. The headers say nothing about earlier attempts, and the marker alone proves nothing. See [refusals that applied nothing](/docs/0.7/api/overview#refusals-that-applied-nothing).
Response headers include `Stream-Incarnation` and `Stream-Next-Offset`, except on a duplicate whose receipt has left the stream's receipt window ([exactly-once writes](/docs/0.7/concepts/exactly-once-writes#how-dedup-works)). When a `Producer-Id` was supplied, the server echoes the accepted `Producer-Epoch` and `Producer-Seq` so the producer can confirm what was durably recorded. `Stream-Closed: true` is set if this request closed the stream. `ETag` is set on reads only, not on appends.
```bash Append binary
curl -X POST http://127.0.0.1:4437/demo/hello \
-H 'Content-Type: application/octet-stream' \
--data-binary 'hello world'
```
```bash Append JSON
curl -i -X POST http://127.0.0.1:4437/demo/hello \
-H 'Content-Type: application/json' \
--data-binary '{"captured_at":"2026-07-18T10:30:00.100Z","event":"click"}'
```
```bash Append multiple JSON messages
curl -i -X POST http://127.0.0.1:4437/demo/hello \
-H 'Content-Type: application/json' \
--data-binary '[{"event":"click"},{"event":"navigation"}]'
```
```bash Close a stream
curl -X POST http://127.0.0.1:4437/demo/hello \
-H 'Stream-Closed: true'
```
## JSON message text
Ursula validates every JSON body and stores each message as you wrote it, minus insignificant whitespace (spaces, tabs and line breaks outside strings), followed by one LF. Member order, duplicate members, number spelling such as `1.50e3` or `1e400`, and string escapes, including escapes of unpaired surrogates, are kept byte for byte, and every read returns that text. A top-level array is split into one message per element. A message may nest at most 127 levels. See [JSON message text](/docs/0.7/specs/extensions#json-message-text). Releases before this rule re-serialized messages; see the [release notes](/docs/0.7/release-notes).
To append only if no other write has committed since you last read the stream, see [conditional writes](/docs/0.7/concepts/conditional-writes).
---
# Read stream
Bucket ID.
Stream ID within the bucket.
Starting offset: `-1` for the beginning, `now` for the current tail, or a `Stream-Next-Offset` value from an earlier response.
On a long-poll or SSE read, the `Stream-Cursor` value from the previous live response. It lets caches tell successive live requests apart. It is not a position and never replaces `offset`.
`local` (default) or `leader`, on catch-up reads only. `leader` makes the read linearizable: it includes every append acknowledged before the request, at the cost of one round trip from the group's leader to a majority of voters. Use it to read back your own acknowledged writes. Combining it with `live` answers `400`. See [Read path](/docs/0.7/specs/extensions#92-leader-consistency).
Live mode: `sse` for Server-Sent Events, `long-poll` for long-polling. Omit for catch-up read.
On an offset read, the maximum bytes to read in one response or SSE data batch. Every response is capped at 8 MiB; a larger value is clamped, and a value that is not a decimal integer is ignored. A shortened response has no `Stream-Up-To-Date` and may end inside a message, on a JSON stream too; continue from `Stream-Next-Offset`. For UTF-8 SSE data, Ursula may shorten the emitted batch so `Stream-Next-Offset` lands on a valid text boundary.
## Read modes
No `live` parameter. Returns all available data from the given offset immediately.
```bash
curl 'http://127.0.0.1:4437/demo/hello?offset=-1'
```
`live=long-poll`. Returns immediately if data is available, otherwise holds the connection until new data arrives or `timeout_ms` elapses (default 1000, at most 60000).
```bash
curl 'http://127.0.0.1:4437/demo/hello?offset=00000000000000000042&live=long-poll'
```
`live=sse`. Opens a persistent Server-Sent Events connection. The server pushes data events as new entries are appended. It sends no heartbeats, so a proxy with an idle timeout may close a quiet connection; reconnect from the last control event's `streamNextOffset`.
```bash
curl 'http://127.0.0.1:4437/demo/hello?offset=-1&live=sse'
```
Optional precondition: the stream's incarnation token, as a previous response's `Stream-Incarnation` header returned it. The read is answered only if the stream that serves it is still that incarnation; otherwise it answers `412`. A long-poll or SSE read with it ends when the stream it was opened against is deleted or recreated (a long-poll answers `404` or `412`; an SSE stream ends with an `error` event). Without the header, a long-poll still ends when the stream it registered against is recreated, usually with `416`; otherwise the request behaves exactly as described here. A malformed value answers `400`. See [`Stream-Incarnation`](/docs/0.7/specs/extensions#24-snapshot-metadata-on-head-and-stream-incarnation).
## Response
| Status | Meaning |
| ------ | ------------------------------------------------------------- |
| `200` | Data returned. A catch-up read at the tail answers `200` with an empty body and `Stream-Up-To-Date: true`. |
| `204` | Long-poll only: `timeout_ms` passed without new data, or the stream is closed and the read is at its tail (answered at once with `Stream-Closed: true`, which signals end of stream). |
| `304` | `If-None-Match` matched the response's `ETag`. |
| `307` | In a cluster, this node could not serve the read as the stream's group leader (a long-poll or SSE read on a follower, or a `consistency=leader` read on a node that lost leadership). `Location` is the same request on the leader. |
| `400` | Invalid offset, live mode or `consistency`, `consistency=leader` with `live`, or a read parameter removed in 0.6.0 (see the [release notes](/docs/0.7/release-notes)). |
| `404` | Stream not found or expired. |
| `410` | Requested offset has been trimmed. `Stream-Next-Offset` is the earliest offset still readable. |
| `412` | `Stream-Incarnation` did not match: the stream was deleted and recreated. The response's `Stream-Incarnation` is the current token. A read that would answer `404`, `410` or `416` without the header still does. |
| `416` | Requested offset is beyond the stream's tail. `Stream-Next-Offset` is the tail. |
| `503` | The group has no known leader, the leader could not confirm its leadership in time (`consistency=leader` and live reads), or this node holds too many live readers. Retry after `Retry-After`. |
Response headers include `Stream-Next-Offset`, `Stream-Incarnation`, `ETag`, `Stream-Up-To-Date`, `Stream-Closed`, and `Content-Type`; the SSE response and the long-poll `204` carry `Stream-Incarnation` too. Long-poll responses for an open stream also carry `Stream-Cursor`.
Streams written with `Content-Type: application/json` are returned as newline-delimited JSON with response `Content-Type: application/x-ndjson`, one stored message per line, exactly as written minus insignificant whitespace ([JSON message text](/docs/0.7/specs/extensions#json-message-text)). On an offset read, `max_bytes` applies to the encoded byte stream, so a response can end mid-line. Resume from `Stream-Next-Offset` and buffer any incomplete trailing line before parsing. `HEAD` reports the configured stream content type, while read responses report the wire representation.
## SSE event format
In SSE mode, the server sends:
- **Data events** (`event: data`): stream payload in the `data` field. The response `Stream-Data-Content-Type` header identifies the data payload type (`application/x-ndjson` for JSON streams, the original content type for other streams). SSE `data:` lines are transport lines, not guaranteed message boundaries. JSON clients should buffer until newline before parsing messages. For binary streams, data is base64-encoded (controlled by the `Stream-Sse-Data-Encoding` header).
- **Control events** (`event: control`): JSON metadata including the current offset and stream state.
```bash Catch-up
curl 'http://127.0.0.1:4437/demo/hello?offset=-1'
```
```bash Long-poll
curl 'http://127.0.0.1:4437/demo/hello?offset=00000000000000000042&live=long-poll'
```
```bash SSE tail
curl 'http://127.0.0.1:4437/demo/hello?offset=-1&live=sse'
```
```bash Tail future appends over SSE
curl -N 'http://127.0.0.1:4437/demo/hello?offset=now&live=sse'
```
> [!NOTE]
> See [read modes](/docs/0.7/concepts/read-modes), [binary SSE](/docs/0.7/concepts/binary-sse), and [offsets](/docs/0.7/concepts/offsets) for more details.
---
# Head stream
Bucket ID.
Stream ID within the bucket.
Optional precondition: the stream's incarnation token, as a previous response's `Stream-Incarnation` header returned it. The `HEAD` answers `200` only if the stream is still that incarnation; otherwise `412`, with the current token in `Stream-Incarnation`. Without the header, the request behaves exactly as described here. A malformed value answers `400`. See [`Stream-Incarnation`](/docs/0.7/specs/extensions#24-snapshot-metadata-on-head-and-stream-incarnation).
## Response
| Status | Meaning |
| ------ | ------------------------------------------------------ |
| `200` | Stream found. |
| `307` | In a cluster, this node could not answer as the stream's group leader. `Location` is the same request on the leader. |
| `404` | Stream not found or expired. |
| `412` | `Stream-Incarnation` did not match. The response's `Stream-Incarnation` is the current token. |
| `503` | The group has no known leader, or the leader could not confirm its leadership in time. Retry after `Retry-After`. |
`HEAD` is linearizable: the leader confirms with a majority of voters that it still leads before it answers, so the response reflects every append acknowledged before the request started. See [Durability and consistency](/docs/0.7/concepts/durability-and-consistency#consistency).
Response headers include:
| Header | Description |
| ------------------------ | --------------------------------------------------------- |
| `Content-Type` | The stream's content type. |
| `Stream-Next-Offset` | The stream's tail: the offset the next append starts at. |
| `Stream-Closed` | Present and `true` if the stream is closed. |
| `Stream-Retained-Offset` | The earliest offset still readable. Reads below it answer `410 Gone`. Before any [retention](/docs/0.7/concepts/snapshots) advance, it is the start of the stream. |
| `Stream-Snapshot-Offset` | The offset of the latest snapshot. Absent when the stream has no snapshot. Read the snapshot with [`GET …/snapshot/{offset}`](/docs/0.7/api/read-snapshot). |
| `Stream-Snapshot-Digest` | The digest of the latest snapshot's content type and body, as its publish returned it. Present with `Stream-Snapshot-Offset`. |
| `Stream-Incarnation` | An opaque token for this incarnation of the stream. It changes when the stream is deleted and recreated at the same URL; compare it only for equality. |
| `Stream-TTL` | The configured TTL in seconds (if a TTL was set). This is the sliding window length, not the time remaining. `HEAD` does not renew it. |
| `Stream-Expires-At` | Expiration timestamp (if set). |
| `Cache-Control` | `no-store`. |
| `Stream-Cold-Hot-Start-Offset` | Diagnostic and unstable: may change or disappear in any release. Data below this offset has been flushed to cold storage. |
```bash Example
curl -I http://127.0.0.1:4437/demo/hello
```
---
# Publish snapshot
Publishes a new checkpoint blob at the given offset. The snapshot replaces any previously published snapshot, but does not delete stream history. Retention advances only through the separate `/retention` endpoint.
Bucket ID.
Stream ID.
Stream offset this snapshot represents: the 20-character zero-padded decimal token.
For an `application/json` stream the offset must be a message boundary: a `Stream-Next-Offset` that Ursula returned, or any offset just after a message's LF.
Content type of the snapshot blob, stored separately from the stream's own content type. Defaults to `application/octet-stream`.
The snapshot blob bytes. Bodies of up to **32 MiB** are stored inline in the stream's replicated state. On a node with a cold backend, a body of at least `runtime.external_payload_min_size` (1 MiB by default) is streamed into the cold tier instead, and bodies of up to **1 GiB** are accepted. Larger bodies return `413`.
Optional precondition: the stream's incarnation token, as a previous response's `Stream-Incarnation` header returned it. The request applies only if the stream is still that incarnation, checked atomically with the write; otherwise it answers `412` and changes nothing. Without the header, the request behaves exactly as described here. A malformed value answers `400`. See [`Stream-Incarnation`](/docs/0.7/specs/extensions#24-snapshot-metadata-on-head-and-stream-incarnation).
## Response
| Status | Meaning |
| ------ | --------------------------------------------------------------- |
| `204` | Snapshot published successfully. |
| `400` | Invalid offset or content type, or, on a JSON stream, an offset inside a message. |
| `404` | Stream not found or expired. |
| `409` | Stale publish (a newer snapshot exists) or offset out of range. |
| `410` | Snapshot offset is older than the retained stream history. |
| `412` | `Stream-Incarnation` did not match: the stream was deleted and recreated. Nothing was published; the response's `Stream-Incarnation` is the current token. |
| `413` | Snapshot body exceeds the maximum allowed size (32 MiB inline, 1 GiB in the cold tier). |
| `502` | The cold backend failed while storing the body. |
| `503` | JSON stream only: Ursula could not read the byte before the offset to check the boundary. Nothing was published; retry. |
Response headers include `Stream-Next-Offset`, `Stream-Snapshot-Offset`, `Stream-Snapshot-Digest`, and `Stream-Incarnation`. Repeating the same offset, content type, and body is idempotent and returns the same digest. Reusing an offset with different content returns `409`.
```bash Example
curl -X PUT 'http://127.0.0.1:4437/demo/hello/snapshot/00000000000000000042' \
-H 'Content-Type: application/json' \
--data-binary '{"state": "aggregated snapshot data"}'
```
## Advance retention
`PUT /{bucket}/{stream}/retention/{offset}` explicitly discards history before a published checkpoint. The boundary must be monotonic, no newer than the visible checkpoint, and, on a JSON stream, a message boundary (`400` otherwise, `503` when it cannot be checked). Retention is exact: the response's `Stream-Retained-Offset` is the requested offset.
After a successful `204`, reads before the retained offset return `410 Gone`. This separate step lets clients publish and verify a checkpoint before making history unreachable.
> [!NOTE]
> Snapshots are immutable at a given offset. A byte-identical retry is idempotent; a different body at the same offset conflicts. Publishing does not advance retention.
---
# Read snapshot
`GET /{bucket}/{stream}/snapshot/{offset}` returns the snapshot blob at a specific offset. To read the latest snapshot, take its offset from the `Stream-Snapshot-Offset` header of [`HEAD`](/docs/0.7/api/head-stream).
Bucket ID.
Stream ID.
Snapshot offset: the 20-character zero-padded decimal token returned by previous reads or `Stream-Snapshot-Offset` headers.
Optional precondition: the stream's incarnation token, as a previous response's `Stream-Incarnation` header returned it. The read is answered only if the stream that serves it is still that incarnation; otherwise it answers `412`. Without the header, the request behaves exactly as described here. A malformed value answers `400`. See [`Stream-Incarnation`](/docs/0.7/specs/extensions#24-snapshot-metadata-on-head-and-stream-incarnation).
| Status | Meaning |
| ------ | -------------------------------------- |
| `200` | Snapshot blob returned. |
| `307` | In a cluster, this node is not the stream's group leader. `Location` is the same request on the leader. |
| `404` | Stream, snapshot, or offset not found. |
| `412` | `Stream-Incarnation` did not match. The response's `Stream-Incarnation` is the current token. |
| `502` | The snapshot body is in the cold tier and could not be read. |
| `503` | The group has no known leader, or the leader could not confirm its leadership in time. Retry after `Retry-After`. |
Response headers include `Content-Type`, `Content-Length`, `Stream-Next-Offset`, `Stream-Snapshot-Offset`, `Stream-Snapshot-Digest`, `Stream-Incarnation`, and `Stream-Up-To-Date: true` when the snapshot offset is the stream's tail. A body stored in the cold tier is streamed from it.
```bash Read specific offset
curl 'http://127.0.0.1:4437/demo/hello/snapshot/00000000000000000042'
```
> [!NOTE]
> Snapshot reads are served by the group's leader after it confirms with a majority of voters that it still leads, so the read reflects every snapshot publish acknowledged before the request. A follower answers `307` to the leader; follow the redirect. See [Durability and consistency](/docs/0.7/concepts/durability-and-consistency#consistency).
> [!NOTE]
> See [snapshots](/docs/0.7/concepts/snapshots) for the snapshot lifecycle.
---
# Bootstrap
Returns the stream's latest snapshot (if any) plus the retained updates after the snapshot offset, packed as a `multipart/mixed` response. When some of those updates are only in cold storage, or they exceed 8 MiB, the response is an honest partial: it stops early and the client reads the rest with ordinary reads. This is the recommended way to initialize a client that needs the complete current state of a stream.
Bucket ID.
Stream ID within the bucket.
Bootstrap does not accept `?live=sse`. Combining the multipart body with an SSE event stream is rejected with `400`. To go live after bootstrap, finish the multipart response, then open a separate `GET /{bucket}/{stream}?offset=&live=sse`.
Optional precondition: the stream's incarnation token, as a previous response's `Stream-Incarnation` header returned it. The read is answered only if the stream that serves it is still that incarnation; otherwise it answers `412`. Without the header, the request behaves exactly as described here. A malformed value answers `400`. See [`Stream-Incarnation`](/docs/0.7/specs/extensions#24-snapshot-metadata-on-head-and-stream-incarnation).
## Response
| Status | Meaning |
| ------ | --------------------------------------------- |
| `200` | Bootstrap response with snapshot and updates. |
| `307` | In a cluster, this node is not the stream's group leader. `Location` is the same request on the leader. |
| `400` | Invalid query parameters (including `live=sse`). |
| `404` | Stream not found or expired. |
| `410` | Requested offset has been trimmed. |
| `412` | `Stream-Incarnation` did not match. The response's `Stream-Incarnation` is the current token. |
| `503` | The group has no known leader, or the leader could not confirm its leadership in time. Retry after `Retry-After`. |
Bootstrap is served by the group's leader after it confirms with a majority of voters that it still leads, so the response reflects every append and snapshot acknowledged before the request. See [Durability and consistency](/docs/0.7/concepts/durability-and-consistency#consistency).
Response headers include:
| Header | Description |
| ------------------------ | ---------------------------------------------------------------- |
| `Content-Type` | `multipart/mixed; boundary=...` |
| `Stream-Next-Offset` | The offset after the last included update, or the snapshot point when the response has no update parts. Continue reading from here. |
| `Stream-Snapshot-Offset` | The snapshot offset, or `-1` if no snapshot exists. |
| `Stream-Incarnation` | The incarnation token of the stream that served the response. |
| `Stream-Up-To-Date` | `true` when the response reaches the stream's tail. Absent on a partial response. |
| `Stream-Closed` | Present and `true` if the stream is closed and the response reaches its tail. |
| `Cache-Control` | `no-store`. |
## Complete and partial responses
Let `S` be the snapshot offset, or the first retained offset when there is no snapshot. Ursula returns one of two forms:
- **Complete.** Every retained byte in `[S, tail)`, with `Stream-Next-Offset` at the tail and `Stream-Up-To-Date: true`.
- **Partial.** The bytes in `[S, P)` for some `P` below the tail, with `Stream-Next-Offset: P` and no `Stream-Up-To-Date`. When bytes after `S` have already moved to cold storage, `P = S` and the response holds only the snapshot part. When the updates exceed 8 MiB, a JSON stream stops at the last message boundary within 8 MiB and always holds at least one message; any other stream answers the snapshot part alone.
Neither form skips bytes. After a partial response, read from `Stream-Next-Offset` with ordinary catch-up reads until `Stream-Up-To-Date: true`, and apply those updates before treating the state as current. Bootstrap never reads cold storage itself.
## Response body
The body is a `multipart/mixed` message:
**Snapshot part**
The first part is the snapshot blob (or an empty part if no snapshot exists).
**Update parts**
Subsequent parts hold the updates starting at the snapshot point, with the stream's content type. For JSON streams, each part is one JSON message. For any other stream, the updates come back as one part that may contain several messages; a part boundary is not a message boundary.
```bash Example
curl 'http://127.0.0.1:4437/demo/hello/bootstrap'
```
> [!TIP]
> After bootstrapping, catch up from `Stream-Next-Offset` if the response was partial, then switch to [SSE reads](/docs/0.7/api/read) with `live=sse` from the latest `Stream-Next-Offset` to receive real-time updates.
> [!NOTE]
> See [bootstrap](/docs/0.7/concepts/bootstrap) and [snapshots](/docs/0.7/concepts/snapshots) for the conceptual model.
---
# Delete stream
Bucket ID.
Stream ID within the bucket.
Optional precondition: the stream's incarnation token, as a previous response's `Stream-Incarnation` header returned it. The delete applies only if the stream is still that incarnation; otherwise it answers `412` and deletes nothing. Without the header, the request behaves exactly as described here. A malformed value answers `400`. See [`Stream-Incarnation`](/docs/0.7/specs/extensions#24-snapshot-metadata-on-head-and-stream-incarnation).
## Response
| Status | Meaning |
| ------ | ----------------- |
| `204` | Stream deleted. |
| `404` | Stream not found. |
| `412` | `Stream-Incarnation` did not match: the stream is another incarnation, which was not deleted. The response's `Stream-Incarnation` is its token. |
```bash Example
curl -X DELETE http://127.0.0.1:4437/demo/hello
```
> [!WARNING]
> Deletion is permanent. The stream's data will be asynchronously garbage-collected after the delete is committed.
---
# ursulactl
`ursulactl` manages the **logical** state of a running cluster over Ursula's admin and metrics HTTP APIs: which node leads which Raft groups, and whether a node is caught up. It executes nothing on hosts. Physical lifecycle belongs to whatever owns the process: Helm and the StatefulSet controller on Kubernetes, systemd on hosts, OpenTofu for the infrastructure underneath. The verbs encode the safety properties an operator otherwise has to remember manually:
- before a node goes down, transfer every Raft group it leads to a healthy successor (`drain`)
- after it comes back, refuse to move on until `last_applied_index` has caught up to peers' `committed_index` (`wait`)
- abort rather than corner a group with no leader
A safe rolling restart wraps the platform's restart in these verbs, one node at a time. On Kubernetes:
```bash
ursulactl drain --config manifest.json --node 3
kubectl delete pod ursula-2 # the platform restarts the pod
ursulactl wait --config manifest.json --node 3
ursulactl undrain --config manifest.json --node 3
ursulactl verify-cluster --config manifest.json
```
On bare metal the restart in the middle is `systemctl restart ursula` on the host. The drain planning and readiness logic is exercised under [deterministic simulation](/docs/0.7/architecture/overview).
> [!WARNING]
> The naive procedure of "restart followers, then leader" does **not** wait for `applied_index` to catch up between steps. A restarted voter that lost entries, for example after a host crash under `raft.wal.fsync = "never"` or with a replaced disk, stays out of elections until its group leader has rebuilt it. Always drain before restart and wait for catch-up before moving to the next voter.
## Verbs
| Verb | Effect |
|------|--------|
| `drain --node N` | Mark the node draining and transfer away every leadership it holds. The mark persists (the node attracts no leaderships) until `undrain`. `--dry-run` prints the transfer plan |
| `undrain --node N` | Clear the drain mark so the node may hold leaderships again |
| `wait --node N` | Block until the node is a voter in every group and within `--lag-tolerance` of peers. Progress-gated: a node that keeps advancing is never timed out |
| `pin-incarnations` | Save required process identities in a manifest for subsequent commands |
| `status` | Per-node group counts and leadership distribution |
| `wait-ready` | Wait for complete maintenance evidence and local serving health on every node |
| `verify-cluster` | Require every configured voter to be present and caught up in two consecutive samples before the next rollout step |
| `verify-quorum` | Confirm every configured group with a fresh ReadIndex round and require every replica to apply its fixed prefix under the same leader term |
| `verify-survivors` | Confirm fresh prefixes on two observed survivors while preserving the three-voter configuration; does not certify full redundancy or physical fencing |
| `backup-create` | Export every Raft group into a backup at a local directory or `s3://bucket/prefix` |
| `backup-verify` | Check a backup's manifest, sizes, checksums and every group snapshot without contacting a cluster |
| `restore` | Restore a verified backup into a fresh, empty cluster with the same group count, once the cluster confirms it holds every cold object the backup references |
Mutating verbs exit `0` on success and `2` on an abort (drain timeout, no safe transfer target, catch-up stall). Anything else is a configuration or transport error with a single-line, machine-greppable message.
## Process identity across commands
Each CLI invocation pins the first observed server instance. A process change makes subsequent observations fail, and mutations retain the original `x-ursula-process-incarnation` header. To preserve the same plan across invocations:
```bash
ursulactl pin-incarnations --config cluster.json > pinned-cluster.json
ursulactl drain --config pinned-cluster.json --node 3
```
The saved nodes carry `expected_process_incarnation`. After an admitted Pod replacement, `pin-incarnations --replace-node 3` can bind that one replacement. Every survivor must still match its saved identity. The Helm rollout persists these pins and the admitted source/replacement Pod UIDs in state schema 3. Resuming the same replacement validates its saved process instead of refreshing it after a container restart. It stops on a missing saved plan, changed survivor, or another unadmitted replacement.
`pin-incarnations` requires a process incarnation from every node, including an explicitly selected replacement. Nodes that do not report one are refused. `verify-quorum` records the identity of every observed process, and reservation steps compare that map with the saved plan. Instance identity does not serialize concurrent executors or prove a failed host is physically fenced.
## Maintenance executor admission
A reviewed cell reservation consumer can add one immutable `maintenance_fence` object at the manifest root: `reservation_id` and `executor_id` are 32 lowercase hexadecimal characters, and `generation` is a nonzero cell-wide monotonically increasing integer. Every saved node carries that same token as `expected_maintenance_fence`; inconsistent or partially populated plans are rejected. The lifecycle commands require saved process identities for every configured voter:
```bash
ursulactl activate-maintenance-fence --config admitted-cluster.json
ursulactl drain --config admitted-cluster.json --node 3
# After replacement and reconciliation of its physical lifecycle:
ursulactl retire-maintenance-fence --config admitted-cluster.json
ursulactl verify-quorum --config admitted-cluster.json --expected-groups 256
```
These commands install or retire the supplied token; they do not acquire a shared reservation, choose generations, authorize Pod deletion, or establish provider-host fencing. A multi-node command may stop after partial acknowledgement. Keep the external operation reserved, resume its same identities, and require all relevant acknowledgements before advancing. Expiry is not authority to select another voter. A new executor uses the higher generation admitted by the external CAS operation; CLI never adopts a token discovered in metrics or refreshes it after 412/428.
Activation waits for admitted mutation handlers and then confirms prior messages were processed by each captured local Raft API queue. Caller cancellation leaves its admitted handler running; abnormal handler termination latches that process closed. Failed or cancelled activation/retirement stays closed until the same pending lifecycle completes or an externally admitted higher generation activates. Retirement retains its generation and cannot be reopened by the same token. This confirms submission ordering, not completion of asynchronous replication, snapshot work, or physical operations. Fresh all-replica prefix certification and external lifecycle reconciliation remain required before releasing the reservation.
New processes start `unclaimed`, explicitly uncertified until admission. An explicit `pin-incarnations --replace-node` can bind only that selected unclaimed replacement while preserving the token. Other voters must still match the active saved authority. Once activated, all mutating admin requests require `x-ursula-maintenance-fence` as `reservation_id:generation:executor_id` in addition to the process-incarnation header. Metrics report `maintenance_fence` and `maintenance_fence_uncertain`. `verify-quorum` schema 3 records `maintenance_executor_certified` only when every observed process matches the same caller-supplied active token. A subsequent proof can inspect the same retired token and reports `maintenance_executor_retired_certified` only when all observed processes have retired it. A mix of active and retired nodes certifies neither state. These flags certify process-local admission or retirement, not the external reservation or physical fencing. Existing chart and Cloud consumers are uncertified until their shared reservation integration is complete.
## The admin plane
Nodes carry **no cluster-mutation surface on the network**. The mutating operator endpoints (raft snapshot/purge/membership/learners/leader-transfer, maintenance drain, cold-flush trigger, bucket purge) plus metrics and usage are served on a separate **admin plane** bound to `server.admin_listen`, which defaults to loopback (`127.0.0.1:4438`). The public client plane (`:4437`) serves only stream traffic and read-only metrics and usage.
Metrics and readiness use an explicit `metrics_url` when configured, then `http_url`, then `admin_url`. Both planes serve them. The chart's rollout hook sets `metrics_url` to its Pod-bound admin tunnel so it can inspect a replacement before cluster DNS becomes reachable. Mutating verbs always use `admin_url`. Learner attachment and Raft handoff retain the advertised `http_url`.
ursulactl is a plain HTTP client and opens no tunnels itself. When the admin plane is not directly reachable, bring your own forward and point the manifest's `admin_url` at it. On Kubernetes, `kubectl port-forward` reaches the loopback-bound plane inside each pod:
```bash
kubectl port-forward pod/ursula-0 5441:4438 &
kubectl port-forward pod/ursula-1 5442:4438 &
kubectl port-forward pod/ursula-2 5443:4438 &
```
```toml
[[nodes]]
id = 1
admin_url = "http://127.0.0.1:5441"
[[nodes]]
id = 2
admin_url = "http://127.0.0.1:5442"
[[nodes]]
id = 3
admin_url = "http://127.0.0.1:5443"
```
On bare metal the same shape works with `ssh -N -L 5441:127.0.0.1:4438 admin@node1` per node.
## When to use ursulactl vs. the other surfaces
| Task | Tool |
|------|------|
| Day-2 logical operations: drain, observe, gate on readiness | **`ursulactl`** |
| Deployment, restarts, upgrades, topology | [Helm](/docs/0.7/deploy-cluster) (systemd on bare metal) |
| Custom operator tooling | The admin-plane HTTP endpoints, reached over your own tunnel |
## Install
Build from the workspace alongside the server:
```bash
cargo build --release -p ursula-ctl --bin ursulactl
```
The binary lands at `target/release/ursulactl`. Drop it on your control machine. It does not need to run on the Ursula hosts themselves.
## Manifest format
Every verb accepts `--config `. The manifest is **TOML, JSON, or YAML** (chosen by file extension, sniffed when read from stdin with `-`) and lists the cluster's nodes.
Prefer generating the manifest from whatever already knows the topology instead of writing it by hand. On Kubernetes the Helm chart renders one into its ConfigMap, and the URLs in it are in-cluster DNS, so pipe it to ursulactl running where those names resolve (the server image contains ursulactl):
```bash
kubectl get configmap ursula -o jsonpath='{.data.cluster-manifest\.json}' \
| ursulactl status --config -
```
For infrastructure provisioned by OpenTofu, emit the manifest as a stack output or a generated file rather than teaching ursulactl to read state files. `tofu output -json | ursulactl status --config -` composes the same way.
Per-node fields, all optional except `id`:
- `admin_port` (default `4438`) or an explicit `admin_url`: the admin plane to reach, directly or through your forward.
- `host`: address shown in reports. Falls back to the admin URL's host.
- `http_url`: advertised client/Raft URL used for learner attachment and survivor handoff, and for metrics and readiness when no explicit `metrics_url` is set.
- `metrics_url`: optional metrics and readiness endpoint, for example the same tunnel as `admin_url`. It overrides that access without changing the peer address persisted in membership.
## Restarting a voter that lost its log
A voter that shut down cleanly comes back with every entry it acknowledged and only catches up on what it missed. A voter that lost entries (a host crash under `raft.wal.fsync = "never"`, a journal I/O error, a wiped or replaced disk) needs no ursulactl verb either: it stays behind its recovery gate, and each group leader rebuilds it by itself, removing the voter, adding it back as a learner and promoting it after catch-up (see [Rejoining a wiped node](/docs/0.7/operations#rejoining-a-wiped-node)). Keep the node drained and run `wait` until every group has promoted it. A group whose majority lost entries at once waits for an operator instead (see [Recovering after a host crash](/docs/0.7/operations#recovering-after-a-host-crash)).
The rebuild of a voter that lost its disk installs snapshots for every group and can take 10+ minutes. `wait` is progress-gated, so no timeout tuning is needed: a rebuild that keeps advancing is never timed out, and `--stall-timeout-secs` (default 90) only aborts a node that stops making progress.
## `status`
Per-node summary of Raft group count and leadership distribution, sourced from every node's `/__ursula/metrics`. Nodes whose metrics fail are reported with `metrics unavailable — …` rather than aborting the report, because `status` is meant to surface partial cluster health.
```bash
ursulactl status --config cluster.json
```
Sample output:
```
node 1 (10.0.0.1): groups=4 leaders={1: 2, 2: 2}
node 2 (10.0.0.2): groups=4 leaders={1: 2, 2: 2}
node 3 (10.0.0.3): groups=4 leaders={1: 2, 2: 2}
```
`leaders={…}` is the count of groups each node is leading from this reporter's perspective. Healthy clusters report the same distribution from every node.
## `wait-ready`
Wait until every node reports `--expected-groups` initialized Raft groups with leaders, a complete uniform voter set in `raft_maintenance`, and no uncertain maintenance fence. Each node must also return 200 from `/__ursula/ready`, read from the same address as its metrics.
```bash
ursulactl wait-ready --config cluster.json --expected-groups 4
```
Exits non-zero with the maintenance report or serving refusal when the timeout passes. Connection failures are retried until the timeout, so the wait can start while a node restarts. Every poll observes the processes afresh. Only a node whose process differs from its pinned `expected_process_incarnation`, or an invalid response, stops the wait early with its original cause. Healthy survivors can serve while a replica rebuilds, but `wait-ready` still rejects a second maintenance operation until full redundancy returns.
## Underlying HTTP surface
For custom tooling, every verb maps onto a small set of HTTP endpoints on each node:
| Verb | Endpoint |
|------|----------|
| `status`, `wait` | `GET /__ursula/metrics` |
| `wait-ready` | `GET /__ursula/metrics` and `GET /__ursula/ready` |
| `drain` / `undrain` (maintenance guard) | `POST /__ursula/leadership-shed/maintenance`, `DELETE` to clear |
| `drain` (transfer step) | `POST /__ursula/raft/{raft_group_id}/leader/transfer/{node_id}` |
| `backup-create` | `GET /__ursula/backup/info`, `GET /__ursula/backup/group/{raft_group_id}` |
| `restore` | `GET /__ursula/backup/info`, `POST /__ursula/backup/group/{raft_group_id}/cold-check`, `POST /__ursula/backup/group/{raft_group_id}/import` |
The transfer endpoint returns a typed rejection: `409 Conflict` for a changed leader or a recovering target, `400` for an invalid target, `404` for an unregistered group, and `500` for a stopped Raft. Drain re-observes and re-plans the two `409` cases; other errors stop the operation.
## Backup and restore
`backup-create`, `backup-verify` and `restore` implement the backup workflow in [Backup and disaster recovery](/docs/0.7/operations#backup-and-disaster-recovery). A backup location is a local directory or `s3://bucket/prefix`.
```bash
ursulactl backup-create --config cluster.json --location s3://backups/ursula/2026-10-07
ursulactl backup-verify --location s3://backups/ursula/2026-10-07
ursulactl restore --config new-cluster.json --location s3://backups/ursula/2026-10-07
```
- `backup-create` writes one MessagePack snapshot per Raft group and `manifest.json`. It asks every node for each group and keeps the answer with the highest commit index. `--created-unix-ms` overrides the manifest timestamp.
- `backup-verify` checks that the manifest lists every group once, and each object's size, BLAKE3 checksum and snapshot. It reads only the backup.
- `restore` verifies the backup, refuses a target whose backup format or group count differs, and then asks the target to check every group's cold references against its cold store. Only when nothing is missing does it import the groups, one replicated write each. A target group that already holds data refuses the import. Restore into a fresh, empty cluster.
`backup-create` and `restore` take `--http-timeout-secs` (default `30`). The cold check looks up every object a group references, 32 at a time, in one request per group, so a group with tens of thousands of cold objects needs a longer timeout. An `s3://` location uses the AWS SDK credential chain and `AWS_REGION`. For MinIO or another S3-compatible store, also set `AWS_ENDPOINT_URL`.
The backup format is the format epoch, 3. `ursulactl` refuses a backup or a cluster of another format epoch.
Each command exits `0` on success and non-zero with an error that names the failed check. A refused `restore` imports nothing.
## `verify-quorum`
This source provides a bounded observation for recovery measurements and maintenance tooling. Supply the deployed group count explicitly; the example below uses 256 groups:
```bash
ursulactl verify-quorum --config cluster.json --expected-groups 256 --timeout-secs 120
```
The command requires each configured voter exactly once, every configured group on every voter, complete uniform membership, no learners, and eligible local participation reports. It confirms each group's leader through a new outbound ReadIndex probe, then waits for all replicas to apply the captured prefix under that leader's term. Continuous writes can advance without moving this target. One absolute deadline covers metrics requests, RPCs and waits. The JSON output includes the observation start/end times, each group's leader/term/prefix and every replica's applied index.
The prefix is confirmed through the incarnation-bound admin route
`GET /__ursula/raft/{group}/quorum`. The CLI uses shared admin protocol types
and has no dependency on `ursula-raft`. Quorum proof and self-election requests
require process identity; servers without the admin protocol are unsupported.
A successful observation is not a maintenance reservation, a continuously refreshed health claim or physical fencing permission. Independent maintenance workflows still need to serialize disruptions and bind replacement to the actual Pod/process/host incarnation.
## `verify-survivors`
After one voter becomes unavailable, this source can separately observe the two survivors. Keep all three voters in the manifest and explicitly exclude exactly one known node:
```bash
ursulactl verify-survivors --config cluster.json --excluded-node-id 3 --expected-groups 256 --timeout-secs 120
```
Every group must still have the complete uniform three-voter membership, no learners and open participation gates on both observed replicas. Both survivors must agree on a reachable leader and apply each newly confirmed fixed prefix. The command rejects a second missing replica, an unknown exclusion or a changed topology. It never shrinks the configured membership to fit observations.
The output is scoped as `surviving_quorum`, records the configured and observed voter sets, and always reports `full_redundancy_restored=false`. Excluding a voter from observation does not prove its old process or host is stopped. Hard fencing and an exclusive maintenance reservation remain prerequisites for physical replacement, and the complete `verify-quorum` check is required before any subsequent disruption.
### Shared reservation proposals
Reservation stores have one schema, version 1, for Pod replacement, host recovery and candidate restaging. Host inventory is optional data, and publishing it keeps the version. `ursulactl` refuses a store with any other version.
`reservation-propose --cell cell.json --snapshot before.json --request request.json`
produces a whole-ConfigMap CAS proposal without contacting Kubernetes.
`reservation-acknowledge` takes the same inputs plus `--response response.json` and
validates the exact successful update receipt. Conflicts stop without refreshing
authority, and missing stores do not initialize themselves.
The policy covers sticky, one-source planned Pod replacement, current-token fresh
prefix admission and all-retired completion after the original Pod UID is gone.
Its inputs are operational evidence, not authentication. The chart's
`maintenance-rollout.sh` Job drives planned Pod replacement through it. No
component drives host recovery or calls a provider to terminate an instance, and
this offline interface alone grants no disruption authority. The full protocol is
documented in `docs/architecture/cluster-membership.md`.
---
# Operations
The first tool to reach for is [`ursulactl`](/docs/0.7/cli). It covers the raft-aware verbs operators run most often: drain, status, readiness gates. This page covers everything **around** it: the metrics shape ursulactl reads, the admin endpoints it (and your custom tooling) can call, and the operational policies ursulactl does not encode (backups, log levels). Deployment and process lifecycle belong to the platform, such as [Helm](/docs/0.7/deploy-cluster).
## Tooling map
| Surface | When to reach for it |
|---------|---------------------|
| [`ursulactl`](/docs/0.7/cli) | Day-2 cluster ops over HTTP: drain, catch-up waits, status, wait-ready. |
| `/__ursula/metrics` and the `/__ursula/raft/...` admin endpoints | Custom tooling. ursulactl uses these underneath. The surface is small and stable enough to script directly. |
There is no Prometheus scrape and no general-purpose orchestrator yet.
## Metrics
```bash
curl http://127.0.0.1:4437/__ursula/metrics | jq .
```
The JSON snapshot covers per-core mailbox depth, append and apply counters,
cumulative timings (`*_ns`), each group's leader, term and applied index
(`raft_groups`), hot bytes waiting for cold flush, cold-flush uploads and
errors, WAL physical/reclaim/recovery work, WAL disk-pressure admission, SSE
counters, live-read waiters, and cold-write backpressure. Start here when
triaging slowness, lag, or `503`s. [Troubleshooting](/docs/0.7/troubleshooting#diagnostic-surface)
lists jq selectors for the common fields. `GET /__ursula/ready` provides the
local serving signal with a typed `reason` on refusal, on the client and admin
planes alike. Healthy surviving voters
can serve during a rebuild while the learner remains unready. Maintenance also
requires a complete `raft_maintenance` report and certain maintenance authority
from metrics. `ursulactl wait-ready` checks both the metrics and this existing
readiness endpoint. `ursulactl status` presents leadership fields across the
cluster.
## Admin endpoints
These are the primitives `ursulactl` and any custom operator tooling builds on. They live on the **admin plane** (`server.admin_listen`, loopback `127.0.0.1:4438` by default), not the public client plane. Nodes expose no cluster-mutation surface on the network. Reach them over a tunnel to the admin port (see [ursulactl's operation providers](/docs/0.7/cli#the-admin-plane)). The examples below assume a forward from local `4438` to the node's admin plane. Each call is local to one node. To act on every group, loop over the IDs in metrics.
Every admin mutation requires `x-ursula-process-incarnation`, pinned from `GET /__ursula/metrics` before the operation. Missing identity returns `428`; a restarted target returns `412` before the handler changes state. Keep the same identity for the entire plan and stop on either response. This precondition is public instance identity, not authentication or an exclusive maintenance reservation. Updated `ursulactl` attaches it automatically and refuses to refresh it during an operation.
```bash
# Observe once for this operation; do not refresh after a 412.
process_incarnation=$(curl --fail http://127.0.0.1:4438/__ursula/metrics | jq -er .process_incarnation)
# Force a cold flush for one stream (skip the timer)
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X POST http://127.0.0.1:4438/__ursula/flush-cold/demo/hello
# Trigger a Raft snapshot for one group
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X POST http://127.0.0.1:4438/__ursula/raft/42/snapshot
# Purge log entries below the last snapshot index for one group
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X POST http://127.0.0.1:4438/__ursula/raft/42/purge
# Add a learner (non-voting replica) to one group without blocking on catch-up
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X POST "http://127.0.0.1:4438/__ursula/raft/42/learners/4?addr=http://node-4:4437&blocking=false"
# Hand leadership of one group to another voter (used by `ursulactl drain`)
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X POST http://127.0.0.1:4438/__ursula/raft/42/leader/transfer/2
# Replace the voter set of one group (call on the group's current leader)
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X POST "http://127.0.0.1:4438/__ursula/raft/42/membership?voters=1,2,3"
# Put this node into maintenance drain / lift it again
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X POST http://127.0.0.1:4438/__ursula/leadership-shed/maintenance
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X DELETE http://127.0.0.1:4438/__ursula/leadership-shed/maintenance
# Accept the loss of one group's unsynced tail on this node, naming the log its metrics show (see Recovering after a host crash)
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -H "content-type: application/json" \
-d '{"expected_last_log_index": 1234, "expected_current_term": 7}' \
-X POST http://127.0.0.1:4438/__ursula/raft/42/recovery/accept-unsynced-loss
```
The snapshot endpoint answers once the group has a snapshot of everything it had applied when the request arrived. It waits while a build of the group runs or other snapshot builds hold all of the node's build permits, and answers `503` with `Retry-After: 1` when no build of the group can start within 10 seconds (see [Manually trigger a snapshot](/docs/0.7/troubleshooting#manually-trigger-a-snapshot)).
The leader-transfer endpoint refuses with `409 Conflict` if the receiving node isn't the current leader and `400 Bad Request` if the target isn't a voter. This is why `ursulactl` is the safer way to chain these calls: it consults metrics first.
The group leader keeps every group at its configured voters: if the membership endpoint drops a configured voter, the leader adds it back as a learner and promotes it again once it has caught up (see [Recovering after a host crash](#recovering-after-a-host-crash)). Membership changes are for repair, not for shrinking a group.
The membership endpoint has the same leader-only contract: it answers `409 Conflict` (with the current leader's id in the body) when the receiving node does not lead the group, and `400 Bad Request` without a non-empty `voters` query parameter. On success it returns the log index at which the new voter set was committed. It replaces the whole voter set in one call, so include every node that should remain a voter.
### Maintenance drain
`POST /__ursula/leadership-shed/maintenance` marks the node as draining: it stops campaigning and accepting transfers, and the background leadership balancer migrates its current leaderships to eligible peers one transfer per tick until the mark is lifted with `DELETE`. Both verbs return the node's leadership-shed state so tooling can confirm the transition took effect:
```json
{
"bits": 1,
"state": "maintenance-drain",
"should_accept_transfer": false,
"should_campaign": false,
"should_shed_current_leaders": true
}
```
`ursulactl drain` wraps exactly this pair: mark first so the node stops re-acquiring groups, then transfer leaderships away explicitly instead of waiting for the balancer. The mark stays set until `ursulactl undrain` clears it. If a rollout ever dies mid-way, a node can be left refusing leadership. Run `ursulactl undrain` (or `DELETE` the mark directly) to recover. Peers read each other's shed state (`GET /__ursula/leadership-shed` on the cluster-plane listener) when deciding which nodes are eligible to pick up leaderships, so a drained node stops attracting groups cluster-wide, not just locally.
### Graceful voter shutdown
On the first SIGTERM or Ctrl-C, the voter marks itself maintenance-drained to
prevent re-acquiring leadership. It keeps Raft transport available while attempting
to hand off each leadership to an eligible voter (see the handoff rule under
[Recovering after a host crash](#recovering-after-a-host-crash)). It waits for
observed leadership changes for up to five seconds, then closes Raft transport and
drains HTTP listeners for up to ten seconds. A connection still open after
that, such as one from a peer that lost power, is abandoned. It then stops its
Raft groups, `fsync`s every journal and records a clean shutdown, so the next
start reads the journals strictly and the node holds every entry it
acknowledged. The whole shutdown has a deadline of
twenty seconds from the first signal. If it has not finished by then, the
process exits without recording a clean shutdown, and the next start treats the
run as a crash. A second signal also exits at once without recording a clean
shutdown. The chart gives voter pods thirty seconds by default.
This reduces planned-stop election pauses. It cannot hand off leadership on
SIGKILL or OOM.
`runtime.node_memory_abort_cap_size` is an emergency process-abort threshold, not HTTP admission control. Exceeding it calls `abort()` without graceful leadership handoff, like an OOM kill. It does not return `503`. Leave room below the container memory limit for normal operation and recovery, and use write admission and backpressure to manage load.
### Restarting a voter
Restart one voter at a time:
1. `ursulactl drain --node N` moves every leadership away and keeps the node from taking one back.
2. Restart the process through the platform (Helm, systemd). The graceful shutdown above keeps every journal intact.
3. `ursulactl wait --node N` returns once the node is a voter of every group again and has caught up.
4. `ursulactl undrain --node N` lets it hold leaderships again.
Do not stop another voter before `wait` returns. Two overlapping outages of a three-voter group leave it without a quorum even if the restarts were issued separately. The chart's graceful-rollout hook runs the same sequence for every pod.
### WAL routing configuration
A WAL root is bound to `runtime.core_count` and `raft.group_count` in
`/raft-log/topology.bin`. Startup holds the WAL lock and checks both values before
recovering any group or writing the run state. A mismatch, missing record on
an existing WAL, or corrupt record refuses startup.
Keep both values explicit and unchanged when moving a persistent volume or
resizing its host. Without explicit values, `core_count` follows the process's
available parallelism and `group_count` defaults to that parallelism times 16
(resource presets set a fixed group count). Helm defaults to 4 and 64.
Changing cores changes the journal directory of some groups. Changing group
count changes stream routing. Neither is an online resharding operation.
For a mismatch, configure the stored counts again. The error reports them. Do
not manufacture a record from the new configuration or remove the check's
files. A replica whose record is missing or corrupt can be rebuilt on an empty
WAL directory from a healthy quorum, with the cluster's routing counts. Follow
the rejoin procedure below and wait for full catch-up before replacing another
replica. A single node has no quorum to rebuild from, so a fresh WAL does not
preserve its data.
### Rejoining a wiped node
A voter whose WAL directory was wiped or replaced, for example after a lost disk, starts with an empty log for every group. For each group:
- **It does not initialize the group again.** A per-group membership initializer (`raft.init_membership_per_group = true`, the chart default) runs `Initialize` only when every other configured voter answers that it holds nothing for the group either and no leader is seen. A fresh install still bootstraps this way. One voter holding entries or following a leader means the group exists, and the node waits to be replicated to. It waits for every voter, not a quorum, so a fresh install with a voter that never comes up stays uninitialized (it logs "waits for every voter before initializing").
- **It refuses replication until the leader proves a quorum without it.** The node lost its vote with its disk, so it cannot tell the current leader from a stale one. It refuses appends, heartbeats, snapshots and votes until the group's leader confirms a fresh ReadIndex barrier with a quorum of the other voters. The node then records that leader's vote as its vote floor and accepts replication. In a 3-voter group that quorum needs both other voters up. While the wiped voter waits and another voter is down, the group has no quorum and cannot commit writes.
- **It stays behind the recovery gate.** It does not campaign and refuses every vote once it knows the group holds entries, until it has applied a fresh barrier from the group's leader (see [Recovering after a host crash](#recovering-after-a-host-crash)).
- **The leader rebuilds it.** The leader sees the node answer that it lacks entries it had acknowledged: it commits a voter set without the node, adds it back as a learner, waits until it has replicated the commit index and promotes it. Each step is read off the current membership, so a leader change or another restart of the node converge. Leadership handoffs (the balancer, the commit-stall watchdog) skip the node until then.
While the other voters are up, no operator step is needed. `ursulactl wait --node N` returns once every group has promoted the node, and `ursulactl verify-cluster` shows it unready until then. Do not stop another voter before that.
If a majority of a group's voters lost their disks at once, the voters that kept their disks are not a quorum. No leader can prove a quorum to the wiped replicas, so they never obtain a vote floor, and `accept-unsynced-loss` cannot replace lost voting history. The group stays stopped. Restore the latest verified backup into a fresh cluster (see [Backup and disaster recovery](#backup-and-disaster-recovery)). If every voter lost its disk, the bootstrap probe sees only empty voters and initializes the groups again, empty: every acknowledged write is lost, and cold objects written before stay in object storage, unreferenced. Start such a cluster over on a new object-storage prefix, as for any fresh install.
### Recovering after a host crash
With the default `raft.wal.fsync = "never"`, a node acknowledges ordinary data appends from the page cache. Membership batches are always fsynced before acknowledgment, so a full-cluster power loss cannot erase the voter configuration. A process crash loses nothing. A host crash or power loss, or a journal I/O failure, can cost the node the unsynced tail of its journals. The node detects it on the next start from its run state (`wal_recovery` in `GET /__ursula/metrics`, the `ursula.wal.recovering` gauge) and keeps the frames that still verify. A node whose run state file is missing while its journals hold records is treated the same way. Every group the node held enters the recovery gate, which is recorded in the node's metadata and survives further restarts:
- The node does not campaign, takes no leadership transfer and grants no vote. A node that led the group starts as a follower. It still accepts appends from a leader of its current term or a later one.
- It asks the group's leader for a fresh ReadIndex barrier, applies the log up to the barrier's index and then opens the gate. Until then readiness answers `503` with reason `recovery_gate_closed`, and `recovery_gates` in `GET /__ursula/metrics` lists the gated groups.
- The leader sees that the node lost entries it had acknowledged and rebuilds it: it removes the voter, adds it back as a learner and promotes it once it caught up. The removal needs a quorum of voters that kept their log and acknowledged the leader within the last 1.5 seconds (the minimum election timeout), counting the leader. Without such a quorum no removal can commit, so the leader resends its own log to the voters that lost entries instead.
Leadership handoffs (draining, balancing, a node that sheds its leaderships, the commit-stall watchdog, the admin transfer endpoint) go only to a voter that kept its log, acknowledged the leader within the last 1.5 seconds and holds every committed entry. A handoff whose target has not taken over within 6 seconds (two maximum election timeouts) is abandoned: the leader steps down and campaigns again.
While at most a minority of a group's voters lost their tail, the other voters hold every acknowledged write and nothing is lost. Wait for every group to open the gate on a node before you stop another voter.
**A majority of a group's voters lost their tail at once.** If the group's leader kept its log, it stays leader and its barrier opens the gated replicas as above. Otherwise the gated voters refuse every vote, so the group elects no leader and writes answer `503`. After `30s` without a leader barrier and without applying anything, each gated replica reports its group stalled: readiness reason `recovery_stalled`, the group under `recovery_stalled_groups` in `GET /__ursula/ready` and under `recovery_gates.stalled` in the metrics, the `ursula.raft.recovery_gates{status="stalled"}` gauge, and an error log naming the group. Writes acknowledged after the last `fsync` of the surviving replicas may be gone. Only an operator can accept that. For each stalled group:
1. On every voter, read the group's gate under `recovery_gates.gated` in `GET /__ursula/metrics` (a group missing there is open), and its `last_log_term`, `last_log_index` and `current_term` under `raft_groups`. The call below succeeds only on a replica that reports the group `stalled`.
2. Rank the stalled replicas by last log id, as a Raft election does: highest `last_log_term` first, then highest `last_log_index`. An empty log (`null`) ranks last. On the first, run the call below through its admin listener. Its body names the `last_log_index` and `current_term` you read on that replica. Repeat down the ranking until the replicas whose gate is open form a majority of the voters. A replica that was not gated counts as open.
3. The group elects a leader among the open replicas, the one with the most up-to-date log, and the remaining gated replicas catch up from it and open their gates.
Stop once the open replicas form a majority, even if the election takes a while. A replica that was not gated may hold the longest log, and then only it can win. Accepting the loss on one more gated replica can let two replicas with shorter logs elect each other and discard the writes only that replica holds.
A voter that was never gated does not report `recovery_stalled`. Its readiness answers `503` with `raft_replica_unready`, and `raft_maintenance.group_issues` shows `leader_unknown` for the group.
Replicas with the same last log id (`last_log_term` and `last_log_index`) hold identical logs, so either one may go first. Raft's log matching property guarantees it: two logs that hold an entry with the same term and index are identical up to that entry.
```bash
# group 42 on this node
metrics=$(curl --fail http://127.0.0.1:4438/__ursula/metrics)
process_incarnation=$(jq -er .process_incarnation <<<"${metrics}")
expected=$(jq -ec '.raft_groups[] | select(.raft_group_id == 42)
| {expected_last_log_index: .last_log_index, expected_current_term: .current_term}' <<<"${metrics}")
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -H "content-type: application/json" \
-d "${expected}" -X POST http://127.0.0.1:4438/__ursula/raft/42/recovery/accept-unsynced-loss
```
It answers `{"raft_group_id": 42, "node_id": 1, "outcome": "gate_opened", "last_log_index": 1234, "current_term": 7}`, or `"already_open"` when the gate was open. The open gate is recorded, so a restart does not close it again. A replica that led the group before the crash rejoins as a follower, also when it restarts after the call.
The call is refused with `409 Conflict`, and changes nothing, when:
- the replica lost its vote and has not established a durable vote floor. Restore connectivity to an intact quorum so it can obtain a fresh barrier. Accepting unsynced data loss cannot authorize a replica to acknowledge an unknown leader.
- the replica's gate is closed but not stalled. It may still open through a leader's barrier without losing anything. Wait for `recovery_stalled`.
- the replica no longer holds the log the body names, because it moved since you read the metrics. Read the metrics again and decide again.
**Many stalled groups at once.** A rack power loss can stall many groups together. The script below runs the steps above for every stalled group from one machine, through a forward to each voter's admin plane (see [ursulactl's operation providers](/docs/0.7/cli#the-admin-plane)). It skips a group while one of its voters does not answer or one of its gated replicas is not stalled yet. Review `plan.jsonl` before the last step, and run the script again until no group is stalled.
```bash
# One admin URL per voter, through your forwards.
admins=(http://127.0.0.1:5441 http://127.0.0.1:5442 http://127.0.0.1:5443)
# 1. Observe every replica of every group once.
for admin in "${admins[@]}"; do
curl -sS --fail "${admin}/__ursula/metrics" | jq -c --arg admin "${admin}" '
.process_incarnation as $incarnation | (.recovery_gates.gated // {}) as $gated
| .raft_groups[]
| {admin: $admin, process_incarnation: $incarnation, raft_group_id, node_id,
voters: (.voter_ids | length), gate: ($gated[.raft_group_id | tostring] // "open"),
last_log_term, expected_last_log_index: .last_log_index,
expected_current_term: .current_term}'
done > replicas.jsonl
# 2. For each group with a stalled replica, plan the stalled replicas to open,
# highest last log id first (last_log_term, then last_log_index), until the open
# replicas form a majority.
jq -sc 'group_by(.raft_group_id)[]
| select(length == .[0].voters and any(.gate == "stalled")
and all(.gate == "open" or .gate == "stalled"))
| ((.[0].voters / 2 | floor) + 1 - (map(select(.gate == "open")) | length)) as $needed
| map(select(.gate == "stalled"))
| sort_by([.last_log_term // -1, .expected_last_log_index // -1]) | reverse
| .[:([$needed, 0] | max)][]' replicas.jsonl > plan.jsonl
# 3. Accept the loss on each planned replica. Stop at the first refusal and observe again.
while read -r replica; do
curl -sS --fail -w '\n' -H "x-ursula-process-incarnation: $(jq -r .process_incarnation <<<"${replica}")" \
-H "content-type: application/json" \
-d "$(jq -c '{expected_last_log_index, expected_current_term}' <<<"${replica}")" \
-X POST "$(jq -r '"\(.admin)/__ursula/raft/\(.raft_group_id)/recovery/accept-unsynced-loss"' <<<"${replica}")" || break
done < plan.jsonl
```
With `fsync = "always"`, acknowledged appends survive a host crash, but the newest journal segment can still hold bytes written after the last `fsync`: an unacknowledged batch, or a commit or truncate marker. Recovery refuses damage in sealed segments. In the newest segment it removes an incomplete final frame without gating anything. A complete frame that fails verification, such as a hole, a zero-filled tail or a torn sector inside an unacknowledged batch, looks the same as media corruption of an acknowledged write. Recovery therefore records the recovery gate of every group on that core before it cuts the frame, and those groups rejoin as described above.
A single-voter group has no other replica to recover from, so it needs this call after every host crash under `never`, once its gate reports the group stalled. Under `always` it needs the call only after a host crash that leaves a complete invalid frame in the newest segment. So does a group whose voters lose power together, for example in a rack power loss, when that gates a majority of them. No acknowledged write is lost in these cases, but only an operator can open the gates.
`POST /__ursula/cluster-probe` also exists on the client plane, but it is machinery rather than an operator surface: nodes send each other heartbeat-sized bodies through it to verify cluster-plane egress, and it bypasses ingress admission.
## Memory without retention
Per-stream replicated state, on every replica, is at most about 32 KiB plus bounded producer state, beyond the unflushed payload itself, without retention. It grows neither with the number of messages nor with the length of the cold history. The bounds that make this hold are always on: a 1,024-receipt producer receipt window per stream with 7-day idle-producer expiry and a 4,096-producer cap, unique stream incarnations with incarnation-scoped cold objects, external payload locators committed first and indexed after, and a compact hot window with no per-message bookkeeping, so cold admission and the flush thresholds charge hot payload only. Snapshot bodies of at least `runtime.external_payload_min_size` go to the cold tier when a cold backend is configured (up to 1 GiB). Retention stays available as a storage-cost tool, but it is not the way to bound memory.
The per-group gauges under `group_state_gauges` in `/__ursula/metrics` show what each group holds.
## Usage export
`GET /__ursula/usage` returns committed counters and current gauges from local replicated state, aggregated by bucket across this node's Raft groups:
```json
{
"version": 1,
"write_unit_bytes": 10240,
"buckets": {
"tenant-a": {
"committed_append_bytes": 24576,
"committed_records": 3,
"committed_write_units": 4,
"retained_bytes": 24576,
"stream_count": 1
}
}
}
```
The raw byte and record counters are pricing-neutral facts. `committed_write_units` is a convenience derived meter: every committed, non-deduplicated create or append contributes at least one unit and rounds its canonical payload up independently by `write_unit_bytes`. Consumers must validate both `version` and `write_unit_bytes` before interpreting it; the unit is data in the contract, not encoded into the counter name. The same unit is persisted in Raft snapshots, and a build refuses to restore a snapshot carrying a different unit. Changing it therefore requires a new contract version and an explicit state migration, never a configuration edit. Ursula does not attach currency, a price, or an account owner.
Counters are monotonic and survive snapshots, stream deletion, and bucket purge. `retained_bytes` and `stream_count` are current-state gauges and may decrease. A node serves local applied state, so replicated consumers should poll every voter and use their own completeness policy rather than treating one response as a cluster-wide linearizable read.
## Tenant offboarding (bucket purge)
`DELETE /__ursula/purge/{bucket}` erases one tenant's complete bucket domain: every stream in every Raft group, the bucket, cold chunks and indexes, external payloads, and any uncommitted object-store staging orphan below `{bucket}/`. The replicated purge also installs a permanent bucket-erasure fence in every group. It retains only the bucket's aggregate usage counters, because deleting a counter before an asynchronous meter observes it would erase committed billable work. Purge is served only on the admin listener (`server.admin_listen`, loopback `127.0.0.1:4438` by default); the client listener answers `404`. On Kubernetes the chart keeps the admin listener on pod loopback, so reach it through `kubectl port-forward pod/ 4438:4438` or `kubectl exec`; any one voter accepts the request for the whole cluster.
The response is a completion report an operator can attest to:
```bash
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X DELETE http://127.0.0.1:4438/__ursula/purge/tenant-a
# {"bucket":"tenant-a","removed_streams":12,"groups_with_streams":[0,3,5],"cold_gc_entries_reclaimed":12,"cold_gc_pending_entries":0,"cold_gc_complete":true,"cold_gc_error":null,"bucket_prefix_absent":true}
```
Semantics:
- **Retry on `503`.** A Raft group whose leader is unknown or changing answers `503` with `Retry-After`, never a redirect to a peer. Because purge is idempotent, retry the same request.
- **Idempotent and resumable.** Re-purging an erased bucket returns the same report shape with zero removals. A purge interrupted mid-way converges on re-run: the permanent fence and stream removal are replicated commands per group, while cold reclamation and the complete bucket-prefix erasure are repeatable object-store operations.
- **Permanently fenced.** Commands linearized after purge cannot create the bucket or append to its streams. The fence survives Raft snapshots and restore; data-plane access continues to conceal the erased namespace as `404 Not Found` instead of recreating it.
- **Isolated.** Other buckets' streams, offsets, checksums, snapshots, and object-store prefixes are untouched. Every cold object, shared packs included, lives under its own bucket's `{bucket}/` prefix.
- **Accounting residue.** The retained entry contains only aggregate counters and zero-valued content gauges, not stream data. Ursula does not currently provide an acknowledgement protocol for erasing it after an external meter persists the final delta. Use opaque bucket IDs if retaining the identifier itself would violate an erasure requirement.
- **Physical absence proof.** `cold_gc_entries_reclaimed` is diagnostic only. After a second replicated purge proves every group's bucket-specific GC queue is empty, Ursula recursively deletes the complete `{bucket}/` erasure domain and lists the authoritative object store to prove it contains no files. Only `cold_gc_complete=true`, `cold_gc_pending_entries=0`, `bucket_prefix_absent=true`, and no error constitute the proof. If object storage is unavailable or still lists a file, the response remains incomplete and a later idempotent request retries the proof.
- **Backups.** A purged tenant may persist inside backups taken before the purge until those backups age out; restore replays whatever the manifest contains. Deleting old backup sets is part of completing an offboarding with erasure obligations.
## Cleaning S3
Cold data lives under the `storage.cold.root` prefix. Benchmark and test runs should use a date-stamped root so cleanup is one prefix delete with standard S3 tooling:
```bash
aws s3 rm "s3://my-ursula-bucket/ursula-test-20260518T000000Z" --recursive
```
There is no automatic retention policy. Application checkpoints and explicit stream-retention advances control which cold objects become eligible for GC. After a retention advance, each group's leader deletes the stream's cold objects that lie wholly below the retained offset (chunks and offloaded external payloads, in whole 64 MiB cold-index pages, so up to one page just below the retained offset can remain until retention moves past it) once a 5-minute grace has passed, so reads planned before the advance still complete; its cold-index cursor visits 16 streams per group per minute, so on groups with many streams collection can lag further. The grace restarts after a leader change. Retention is a storage-cost tool only: replica memory does not depend on it (see [Memory without retention](#memory-without-retention)).
## Backup and disaster recovery
`ursulactl` provides a verifiable whole-cluster backup and restore workflow:
```bash
# Create: one MessagePack snapshot object per raft group + manifest.json,
# into a local directory or s3://bucket/prefix.
ursulactl backup-create --config cluster.yaml --location s3://backups/ursula/2026-07-25
# Verify: manifest completeness, byte sizes, BLAKE3 checksums, and deep
# state-machine validation of every snapshot. Touches no cluster.
ursulactl backup-verify --location s3://backups/ursula/2026-07-25
# Restore: into a FRESH, EMPTY cluster with the same raft group count.
ursulactl restore --config new-cluster.yaml --location s3://backups/ursula/2026-07-25
```
An `s3://` location uses the AWS SDK credential chain and `AWS_REGION`. For MinIO or another S3-compatible store, also set `AWS_ENDPOINT_URL`, for example `AWS_ENDPOINT_URL=http://minio.internal:9000`.
The recovery contract:
- **Consistency boundary.** Each group export is the deterministic state-machine snapshot the raft snapshot path persists: internally consistent per group while writes continue. Cross-group consistency is not promised. The boundary is per stream. Acknowledged writes in the exporting replica's applied state are included whether or not they were cold-flushed, so **RPO is bounded by export time**, not by the cold-flush interval. `backup-create` asks every node and keeps the reply with the highest group commit index.
- **Fresh identity on restore.** Restore replays each snapshot as one replicated write on the target cluster, which keeps its own raft membership and log identity. Nothing from the source cluster's raft metadata is reused. Groups that already hold buckets refuse the import with `409`, and group-count mismatches fail closed before the first import. Group commit indexes and per-stream append counters restart under the new identity. Stream bytes, offsets, close/TTL state, published snapshots, retention floors, and producer dedup states are preserved exactly.
- **Cold objects are part of the backup set.** Snapshots reference cold-tier chunks, cold-index pages and external payloads by object key. The backup holds the references, not the objects. The restored cluster's cold store must hold the same objects under the same keys: point it at a copy (see [Copying cold objects](#copying-cold-objects)). `backup-verify` validates snapshots but does not dereference cold objects. `restore` asks the target cluster, before the first import, to check every group's references against its own cold store (`POST /__ursula/backup/group/{group}/cold-check`). If any object is missing, it stops with nothing imported and prints how many are missing and the first keys.
- **Format compatibility.** The backup format is the format epoch: `format_version` 3 in the manifest and in `/__ursula/backup/info`, and `format_epoch` 3 in every group object. A backup restores only into a cluster of the format epoch that wrote it. `ursulactl` refuses a manifest or a target cluster of another version before the first import, and a server refuses to import a group of another format epoch (`400`).
- **RTO** is dominated by transferring and importing the group snapshots. Each import is a single replicated write per group.
The recovery drill runs in CI (`backup_restore_drill_preserves_streams_and_allows_continued_appends`): write, snapshot, retain and close, back up, destroy the cluster, restore, verify checksums, offsets and boundaries, and continue appending.
What you can additionally rely on:
- **Quorum durability.** Acknowledged writes survive as long as a majority of voters survives. Three voters across AZs tolerate any single-AZ outage.
- **Cold-tier durability.** Once flushed to S3, a chunk inherits S3-grade durability. The unflushed window is bounded by the flush interval (seconds by default).
- **Format epoch 3, no migration.** The Raft WAL, snapshots, backups, the
object-storage prefix and the Raft gRPC protocol carry format epoch 3. A node
refuses data, backups and peers of another format epoch and does not migrate
them.
Node-level loss: replace the host with the same `node_id` and cluster config. Each group's leader rebuilds the replacement from the other voters (see [Rejoining a wiped node](#rejoining-a-wiped-node)). `ursulactl wait --node N` returns once the replacement is a voter of every group and has caught up, and `ursulactl verify-cluster` then requires every configured voter to be present and caught up. `ursulactl wait-ready` checks the whole cluster: leaders for every group, complete voter sets and `200` from every node's `/__ursula/ready`. Run it before declaring the recovery done. Total-cluster loss: restore the latest verified backup into a fresh cluster as above.
### Copying cold objects
A restored cluster reads cold data under the keys the source cluster wrote, relative to `storage.cold.root`. Copy the source's objects into the target's root after the target has started and written its `URSULA_FORMAT_EPOCH` marker, and before `restore`. Skip two things:
- `URSULA_FORMAT_EPOCH`, the marker. The target's own marker stays.
- `snapshots/`, the source's Raft snapshots and their references. The target writes its own there. With a custom `storage.snapshot.s3_prefix` inside the cold root, skip that prefix instead.
With the AWS CLI, from the source root to the target root:
```bash
aws s3 sync s3://my-bucket/source/cold/ s3://my-bucket/target/cold/ \
--exclude 'URSULA_FORMAT_EPOCH' --exclude 'snapshots/*'
```
For MinIO or another S3-compatible store, add `--endpoint-url`. The copy must match the backup, so the source must not change its cold objects in between. Stopping writes is not enough: an idle cluster still compacts small cold chunks and deletes the ones it replaced a few minutes later. If the source cluster still runs, stop its nodes once the backup verifies, then copy. `restore` checks that the copy is complete.
Never point two clusters at one prefix, and never point a new cluster at the source's prefix in place. After a restore both clusters hold the same streams under the same keys, but each one deletes cold objects by its own state alone: the cold GC after a delete, a retention advance or a compaction, the orphan sweep for unreferenced objects older than a day, and the pruning of `snapshots/` by the cluster's own references. Two clusters on one prefix delete objects the other still reads. A copy also leaves the source cluster and its prefix as they were, so the source stays a way back until clients switch.
## Upgrading to 0.7
**0.7.0 is a breaking release.** It changes the Raft WAL format (format epoch 3) and removes the memory WAL, so the per-core disk journal is the only Raft WAL. 0.7 cannot read 0.6 data: a node refuses 0.6 Raft WAL directories and object-storage prefixes at startup, `ursulactl restore` refuses 0.6 backups, and a node will not join a cluster with 0.6 nodes. There is no in-place upgrade, no automatic migration and no rollback.
- **If you run 0.6 with data you need to keep, stay on 0.6.2.** 0.7 provides no migration path from 0.6.
- **Deploy 0.7 fresh:** empty PVCs and a never-used object-storage prefix.
- **`helm upgrade` from 0.6 stops before a second voter is replaced.** Recover with `helm rollback`.
Every behavior change is listed in the [0.7.0 release notes](/docs/0.7/release-notes#070).
0.7.6 upgrades 0.7.0, 0.7.1, 0.7.2, 0.7.3, 0.7.4 or 0.7.5 in place with an ordinary rolling restart (`helm upgrade`), with no fresh install. See the [0.7.6 release notes](/docs/0.7/release-notes#076).
For a patch upgrade, upgrade gateways before nodes, and roll back nodes before gateways, so that clients keep the benefit of the `x-ursula-not-applied` marker throughout. The order is a recommendation, not a safety requirement. A client trusts the marker only together with the vouch that gateways from 0.7.3 on add, so whatever order a `helm upgrade` or `helm rollback` replaces gateways and voters in, no client counts a committed write as never applied. See [refusals that applied nothing](/docs/0.7/api/overview#refusals-that-applied-nothing).
To install 0.7:
1. **Install fresh.** Install with a new `s3.prefix` and empty PVCs. With cold storage off and S3 snapshots, also use a new `snapshotStore.prefix`. With the event-time indexer, use a new or emptied `indexer.s3.prefix` and register the indexes again.
2. **Never reuse a prefix** that an earlier release, or a failed upgrade attempt, wrote to.
3. **Update the configuration.**
- Remove `raft.wal.backend`, `raft.wal.allow_volatile_multi_peer`, `raft.memory_bootstrap_marker_dir` and `raft.rejoin_probe`. A config file that still sets one fails to load.
- Set `raft.wal.path` on every node of a multi-node cluster. 0.6 defaulted to the memory WAL, and a 0.7 node with `raft.peers` refuses to start without a WAL path.
- Choose `raft.wal.fsync` (Helm `raft.walFsync`). The default `never` acknowledges a write once a quorum of voters holds it in the page cache, and a host crash can cost a replica its unsynced tail (see [Recovering after a host crash](#recovering-after-a-host-crash)). Set `always` to keep the 0.6 disk WAL behavior of one `fsync` per batch.
- With Helm, remove `raft.storageMode`, `raft.allowVolatileMultiPeer` and `persistence.enabled`. Every pod gets a PVC for its WAL. The values schema refuses unknown keys under `raft` and `persistence`.
- Keep `runtime.core_count` and `raft.group_count` explicit and unchanged (see [WAL routing configuration](#wal-routing-configuration)).
4. **Update runbooks and tooling.** `ursulactl` 0.7 removes the memory-WAL restart verbs `prepare-restart`, `restart-quiesce-capability`, `finish-prepared-restart`, `abort-prepared-restart`, `prepare-amnesiac-restart`, `repair-restarted-voter`, `prepare-recovery-handoff` and `classify-amnesiac`. A planned restart is drain, restart, wait and undrain ([Restarting a voter](#restarting-a-voter)), and a voter that lost entries is rebuilt by its group leaders. Custom tooling that calls admin mutations must send `x-ursula-process-incarnation` ([Admin endpoints](#admin-endpoints)).
A values file that still sets a removed chart value fails validation, and nothing changes. Otherwise `helm upgrade` from 0.6 replaces the first pod, with the default `RollingUpdate` strategy and also with `OnDelete` plus the graceful rollout. That pod finds the 0.6 data on its volume and exits at startup, so no second voter is replaced. Recover with `helm rollback`. With `OnDelete`, also delete the refused pod after the rollback so that it starts from the 0.6 template again. The refused pod wrote nothing, so the old pods are unaffected.
How a 0.7 node refuses data of another release, and what it prints, is listed under [Startup failures](/docs/0.7/troubleshooting#startup-failures). Every startup refusal happens before the node writes anything:
- **Raft WAL directory.** `{raft.wal.path}/raft-log/FORMAT_EPOCH` records the epoch. A directory that holds data without the marker, or a marker of another epoch, is refused. The marker is written after every check passes.
- **Object storage.** `URSULA_FORMAT_EPOCH` sits at `storage.cold.root` when the cold backend is S3, otherwise at the S3 snapshot namespace (`storage.snapshot.s3_prefix`) when snapshots live on S3. A namespace that holds objects without the marker is refused, so it must be used only by this cluster. With a memory cold store and inline snapshots there is no object-storage check.
- **Peers.** Before it writes the markers, a node with configured peers asks each one, in parallel (2 seconds to connect plus 2 seconds for the answer, about 4 seconds at most), whether it speaks Raft protocol 3. A peer that refuses ends startup. An unreachable peer is skipped, because a fresh cluster starts its pods in parallel.
- **Readiness.** Any later Raft protocol mismatch, inbound or outbound, is logged at error level and makes `/__ursula/ready` answer `503` with `"reason": "format_epoch_mismatch"` until the node restarts. This also holds for a 0.6 node that a 0.7 node contacts, including through the startup probe. Keep the peer lists of 0.6 and 0.7 nodes apart.
- **Backups.** Backups use format version 3. `ursulactl restore` and `backup-create` refuse a cluster that reports another version, and a server refuses to import a group of another format epoch (`400`).
Bucket IDs `snapshots` and any ID starting with `__` are reserved (`400`): `snapshots` is the default raft-snapshot namespace in the object store.
## Logs
`RUST_LOG=ursula=info,ursula_runtime=info,ursula_raft=info` is the baseline. Bump to `debug` for one crate when chasing a subsystem:
```bash
RUST_LOG=ursula_raft=debug ./target/release/ursula server ...
```
`debug` is verbose under sustained load, so redirect to a file.
## Committed application failures
A stopped group requires [corrected-code replay](/docs/0.7/apply-failure-recovery).
Retain the original WAL and referenced snapshot objects. Never skip a committed
record to recover readiness.
---
# Recovering stopped Raft groups
A panic or invariant violation while applying a committed command stops that
local Raft group. Other groups remain running. Isolation relies on Rust panic
unwinding, so a process abort, OOM kill or machine failure is outside it. The
failed command receives no successful application response, and the applied
index does not advance past it. A normal stream rejection (for example,
deleting a missing stream) remains a deterministic application result and does
not stop the group.
`/__ursula/metrics` reports `raft_groups[].apply_failure` with the log term,
index, failure kind and diagnostic message. The affected group's
`maintenance.running` is false. The maintenance report names the group with
`apply_stopped`, so `ursulactl wait-ready` and rollouts refuse another
disruption. The node's `/__ursula/ready` stays 200, its other groups keep
serving, and planned leadership handoffs of those groups continue. Requests to
the stopped group answer a typed `503`. Invariant violations can have partially
modified memory: that state must not be read, flushed, or made into a snapshot.
A previously captured snapshot only contains its earlier successful prefix.
## Supported recovery: corrected-code replay
This procedure requires the original durable WAL and snapshot/cold objects.
It does not repair physical corruption or recover data lost beyond the chosen
WAL fsync policy. Do not truncate a committed record, edit its bytes, reset its
applied index, copy a snapshot made from failed memory, or use an unsynced-loss
operation to bypass a poison record.
1. Record the group, failing log term/index, binary revision and diagnostics on
every replica. Stop writes to affected streams. A failed request may already
be committed, so retain its producer sequence and idempotency information.
2. Preserve the WAL directories, snapshot pointers and referenced cold objects.
Have a corrected binary ready before stopping the affected server to take
a consistent filesystem backup. Keep node IDs, membership and storage paths
unchanged. Do not delete the failed group's data to make the pod healthy. A
restart with the faulty binary keeps the stopped group stopped and the other
groups serving, so it is safe but does not repair anything.
3. Correct the deterministic application bug, keeping command encoding and
semantics compatible with the retained history. Reproduce the failing
record on an isolated copy. Verify the last acknowledged records and the
failing command are replayed exactly once into fresh in-memory state. A
deployment of the same faulty code is expected to stop at the same record.
4. Deploy the corrected binary to the affected replicas using their original
volumes. A group with a live quorum can resume by normal replay and
catch-up. If every replica stopped, bring back enough corrected replicas for
quorum.
Never create a replacement cluster or silently skip a committed command.
5. Before restoring traffic, require no `apply_failure`, running/recovery-ready
group metrics, a current leader and applied progress through the recorded
failing index. Read and compare previously acknowledged payloads and the
failed request using its original identity before deciding whether to retry.
If the failure occurs during initial WAL replay, the owner retains an
`ApplyStopped` terminal group instead of a live Raft handle. Other groups can
start and serve. Metrics retain the failing committed term and index and the
last successfully applied prefix. Maintenance readiness refuses disruption
while the node stays ready. Requests to the stopped group return
`ApplyStopped` without repeatedly opening its WAL. A corrected compatible binary must restart and replay the original
history before that group can serve again. Ordinary configuration, WAL-open
and snapshot-restore errors still fail startup. They are not application
failures and are not silently converted to isolated groups.
There is no online skip/quarantine command in this recovery contract. Repair
requires a corrected compatible binary. If none is available, keep the group
stopped and preserve its data. Rolling a snapshot or command format to an
incompatible binary is not a recovery method.
## Local drill
Run `cargo test -p ursula-raft --lib apply_failure_tests`. It uses real disk WALs
with `fsync=always`, a partial-mutation panic and an invariant violation,
and a three-replica in-process Raft network. It verifies stopped-group
isolation, unchanged applied boundary, retained records, same-code replay
failure with healthy-group reads and writes on the same owner thread, and
payload preservation after corrected-code replay. The three-replica drill waits
for a surviving follower to elect a new leader and encounter the committed
poison through normal Raft replication.
The deterministic simulation `a_poison_record_isolates_its_group_until_corrected_replay`
(`RUSTFLAGS="--cfg madsim" cargo test -p ursula-sim --lib disk_tests`) runs the
same poison on every voter of one group while another group shares each node's
core journal. It restarts a node with the faulty code and then replays every
journal with corrected code. These are application-failure drills, not
power-loss or Kubernetes Chaos qualifications.
## WAL retention cost
A stopped group is excluded from snapshot selection and the lagging-snapshot
queue. Reclaim moves each retained payload once into an immutable `frozen-*.seg`
file beside the journal segments, using the existing journal encoding and disk
interface. The shared journal keeps checksummed references. Later reclaim passes
copy only those references, so the stopped group does not hold healthy groups'
segments or repeatedly copy its payloads.
The archive is synced and its directory entry made durable before the journal
reference is synced. Only then can reclaim discard the original records.
Recovery verifies every referenced archive and the exact retained log IDs.
Missing or damaged archives fail startup. Segment continuity checks still apply.
Preserve the entire WAL directory, including the frozen files, during repair.
`wal_stopped_rewritten_bytes` counts payload bytes moved into archives.
`wal_frozen_reference_rewritten_bytes` counts later reference copying.
`wal_physical_bytes` includes retained archive files. Alert on
`ursula.raft.apply_stopped` and monitor retained disk space while repairing the
binary. The payload counter should stop growing once that group's retained
records have been archived, even while healthy groups keep writing.
Corrected code replays the same committed entries through the references.
Archives are collected after no remaining physical journal record references
them and the old segments' deletion is durable. Cleanup failures are retried
without blocking healthy journal reclamation. During an unverified-tail repair run,
cleanup waits until the affected segment has been durably reclaimed. A later
clean startup validates the durably repaired journal before collecting orphans.
A binary that predates frozen references rejects them on replay, including in
the newest segment. Once references have been written, repair with compatible
code rather than downgrading or deleting the archives.
---
# Observability
Ursula emits OpenTelemetry traces and metrics over OTLP, and keeps a built-in
JSON metrics endpoint for quick inspection. Telemetry export is **off by
default**: with no collector configured the server runs exactly as before and
the hot read/write path pays nothing.
## Enabling OTLP export
Set the standard OpenTelemetry OTLP exporter settings before starting `ursula`:
```bash
# Send traces and metrics to a collector (Tempo, Jaeger, the OTel Collector, …).
export OTEL_EXPORTER_OTLP_ENDPOINT="http://otel-collector:4318"
# Optional: head-sampling ratio for new root traces (0.0–1.0, default 1.0).
export OTEL_TRACES_SAMPLER_ARG="0.1"
cat > /tmp/ursula-otel.toml <<'TOML'
[server]
listen = "0.0.0.0:8080"
TOML
ursula --preset default --config /tmp/ursula-otel.toml
```
The exporter uses OTLP over HTTP/protobuf (port `4318`), reusing the server's
HTTP stack rather than opening a second gRPC tree alongside Raft. When the
endpoint is unset, the process stays on the stderr `fmt` logger driven by
`RUST_LOG` (default `info`).
Spans batch-export continuously and metrics export on a fixed interval
(`OTEL_METRIC_EXPORT_INTERVAL`, milliseconds). A final flush on clean shutdown
is only a backstop. Don't rely on it to observe data.
### Identifying the node
Every span and metric carries the process's OpenTelemetry **resource**. The
server sets `service.name=ursula` and, when running with a Raft node id,
`service.instance.id=` so a cluster's nodes are distinguishable. Add
more resource attributes (e.g. `host.name`, region, AZ) with the standard
variable. They stay bounded by cluster size, so this is a resource dimension,
not a high-cardinality label:
```bash
export OTEL_RESOURCE_ATTRIBUTES="host.name=$(hostname),deployment.environment=prod"
```
## Traces
A single request produces one boundary span (`http.append`, `http.read`, or
`http.head`) tagged with `bucket` and `stream`, never the payload. That span
is propagated across the internal actor mailboxes, so on-core work
(`core.append`, `core.read`, …) nests underneath it. When a follower forwards a
read to the leader, the W3C `traceparent` rides the Raft gRPC request, so the
follower and leader appear in the **same trace** end to end.
Trace context travels only in gRPC transport metadata. It is never written to
the Raft log, so state-machine determinism and replay are unaffected.
### Sampling and the hot path
Request-boundary spans (`http.*`) are `info` level, so production traces always
carry one span per request. The on-core spans below them (`core.append`,
`core.read`, …) are `debug` level: filtered out at the default `info` and
compiled out of release builds entirely (`release-max-info`, on by default), so
the hot read/write path pays nothing for them. Measured cost:
| Operation | Cost |
| ------------------------------------------- | ------- |
| `info` boundary span (export off, fmt only) | ~285 ns |
| `debug` span filtered at `info` | ~1 ns |
| cross-mailbox context capture | ~5 ns |
For high-throughput deployments, lower `OTEL_TRACES_SAMPLER_ARG` to bound
export volume.
### Seeing on-core detail
The `core.*` spans only materialize in a build without `release-max-info` (e.g.
a debug build), and only when their target is enabled. Use a **crate-scoped**
filter, never a global `debug`:
```bash
# Good: only Ursula's own crates emit debug spans.
export RUST_LOG="info,ursula_runtime=debug,ursula_raft=debug"
```
A global `RUST_LOG=debug` turns on debug spans/logs from OpenRaft, the
OpenTelemetry SDK, reqwest, and every other dependency, which floods the
collector and bloats the OTLP payload. Scope the filter to the `ursula_*`
crates instead.
> `tokio-console` needs tokio's `TRACE`-level instrumentation, which
> `release-max-info` compiles out. Build with
> `--no-default-features --features tokio-console` when using it.
## Metrics
Two surfaces expose the same counters:
- **OTLP**: when an endpoint is configured, runtime metrics are exported
periodically as OpenTelemetry instruments (`ursula.appends.accepted`,
`ursula.mutations.applied`, `ursula.raft_apply.ns`, `ursula.wal.records`,
`ursula.group_mailbox.depth`, …), ready for Prometheus/Grafana via a
collector.
- **JSON**: `GET /__ursula/metrics` returns the full per-core/per-group
snapshot for quick inspection and is what `ursulactl status` reads.
For redundancy alerts, inspect the JSON metric `raft_maintenance.group_issues`.
Alert on `incomplete_voter_set` or `joint_membership`, and treat a missing
maintenance report as unavailable evidence. Also alert on nonempty
`raft_maintenance.node_issues`. These fields remain strict when healthy
survivors return 200 from `/__ursula/ready` during a replica rebuild.
They are JSON report fields, not new OTLP instrument names.
```bash
curl --fail http://NODE:4437/__ursula/metrics | jq '
.raft_maintenance | {node_issues, group_issues}'
```
Metrics are gathered on the hot path with lock-free per-core counters. The OTLP
layer is an *export-time bridge*: it reads a snapshot at the collection
interval rather than calling the metrics SDK per record, so enabling export
adds nothing to the append/read path.
For WAL operations, the JSON snapshot includes per core the journal's size
and segments (`wal_physical_bytes`, `wal_segments`), rotations
(`wal_rotations`), segments deleted and their bytes (`wal_reclaims`,
`wal_reclaimed_bytes`), the time reclaim took and the passes that stopped on an
error (`wal_reclaim_ns`, `wal_reclaim_failures`), live entry bytes copied out of
old segments (`wal_rewritten_bytes`), and the groups reported lagging to the
snapshot driver with the segments kept only for them (`wal_lagging_groups`,
`wal_pinned_segments`). Reads report entries served from the cache and read
from disk (`wal_cache_hits`, `wal_cache_misses`) and the frames and bytes read
(`wal_disk_reads`, `wal_disk_read_bytes`), per core and per group. Each group
reports its cached bytes and indexed entries (`wal_cache_bytes`,
`wal_indexed_entries`). Startup scan work is under `wal_recovery_ns`,
`wal_recovery_records`, `wal_recovery_bytes`, and `wal_recovery_live_entries`.
Free-space admission is reported separately as `wal_available_bytes`, its two
watermarks, `wal_disk_pressure`, and `wal_disk_stat_errors`. These are bounded
per-node, per-core and per-group series.
For the snapshot driver, the JSON snapshot includes the node's unsnapshotted
Raft log and the most it has held (`raft_log_bytes`,
`raft_log_bytes_high_watermark`), its pressure level against
`raft.snapshot_log_budget` (`raft_snapshot_pressure_level`: 0 below three
quarters of the budget, 1 up to the budget, 2 past it), the snapshot builds a
pass keeps in flight at that level (`raft_snapshot_pressure_builds`), and the
builds that run now and the most that ran at once
(`raft_snapshot_builds_in_flight`, `raft_snapshot_builds_in_flight_max`). Its
pressure passes that planned a build or changed the groups held out (a group
whose trigger went unclaimed is held out for 0.5 s, doubling for each such
trigger in a row up to 16 s, until a trigger of it is claimed) add up in
`raft_snapshot_pressure_passes`, the builds they started in
`raft_snapshot_pressure_groups`, and their time in
`raft_snapshot_pressure_pass_ns`.
The OTLP collection interval defaults to 60s. For a quick check, shorten it so
readings arrive without waiting (and without relying on the shutdown flush):
```bash
export OTEL_METRIC_EXPORT_INTERVAL=1000 # milliseconds
```
---
# Troubleshooting
Start at `/__ursula/metrics`. Most operational symptoms have a clear signal in the JSON snapshot.
## Diagnostic surface
```bash
curl http://NODE:4437/__ursula/metrics > metrics.json
```
Useful jq selectors. Arrays named `per_core_*` are indexed by core, and arrays named `per_group_*` by Raft group ID.
```bash
# Leader, term and applied index of each group. current_leader is null while the group has no leader.
jq '.raft_groups[] | {raft_group_id, current_leader, current_term, last_applied_index}' metrics.json
# Groups without a leader
jq '[.raft_groups[] | select(.current_leader == null) | .raft_group_id]' metrics.json
# Hot bytes waiting for cold flush, per group
jq '.per_group_cold_hot_bytes' metrics.json
# Writes refused by cold-write backpressure, in total and per group
jq '{total: .cold_backpressure_events, per_group: .per_group_cold_backpressure_events}' metrics.json
# Mailbox depth and capacity, per core
jq '{depths: .mailbox_depths, capacities: .mailbox_capacities}' metrics.json
# Live reads waiting for new data, per core
jq '.per_core_live_read_waiters' metrics.json
# SSE streams opened and SSE error events
jq '{opened: .sse_streams_opened, errors: .sse_error_events}' metrics.json
# WAL disk pressure
jq '{available: .wal_available_bytes, minimum: .wal_min_available_bytes, pressure: .wal_disk_pressure}' metrics.json
# WAL reclaim and restart work
jq '{physical: .wal_physical_bytes, segments: .wal_segments, reclaims: .wal_reclaims, lagging: .wal_lagging_groups, recovered_bytes: .wal_recovery_bytes, recovery_ns: .wal_recovery_ns}' metrics.json
# How the WAL opened, and the groups behind a recovery gate
jq '{opening: .wal_recovery, gates: .recovery_gates}' metrics.json
# Local Raft issues that make the node unready or block maintenance
jq '.raft_maintenance | {node_issues, group_issues}' metrics.json
```
`GET /__ursula/ready` is the process readiness probe. It answers `200` when the
node may serve, and otherwise `503` with a typed `reason`. When several apply,
it reports the first of:
| `reason` | Meaning |
| -------- | ------- |
| `format_epoch_mismatch` | The node exchanged a Raft RPC with a peer of another format epoch. It stays unready until it restarts |
| `wal_disk_pressure` | The WAL filesystem is below `raft.wal.min_available_size`, or its free-space check failed |
| `recovery_stalled` | A group's recovery gate on this node is stalled. The groups are listed under `recovery_stalled_groups` |
| `recovery_gate_closed` | A group's recovery gate on this node is closed until the replica has applied a fresh barrier from the group's leader |
| `raft_replica_unready` | A local replica is not a healthy voter. `raft_maintenance.group_issues` names the issue |
Ursula does **not** expose `/healthz`, `/readyz`, Prometheus `/metrics`, or
`/cluster/status`. Use the JSON snapshot for detailed diagnosis and
`ursulactl wait-ready` for cluster-wide Raft readiness.
### Serving during a replica rebuild
`raft_replica_unready` means local Raft participation is not safe for serving.
Inspect `raft_maintenance.group_issues` for `local_replica_not_voter`,
`membership_not_applied`, `not_applied`, `apply_lag` or `leader_unknown`. A
wiped replica stays 503 while its recovery gate is closed and while it is a
learner. Healthy surviving voters can return 200 during the repair. A voter
leaving a joint membership also stays unready once any configuration excludes
it. A replica whose own gate is open reports `leader_unknown` for a group that
has no leader, for example a group stalled because a majority of its voters
are gated.
A 200 response does not certify full redundancy. Keep maintenance blocked until
`ursulactl wait-ready` succeeds. Alert separately on incomplete voter sets in
the maintenance report, as described in [Observability](/docs/0.7/observability).
### `503` with `ApplyStopped` from one group
A committed command failed to apply and stopped that group on this replica.
The node stays ready and serves its other groups. Its maintenance report names
the group with `apply_stopped`, so `ursulactl wait-ready` refuses disruption
until the group recovers. Follow [Recovering stopped Raft
groups](/docs/0.7/apply-failure-recovery) and never skip the committed record.
## Startup failures
### WAL topology mismatch or missing topology
`topology.bin` binds a WAL root to `runtime.core_count` and `raft.group_count`.
For a mismatch, restore the stored counts reported in the error. A missing
record on an existing WAL or a corrupt record also refuses startup. Do not
replace it with the current configuration. See [WAL routing
configuration](/docs/0.7/operations#wal-routing-configuration) for rebuilding a
replica.
### `raft.wal.path is required when raft.peers is set`
Every node of a static cluster keeps its Raft WAL in `raft.wal.path`. Set it to a directory on a persistent volume. Only a single node without `raft.peers` may omit it, and then its WAL lives in a temporary directory that is removed on clean shutdown.
### `raft.node_id must be non-zero`
Static Raft mode requires a unique `raft.node_id` in the config file or a `--node-id` override.
### `raft.peers` must include this node id
The local `raft.node_id` must appear in `[[raft.peers]]`. The peer list always includes self.
### `storage.cold.s3.bucket is required when cold backend is 's3'`
Set `storage.cold.s3.bucket`, or set `storage.cold.backend = "none"` if you don't want cold storage.
### Port already in use
`server.listen` is taken. Public HTTP and inter-node gRPC share the same port unless `server.cluster_listen` is configured separately.
### `raft WAL directory … holds data without a format marker`
The data directory was written by Ursula 0.5.x or earlier (format epoch 1). This release reads format epoch 3 only and has no in-place upgrade. Start on an empty `raft.wal.path` and a new object-storage prefix, or run the release that wrote it.
### `raft WAL directory … is format epoch N`
The directory's `FORMAT_EPOCH` marker names another epoch. This release reads format epoch 3 only. Run the release that wrote it, or start on an empty directory.
### `object storage s3://… is not empty and has no URSULA_FORMAT_EPOCH marker`
The cold root (or, with cold storage off, the S3 snapshot namespace) holds objects from Ursula 0.5.x or from another application. Point `storage.cold.root` (Helm: `s3.prefix` or `coldStorage.prefix`), or `storage.snapshot.s3_prefix` when only snapshots use S3, at an empty prefix used only by this cluster.
### `object storage s3://… is format epoch N`
The prefix's `URSULA_FORMAT_EPOCH` marker names another epoch. Use a new prefix and never reuse one across epochs.
### `journal … uses Ursula WAL version N`
A Raft WAL journal of another format epoch: version 1 is Ursula 0.5.x, version 2 is 0.6. This release reads format epoch 3 only. The directory marker normally refuses the directory first, and the journal check is the backstop. Start on an empty `raft.wal.path`.
### `peer N (…) refused Raft protocol 3`
A configured peer runs a release of another format epoch (Ursula 0.6 speaks protocol 2, 0.5.x protocol 1). A node cannot join a cluster of another format epoch. Install this release as a new cluster.
### `/__ursula/ready` answers `503` with `format_epoch_mismatch`
The node exchanged a Raft RPC with a peer on another format epoch: a peer that was unreachable during the startup probe, or a node of another release that has this node in its peer list. The node stays not ready until it restarts. Remove the mismatched node from the peer lists and restart.
### `/__ursula/ready` answers `503` with `recovery_gate_closed`
The node's logs may be missing entries it acknowledged: it started after a host
crash or power loss under `raft.wal.fsync = "never"`, after a journal I/O error,
or without its run state file, or with an emptied WAL directory. Under
`raft.wal.fsync = "always"`, a host crash gates the groups of a core whose newest
journal segment ended in a complete invalid frame, although no acknowledged
write was lost. Each
gated group waits until it has applied a fresh barrier from the group's leader,
and its leader rebuilds the replica meanwhile. `recovery_gates` in
`/__ursula/metrics` lists the gated groups. This normally clears in seconds
once a leader is reachable. See
[Recovering after a host crash](/docs/0.7/operations#recovering-after-a-host-crash).
### `/__ursula/ready` answers `503` with `recovery_stalled`
A gated group on this node has seen no leader barrier and applied nothing for
`30s`. A majority of the group's voters are probably gated, so the group has no
leader and refuses writes. The groups are listed under
`recovery_stalled_groups`. Restore connectivity to a healthy quorum first.
If the stalled replicas retain their voting history, an operator can accept
the loss of writes acknowledged after the last `fsync`, with
`POST /__ursula/raft/{group}/recovery/accept-unsynced-loss` on the gated
replicas with the most up-to-date last log id (highest `last_log_term`, then
highest `last_log_index`), until the open replicas form a majority. A replica without a durable vote floor refuses this call and still
needs a fresh quorum proof. The request body names the replica's
`last_log_index` and `current_term` from `/__ursula/metrics`. A voter of the
group that was never gated does not report `recovery_stalled`. It answers
`raft_replica_unready` with `leader_unknown` for the group. See
[Recovering after a host crash](/docs/0.7/operations#recovering-after-a-host-crash).
### `accept-unsynced-loss` answers `409 Conflict`
Nothing changed. A replica that lost its voting history must first obtain a
durable vote floor from a fresh quorum proof, and the call cannot replace that
proof. Restore connectivity to intact voters.
The call also refuses when the replica's recovery gate is not
stalled, so it may still open through a leader's barrier without losing
anything, or the replica no longer holds the `last_log_index` and
`current_term` the request names. Wait for `recovery_stalled`, read
`/__ursula/metrics` on the replica again, and decide again with what it shows.
### I/O error on `raft.wal.path`
The directory must be writable and owned by exactly one Ursula process. Each
core journal holds an advisory `journal.lock` containing the owner PID. Do
not remove it while that process is alive; stop the conflicting process or fix
the volume mount. A stale lock file is harmless because ownership is enforced
by the OS lock, not the file's existence.
## Backup and restore
### `backup manifest format_version N` or `target cluster speaks backup format N`
The backup or the cluster is of another format epoch. This release reads and writes backup format 3 only, so a backup of another epoch cannot be restored, and a cluster of another epoch cannot be backed up or restored into.
### `… cold objects this backup references are missing from the target cluster's cold store`
The backup references cold objects that the target's `storage.cold.root` does not hold. `restore` stopped before the first import, so the target is still empty. Copy the source cluster's cold objects into the target's root (see [Copying cold objects](/docs/0.7/operations#copying-cold-objects)) and run `restore` again. The error names the first missing keys. A key under `cold-index/` is an index page, so everything that page lists is unchecked until the page is there.
## Bootstrap and cluster join
### A fresh cluster never elects leaders
`/__ursula/metrics` on each node should show a non-null `current_leader` and a stable `current_term` for every group in `raft_groups`. If groups stay leaderless:
- First start must include `raft.init_membership_per_group = true` (or `raft.init_membership = true`) on every voting node.
- Every peer URL must be reachable from every node.
### Replacing or restarting a node leaves it leaderless
Restart with the **same** `raft.node_id` and peer list. A fresh `raft.wal.path` means rehydrating from peers, which can be slow on cold-storage-only history (see [Rejoining a wiped node](/docs/0.7/operations#rejoining-a-wiped-node)).
## Write path
### `503` with `Retry-After` on append
Inspect the JSON error. `WalDiskPressure` means the WAL filesystem is below
`raft.wal.min_available_size` (or its free-space stat failed); the node also
becomes unready and attempts to yield its leaders. Free space or expand the
volume. Admission resumes only at `raft.wal.resume_available_size`, preventing
probe flapping.
`RaftLogPressure` means the node's unsnapshotted Raft log is over twice
`raft.snapshot_log_budget`. The node refuses writes with a body until
snapshots bring the log below 1.5 times the budget. Before it gets there, the
snapshot driver runs more snapshot builds at once the further the log is past
the budget, up to `raft.snapshot_build_max_concurrency`. When writes still
reach the limit, the node's metrics show how far the log got
(`raft_log_bytes_high_watermark`), whether the driver used every build
(`raft_snapshot_builds_in_flight_max`), and how long a build's upload takes
(`raft_snapshot_build_ns` over `raft_snapshot_builds`). If every build was in
use, raise `raft.snapshot_build_max_concurrency` (the chart's
`raft.snapshotBuildMaxConcurrency`) or the budget. Each build holds its group's
captured state and, while it uploads, the compressed snapshot, so raise the
builds only as far as the node's memory holds that many builds of its largest
groups. Set it to `1` to keep one build at a time, as before 0.7.4.
`IngressBodyBytesLimitReached` means the write bodies in flight on the node
leave too little of `server.http_inflight_body_size` for this one.
`WalDiskPressure`, `RaftLogPressure` and `IngressBodyBytesLimitReached` are
write admission refusals. Their responses also carry the code in
`x-ursula-not-applied`: the node refused the request before it proposed
anything of it. A client can count the attempt as never applied only when
the response also carries `x-ursula-not-applied-checked`, which a gateway
at 0.7.3 or newer adds. See
[refusals that applied nothing](/docs/0.7/api/overview#refusals-that-applied-nothing).
Other `503`s come from the stream's Raft group, and their text names the
cause. `ColdBackpressure` means that the group's hot ring would exceed
`storage.cold.max_hot_size_per_group`: cold flush isn't keeping up, or the
cold backend is slow or erroring out. Raise the per-group ceiling, lower
`storage.cold.flush_interval`, raise `storage.cold.flush_max_concurrency`, or
fix the cold backend. `RaftUncommittedBackpressure` means that the group's
write bytes that are proposed but not yet applied would exceed
`raft.max_uncommitted_size_per_group`.
The node refuses both before it proposes the append, so from 0.7.3 their
responses also carry the name in `x-ursula-not-applied`, like the admission
refusals above. A `503` while the group has no known leader, for example
during an election, carries `NotLeader` when the node found that before it
proposed the append. A `503` from a node that does not host the stream's
group and knows no node that does carries `GroupNotHosted`: the node refuses
before any engine runs.
### `409 Conflict` with `producer-expected-seq` / `producer-received-seq`
Out-of-order producer headers. Response tells you what was expected:
- `producer-expected-seq: N` is the next allowed sequence
- `producer-received-seq: M` is what the client sent
Common causes:
- Producer restart without bumping `Producer-Epoch`. Bump every restart.
- Two writers share the same `Producer-Id`. Use distinct IDs.
- A retry skipped a sequence number. Producer sequences must be contiguous within an epoch.
### `409 Conflict` from `Stream-Seq`
The supplied `Stream-Seq` is not bytewise greater than the last value accepted on this stream: another writer committed first, or the values are not fixed-width (`"10"` sorts before `"9"`). The response's `Stream-Next-Offset` is the tail. Read what you missed, re-derive the value and retry. See [Conditional writes](/docs/0.7/concepts/conditional-writes).
### `404 Not Found` on `POST /{bucket}/{stream}`
The stream hasn't been created. Create with `PUT /{bucket}/{stream}` first. There is no implicit creation on POST.
### `400 Bad Request` on JSON appends
JSON streams are validated on the server. Malformed JSON, invalid UTF-8, messages nested deeper than 127 levels, and empty arrays without `allow_empty_array` are rejected. Send valid JSON or switch to `application/octet-stream`.
## Read path
### No `Stream-Up-To-Date` on a read
The response is partial: cut by `max_bytes` or the 8 MiB cap, or served by a follower, which never sends it with data. Read on from `Stream-Next-Offset`. Not an error.
### SSE drops after 30-60 s idle
A proxy or load balancer is closing idle TCP. Raise its idle timeout or enable TCP keepalive on the proxy-to-Ursula leg. Ursula doesn't emit SSE keepalive comments.
### SSE on a binary stream returns base64 text
Expected. SSE wire format is text-only, so binary stream data events carry raw base64 text and the `Stream-Sse-Data-Encoding: base64` header signals it. See [binary SSE](/docs/0.7/concepts/binary-sse).
### Reads stall for hundreds of ms for cold offsets
The first read of a cold offset triggers an S3 `GetObject` range read. It runs off the actor turn so it doesn't block other commands, but the request still pays S3 latency.
## Replication and consensus
### A group has no leader (`current_leader: null`)
Either an election is in progress or quorum is lost for that group. Only streams hashed to that group are affected.
- Multiple nodes briefly claim leader: election is flapping. Check peer reachability and CPU pressure.
- No node claims leader: fewer than `n/2+1` voters are reachable, or a majority of the group's voters are behind their recovery gates. Restore reachability. For gated voters, see [Recovering after a host crash](/docs/0.7/operations#recovering-after-a-host-crash).
### One follower lags behind
State-machine apply runs on the follower's owner core. If CPU is pinned, `last_applied_index` trails `committed_index`. Check:
- Per-core mailbox depth (`mailbox_depths`). Sustained queueing means saturation.
- Disk pressure on the WAL volume (`fsync` latency, in particular with `raft.wal.fsync = "always"`).
- A pathological hot stream forcing constant cold flushes (`per_group_cold_hot_bytes`).
### Manually trigger a snapshot
These are admin-plane endpoints (`server.admin_listen`, loopback `:4438` by default), reached over a tunnel (see [ursulactl's operation providers](/docs/0.7/cli#the-admin-plane)). The examples assume a forward from local `4438` to the node's admin plane.
```bash
process_incarnation=$(curl --fail http://127.0.0.1:4438/__ursula/metrics | jq -er .process_incarnation)
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X POST http://127.0.0.1:4438/__ursula/raft/{group_id}/snapshot
curl --fail -H "x-ursula-process-incarnation: ${process_incarnation}" -X POST http://127.0.0.1:4438/__ursula/raft/{group_id}/purge
```
The snapshot request answers once the group has a snapshot of everything it had applied when the request arrived. It waits while a build of the group runs and while other snapshot builds hold all of the node's build permits, and a snapshot that a build begun since the request made answers it too. When no build of the group can start within 10 seconds, it answers `503` with `Retry-After: 1`. Send it again then. Unless the group's published snapshot already covers the request, it then waits up to 10 seconds for the group's running build to end and up to 10 more for the snapshot to show, and answers `500` when it does not.
Purge only after the snapshot replicates. Otherwise a slow follower may need log entries you just dropped.
## Cold flush issues
### Writes succeed but data never reaches S3
- `storage.cold.backend` is `none`, or set to `memory` when you expected `s3`.
- IAM permissions missing. Required: `s3:GetObject`, `s3:PutObject`, `s3:ListBucket`, `s3:DeleteObject`.
- `storage.cold.s3.endpoint` unreachable.
- Cold flush failing or falling behind (`cold_flush_write_errors` or `cold_backpressure_events` climbing, `cold_hot_bytes` growing).
If hot bytes grow unbounded, you'll eventually hit `storage.cold.max_hot_size_per_group` and writes start returning `503`. Fix the backend before that.
## SQLite VFS
These entries cover the `sqlite-ursula-vfs` extension and `@tonbo/sqlite-ursula`. See [SQLite on Ursula](/docs/0.7/examples/sqlite-vfs).
### A commit or attach is refused with error code 7178
`SQLITE_IOERR_AUTH` (`errcode` 7178 in `node:sqlite`, `UrsulaWriteRefusedError` from `open`, its database, `attach` and `openUrsulaPiStorage`) means that a write was refused before it reached the stream, so nothing of it was applied. A refused commit was rolled back, but the file is not poisoned. Reads go on, and the next commit succeeds once the refusals stop, with no new attach. A refused attach claimed nothing. Attach again later, or attach read-only to read the database meanwhile. An attach whose claim a `402` refuses fails with `SQLITE_AUTH` (`errcode` 23) instead, and `open` and `attach` throw `UrsulaPaymentRequiredError`. For a refused commit, `status(file)` (`ursula_status`) reports `writes_refused: true` and names the refusal:
| `refused_kind` | Refusal | What to do |
| -------------- | ------- | ---------- |
| `payment_required` | A layer in front of the stream answered `402` | Settle what the layer asks for. `refused_reason` carries its explanation |
| `unauthorized` | The token was refused (`401`) until `URSULA_VFS_RETRY_MS` ran out | Set a fresh token with `setToken`, or fix the file `URSULA_VFS_TOKEN_FILE` names |
| `forbidden` | The gateway's authorizer denied the write with a `403` | Allow the write in the authorizer |
| `rate_limited` | `429` until `URSULA_VFS_RETRY_MS` ran out | With `refused_reason` `tenant request rate limit exceeded`, lower the write rate or raise the bucket's `requests_per_sec` in the gateway's `--quota-policy`. With `tenant concurrent upload limit exceeded` or `tenant upload bytes limit exceeded`, the bucket's large bodies under way reached its `max_concurrent_uploads` or `max_upload_bytes` there: raise them, or keep commits at or below the gateway's `--small-body-max-bytes` (1 MiB by default), which never count |
| `not_applied` | Any other refusal marked `x-ursula-not-applied`, vouched for by `ursula gateway` 0.7.3 or later | See `refused_reason` below |
| `too_large` | The transaction's commit frame is over `URSULA_VFS_MAX_FRAME_BYTES` (32 MiB by default), so the extension never sent it. Or a lower limit in front of the stream, such as the gateway's `--max-request-body-bytes` or a bucket's body limit, refused it with a `413` that carries both headers | Split the transaction. Other commits succeed as usual, and the next one that does clears the refusal. See [Rules](/docs/0.7/examples/sqlite-vfs#rules) |
| `unreachable` | From 0.7.4: until `URSULA_VFS_RETRY_MS` ran out, every attempt failed before any byte of it went out, or was refused as above, and the last one failed before it went out. The stream URL's host did not resolve, or no connection to it was made: refused, timed out, or a failed TLS handshake. Nothing of the commit was sent | Check that the stream URL's host resolves from the client and that the gateway or the server behind it is up and reachable, through the proxy in `HTTPS_PROXY`, `HTTP_PROXY` or `ALL_PROXY` when one is set (`NO_PROXY` excludes hosts). A failed TLS handshake shows in `refused_reason` as rustls reports it. `io: invalid peer certificate: UnknownIssuer` means the client does not trust the endpoint's certificate authority: point `URSULA_VFS_CA_FILE` at it. `io: invalid peer certificate: certificate not valid for name ...` means the stream URL names a host the certificate does not cover: use one it covers. `io: received corrupt message of type InvalidContentType` means an `https://` URL points at a port that speaks plain HTTP. `io: received fatal alert: ...` means the endpoint refused the handshake, for example over its TLS versions or cipher suites. Commits succeed again once the endpoint is reachable |
`refused_reason` is the refusing layer's response body, or the value of `x-ursula-not-applied` when the body is empty, and always that value for `not_applied`. For `too_large` it is the frame's size and the limit, such as `41943104-byte frame over the 33554432-byte frame limit`, or the value of the `413`'s `x-ursula-not-applied`. For `unreachable` it is the last attempt's failure, such as `timeout: resolve` or `io: Connection refused (os error 111)`. The values Ursula sends for `not_applied`:
| `refused_reason` | Meaning |
| ---------------- | ------- |
| `RaftLogPressure` | The server's Raft log is over its hard limit under sustained write load. Writes resume once snapshots catch up |
| `WalDiskPressure` | The server's WAL filesystem is below `raft.wal.min_available_size`. Free space or expand the volume |
| `IngressBodyBytesLimitReached` | The server holds as many request bytes in flight as it admits |
| `RaftUncommittedBackpressure` | The stream's Raft group holds more proposed but unapplied write bytes than `raft.max_uncommitted_size_per_group` allows |
| `ColdBackpressure` | The stream's Raft group holds more hot bytes than `storage.cold.max_hot_size_per_group` allows, because cold flush is behind or the cold backend is slow |
| `NotLeader` | A `503`: the server does not lead the stream's Raft group and knows no leader yet, for example during an election. For the same reason, the redirects it sends before it proposes a write carry the same marker, and the gateway follows them. Writes resume once the group has a leader |
| `GroupNotHosted` | A `503`: the server does not host the stream's Raft group and knows no server that does. Its redirects to a server that does carry the same marker, and the gateway follows them |
| `RequestBodyUnreadable` | The gateway could not read the request body, so it sent nothing on |
| `UploadBudgetExhausted` | A `503`: the gateway's request body budget (`--body-budget-bytes`) stayed spent for `--body-budget-wait-ms`, so it sent nothing on. Commits of at most `--small-body-max-bytes` have headroom of their own, so this hits large commits under heavy upload load. Writes resume once uploads under way end. The gateway's `upload_refusals_budget` counter shows these refusals |
| `RequestBodyTooSlow` | A `408`: no byte of the body reached the gateway for `--body-stall-timeout`, or less than `--body-min-bytes-per-sec` arrived over one such window, so the gateway cut it and sent nothing on. With the defaults (30 s, 1 KiB/s) this does not happen to a commit, since the extension gives up on a commit after 10 s. Over HTTP/1.1 the gateway closes the connection with the `408`, so a client still sending usually meets the closed connection instead. Keep `--body-stall-timeout` above 10 s, or a commit still being sent can meet that closed connection on every attempt and poison the database. The gateway's `upload_refusals_too_slow` counter shows the cuts |
| `NotFound` | The gateway's access policy does not serve the stream to these credentials: no token, or one the policy does not allow. The cause is the credentials or the policy, not load |
| `AuthenticationUnavailable`, `AuthorizationUnavailable` | The gateway could not verify the token or reach a decision, for example while its token issuer is down |
| `NoUpstream`, `UpstreamUnreachable` | The gateway could not reach a server, so it sent nothing on. After redirects, the nodes it reached before refused the write as not their own to propose |
| `LeaderNotFound` | The gateway found no node that leads the stream's group among its upstreams within three hops and 5 s, and every node it reached refused the write before proposing it. Writes go through once the group has a leader. If it persists, check that `--upstream` reaches every voter, by its peer URL where possible |
[Refusals that applied nothing](/docs/0.7/api/overview#refusals-that-applied-nothing) lists every value. A commit fails with 7178 only when every attempt of it was refused this way, or failed before any byte of it went out. One attempt that went out and got no answer, a `307`, or a `5xx` without both headers, poisons the file instead, because that attempt may have been applied.
### The file is poisoned at the commit after a refused one
`status(file)` reports `poisoned: true` with a `reason` that starts with `a commit refused as never applied may have been applied after all`. A commit failed with 7178, refused with an answer that says nothing was applied (a `401`, `402` or `429`, or a refusal marked `x-ursula-not-applied` with the gateway's vouch). The next commit went out with the same producer sequence, and the stream answered it as a duplicate. So the stream holds a commit at that sequence that the file lacks: either the refused commit, applied after all, or the next commit, sent again by a layer in front of the stream. The answer does not tell which, so the extension poisoned the file instead of acknowledging a commit that may never have reached the stream.
Attach the file again. The attach replays whichever commit the stream holds. Check which of the two transactions the database then holds before you run either again. When it holds the refused one, although its commit failed, a layer broke the rule in [Writes refused before they reach the stream](/docs/0.7/examples/sqlite-vfs#writes-refused-before-they-reach-the-stream). Find it:
- A proxy or service mesh that retries writes. Istio before 1.24 retries a `503` twice by default, POSTs included, and Istio 1.24 to 1.26 does the same when istiod runs with `EXCLUDE_UNSAFE_503_FROM_DEFAULT_RETRY=false`, which `compatibilityVersion=1.23` sets. Any version retries on a route whose `retryOn` includes `503`, `5xx`, `gateway-error` or `reset`. Set `retries: attempts: 0` on the routes that carry stream writes, to the gateway and from the gateway to the servers.
- A proxy that follows redirects for writes.
- A layer between `ursula gateway` and the servers that answers `401`, `402` or `429`.
- A layer that refuses a write after it passed the write on, such as metering that answers `402` after the stream answered.
When the database holds the next transaction instead, a gateway sent that commit again to a new leader, after the old leader proposed it and redirected the gateway. No layer broke the rule, and nothing is lost.
### Reads of a fenced file fail with error code 266
From 0.7.5, once an owner knows that another owner claimed its stream (its commit failed, and `status(file)` reports `poisoned: true` and `fenced: true`, or `error.kind` `fenced`), every new read transaction on the file fails with `SQLITE_IOERR_READ` (`errcode` 266 in `node:sqlite`, `disk I/O error`). `open`'s database and `openUrsulaPiStorage` report these reads, those inside a transaction included, as `UrsulaFencedError` (an `UrsulaReplicationError` with `fenced: true`, whose `action` is `stop`) with a message that starts with `read refused`. The file no longer holds what the stream does, so it serves nothing. A read transaction already open when the fence landed keeps its snapshot until it ends. Find the other owner. Attaching the file again takes the stream back from it, so do that only for a deliberate takeover.
### A file opens with "unable to open database file" after a failed attach
From 0.7.5 a file whose attach failed in the process (`SQLITE_CANTOPEN`, `errcode` 14) opens only after an attach of it succeeds, whether or not it has a sidecar `-ursula`, so that no commit bypasses the stream. `status(file)` throws an error that names the failure: `ursula_status: attach_failed: : is not attached: its last attach failed ()`. The kind says what to do, as in `ursula_attach`'s own error: `invalid_name` or `invalid_url` (fix the stream URL), `unreachable` or `unavailable` (attach again later), `local_setup` (the directory refuses the file: fix its permissions, its mount or its free space), `outcome_unknown` (the attach failed after its claim, which may have fenced the earlier owner: attach again), `fenced` (another owner claimed the stream during the attach). A failed attach that wrote nothing into a new file removes the sidecar it created. An attach refused because of the file itself, before it changed anything, records no failed attach and changes nothing. It fails with `in_use` while the process has the file, its `-wal`, `-shm` or `-journal` open outside the extension (a connection opened before the extension was loaded, or through another VFS, read-only ones included), or another process has the file open: close those connections, or that process, and attach again. The process's own descriptors are checked on Linux with `/proc` mounted and on macOS only. It fails with `unreplicated_file` when the file holds data but no sidecar: the file still opens, so copy its rows into a newly attached file, or move it and its `-wal` aside to attach the path. An attach that meets another process holding the file only after it changed the local files is a failed attach (`attach_failed: in_use`).
## Still stuck?
Open a [GitHub issue](https://github.com/tonbo-io/ursula/issues) with:
- `/__ursula/metrics` from every node (with timestamps)
- Config file and CLI overrides per node (redact credentials)
- Last ~200 log lines at `RUST_LOG=ursula_runtime=debug,ursula_raft=debug,ursula=info`
---
# Overview
## The problem
Building a durable stream directly on S3 hits two walls: S3 has no append (every write either pays per-PUT or batches and eats the latency), and its conditional writes are optimistic compare-and-set, so concurrent writers retry-storm with exponential latency ([Chroma's wal3 post](https://www.trychroma.com/engineering/wal3) covers the mechanics). The market splits on which wall to lean against. [S2](https://s2.dev/blog/intro) writes through S3 Express to get under 50 ms but pays around 7x per-GB, while [WarpStream](https://www.warpstream.com/blog/minimizing-s3-api-costs-with-warpstream) batches to S3 Standard and accepts 250 ms+ p50.
## How Ursula works
Ursula keeps S3 off the write path entirely. Writes go to a cluster of nodes that coordinates through [Raft consensus](https://en.wikipedia.org/wiki/Raft_(algorithm)). Once a majority of replicas has persisted a record, it is committed, with no compare-and-set races and no retry storms. S3 is the cold tier.
Two structural choices shape the rest of the design:
1. **[Thread-per-core](https://seastar.io/shared-nothing/) shard ownership.** Every stream is statically hashed to one specific CPU core on each node. That core handles all reads and writes for its streams, in order, with no cross-core synchronization on the hot path.
2. **[Multi-Raft](https://tikv.org/deep-dive/scalability/multi-raft/).** Ursula runs hundreds to thousands of small Raft groups per cluster, not one log per node. Each stream hashes to a group, each group pins to a core. An unhealthy follower for one group does not stall traffic for another.
Together, write throughput scales with healthy cores across the cluster instead of with any single leader's bandwidth, and failure domains stay per-group rather than per-node.
## The stack at a glance
Ursula is async Rust. The pieces that show up in the diagrams below:
- **axum** is the HTTP server library that terminates public client requests.
- **tokio** is the async runtime. Each core runs its own single-threaded executor (`tokio::current_thread`) so work on that core never migrates.
- **tonic** is the gRPC library used for peer-to-peer Raft traffic between nodes.
- **OpenRaft** is the Raft consensus implementation Ursula plugs into.
The Rust-named components in diagrams (`ShardRuntime`, `GroupActor`, `GroupEngine`, `StreamStateMachine`) are the structures inside one node, glossed in the sections below.
## Interface
The API is the [Durable Streams protocol](https://durablestreams.com/), an open MIT-licensed spec published by [Electric](https://electric-sql.com/): create a stream, append, read from any offset, tail in real time over Server-Sent Events (SSE), with exactly-once write semantics. Plain HTTP plus SSE, with no custom binary protocol or mandatory client library. See the [API overview](/docs/0.7/api/overview) and the [extensions spec](/docs/0.7/specs/extensions) for what Ursula adds to it.
## Cluster topology
```
Clients (HTTP / SSE)
│
┌──────────────────┼──────────────────┐
v v v
┌──────────┐ ┌──────────┐ ┌──────────┐
│ Node A │ │ Node B │ │ Node C │
└────┬─────┘ └────┬─────┘ └────┬─────┘
cores 0..N cores 0..N cores 0..N
│ │ │
└─── gRPC peer-to-peer Raft traffic ──┘
│
v
┌────────────────────────┐
│ Cold storage (S3) │
│ per-stream chunks │
└────────────────────────┘
```
Every stream is replicated to all voting members of its Raft group. Hot data lives in memory on each replica. Cold data lives in a shared object store any replica can read from on demand.
## Inside a node
```
HTTP / SSE request
│
v
route(bucket_id, stream_id)
│
v
owner core
│
v
Raft group
│ │
│ └── replicate to peer nodes
│
├── hot state and live watchers
│
└── cold chunks in S3
```
A request enters the HTTP server, routing maps the bucket and stream name to one owner core and one Raft group, and the rest of the request runs inside that group. The group owns its hot bytes, live-tail watchers, producer dedup state, and Raft state machine, so mutable stream state does not cross cores after routing.
## The replaceable group engine
The `GroupEngine` trait is the seam between the runtime and a group's storage and replication strategy. Two engines ship today:
- **In-memory, non-replicated.** Used in tests, as a performance baseline, and
by the zero-config development node.
- **Raft engine** over the disk WAL (`raft.wal.path = "DIR"`). OpenRaft groups
on the same core share one checksummed, versioned journal under
`DIR/raft-log`, and cross-group fsync batching avoids one writer per group.
The journal is a run of segments: purge deletes whole segments once no group
needs them, and the few entries quiet groups leave in old segments are
copied forward. Each group keeps an index of where its entries are and a
small cache of recent ones, and reads older entries from disk. Snapshot plus
committed-tail replay rebuilds the in-memory state machine after restart. A
single node without `raft.wal.path` keeps the journal in a temporary
directory that is removed on clean shutdown.
A factory selected at startup picks which engine each group opens, leaving the runtime code identical across modes.
## Write path
```
HTTP ---> Runtime ---> core mailbox ---> GroupActor
│
├─ leader? ──no─---> 307 redirect to leader
│
v yes
Raft.client_write
│
v
WAL append (fsync per policy), replicate to followers
│
v commit
StateMachine::apply
│
v
notify_read_watchers (same turn, no lock)
│
v
AppendResponse -> client
```
The leader appends the entry to its write-ahead log (WAL) and replicates it to followers in parallel. A replica acknowledges the entry once its journal holds it: in the page cache with the default `raft.wal.fsync = "never"`, after an `fsync` with `always` (see [Configuration](/docs/0.7/configuration#raft)). Once a quorum holds the entry, the state machine applies it and the response goes back to the client. Producer dedup headers ([exactly-once writes](/docs/0.7/concepts/exactly-once-writes)) are checked inside the state machine, so retries of an already-applied append return the original offset rather than appending again. `Stream-Seq` is checked in the same place, after producer dedup, as a per-stream compare-and-set ([conditional writes](/docs/0.7/concepts/conditional-writes)).
## Storage layout
Each Raft group has two storage planes: a replicated log and a stream state machine.
```
Raft group
│
┌─────────┴─────────┐
v v
replicated log state machine
command order streams in this group
quorum durability offsets / metadata / dedup
│
┌─────────┴─────────┐
v v
hot ring memory cold chunk refs
recent bytes S3 objects
```
The Raft log is the ordering and durability boundary for writes. A write is acknowledged only after a majority of that group's replicas hold the entry in their WAL. The entry carries the stream command and metadata needed to apply it deterministically.
The stream data plane lives in the group's state machine. A group owns many streams, keyed by `(bucket_id, stream_id)`. For each stream it tracks metadata, offsets, producer dedup state, live snapshots, hot byte segments, and cold chunk references.
Recent bytes stay in an in-memory hot ring on every replica. When a group's hot bytes exceed the configured cold-flush threshold, a background worker uploads older contiguous segments to the cold backend, usually S3. After upload succeeds, Ursula commits a metadata update through Raft that publishes the cold chunk reference and advances the stream's hot start offset.
Reads assemble one logical stream from both tiers. The state machine first builds a read plan: which byte ranges are still hot and which ranges are cold chunk references. Hot bytes are copied from memory, and cold ranges are fetched with object-store range reads. Any replica that has applied the cold chunk metadata can serve those cold reads, as long as all nodes point at the same S3 bucket/prefix.
## Read path and cold offload
```
HTTP ---> Runtime ---> core mailbox ---> GroupActor
│
├─ leader? ──no─---> forward to leader
│
v yes
engine.read_stream_parts (under state-machine lock)
│
v release lock
GroupReadStreamParts { plan, cold_store }
│
v tokio::spawn (off the actor turn)
materialize: hot bytes + S3 range reads
│
v
ReadStreamResponse -> client
```
Under the state-machine lock the runtime computes only a read plan: which segments are still in the hot ring and which live as cold chunks in S3. Then it releases the lock. Materialization, including any S3 `GetObject` range reads, runs in a separately spawned task, so a slow cold fetch never blocks other commands waiting on the same group. Followers serve replicated historical catch-up bytes from their local applied state when the read does not need a leader-owned access mutation. Tail-sensitive responses such as `HEAD`, open-stream `Stream-Up-To-Date: true`, and live reads stay on the leader path (see [offsets](/docs/0.7/concepts/offsets)).
## SSE live tailing
When a read finds no new data and the response carries `Stream-Up-To-Date: true`, the runtime registers a watcher inside the owning group actor. The watcher map is per-actor and mutated only inside that actor's turn, with no mutex and no awaiting while holding a lock. When a later append commits on the same group, the actor wakes the matching watchers in the same turn, deduplicates them by request shape, and dispatches materialization through the same fast path the read uses.
For deployment topology (voting layout across availability zones, non-voting replicas, S3 prefix layout), see the [operations guide](/docs/0.7/operations).
---
# Comparisons
Ursula is for streams that diverse clients reach over the public internet with plain HTTP. One durable timeline per resource, and many small streams rather than a few big ones.
Most streaming systems are built for a different shape. They assume an SDK client inside your network and a few high-throughput topics. That assumption decides most of the comparison below, so start there.
## Start with the access model
Two questions decide whether Ursula fits.
**Where do clients run, and how do they connect?** Ursula serves plain HTTP and SSE. A browser, a mobile app, a serverless function, or a third party can read, write, and tail a stream over the public internet with no SDK. Broker-style systems expect a client library inside the same network.
**What is one stream?** In Ursula one stream maps to one resource: a document, a session, a task, an agent run. You address it by URL and keep thousands of them. Broker-style systems multiplex many resources into a few topics and partitions, and you rebuild per-resource state downstream.
If your clients are in-network services and your data is a few high-throughput pipelines, Ursula is the wrong shape. If your clients are on the open internet and your data is many small per-resource timelines, the rest follows.
## Kafka / Redpanda
Kafka and Redpanda are built for high-throughput pipelines. Topics, partitions, consumer groups, and SDK clients inside the cluster network. They are mature and fast at that job.
They are not built for public-internet HTTP access or for thousands of small, independently addressable streams. There is no plain-HTTP read, write, and tail for an arbitrary client, and the unit of work is the topic, not the resource. Pick Kafka or Redpanda for a few high-throughput topics with in-network consumers. Pick Ursula for many small streams with open-internet clients.
## S2 / S2 Lite
S2 is an object-storage-backed stream API and the closest alternative to Ursula. Acknowledged writes go through S3 Express One Zone, and the cold tier stays on Express-class storage. Self-hosted S2 Lite is a single serving process. The managed product uses its own API, not Durable Streams.
S3 Express looks fine on its own. The point is that you do not need it. Ursula gets its write latency from an in-memory hot ring replicated by Raft quorum, which has nothing to do with the storage tier, and it keeps data on plain S3 Standard. In our 3-node benchmark Ursula commits appends faster than S2 Lite at every concurrency, since S2's per-append latency is bound by the S3 PUT round-trip. So S2 pays for a premium storage tier and still writes slower.
| | Ursula | S2 |
| --- | --- | --- |
| Protocol | Durable Streams over plain HTTP and SSE, portable clients | Custom API, locked to one vendor |
| Write latency | Single-digit to tens of ms p99 append | Hundreds of ms p99, bound by the S3 PUT |
| Storage tier | Plain S3 Standard, commodity per-GB price | S3 Express, around 7× the per-GB storage bill |
| Self-host | Raft cluster with quorum HA | Single process (Lite), no failover |
On a workload that accumulates terabytes in the cold tier, the storage gap is the larger line on your bill, and it grows every month. Pick S2 for a managed service when you can adopt its API and pay for Express-class storage. Pick Ursula for lower-latency writes on an open protocol with a commodity S3 bill.
## S3 directly
S3 is great for durable immutable blobs. It is a poor append log. Every append is a new object. Conditional writes are optimistic and retry-storm under load. Live readers have to poll.
Ursula uses S3 for the cold tier and snapshots, never the write path. Appends commit at Raft quorum in an in-memory hot ring, and a background flush moves older data to S3. One read serves hot and cold transparently. Use S3 directly for immutable blobs with no append or tail. Use Ursula when you need an append log with live readers.
## etcd
etcd is strongly consistent key-value over a single Raft log. It is excellent for small consistent state. Throughput is capped at one leader's serialization.
Ursula runs many small Raft groups across cores and nodes, so throughput scales with the cluster. There is no single-log order across the whole namespace, but each stream stays linearizable. Use etcd for small consistent cluster-wide state. Use Ursula for many independent durable streams.
## When to pick which
- Many small per-resource streams, open-internet HTTP clients, replay and live tail in one primitive: **Ursula**.
- A few high-throughput topics with consumer groups: **Kafka / Redpanda**.
- Managed service, willing to change API: **S2**.
- Durable immutable blobs, no append or tail: **S3 directly**.
- Small consistent key-value across the cluster: **etcd**.
---
# Durable Streams Protocol
> [!NOTE]
> This page is a verbatim mirror of the upstream Durable Streams Protocol specification, authored by ElectricSQL. The authoritative source is [github.com/durable-streams/durable-streams](https://github.com/durable-streams/durable-streams/blob/main/PROTOCOL.md). Please open issues and pull requests against that repository, not Ursula's. Ursula's own [extensions](/docs/0.7/specs/extensions) are documented separately.
**Document:** Durable Streams Protocol
**Version:** 1.0
**Author:** ElectricSQL
---
## Abstract
This document specifies the Durable Streams Protocol, an HTTP-based protocol for creating, appending to, and reading from durable, append-only byte streams. The protocol provides a simple, web-native primitive for applications requiring ordered, replayable data streams with support for catch-up reads, live tailing, and explicit stream closure (EOF). It is designed to be a foundation for higher-level abstractions such as event sourcing, database synchronization, collaborative editing, AI conversation histories, and finite response streaming.
## Copyright Notice
Copyright (c) 2025 ElectricSQL
## Table of Contents
1. [Introduction](#1-introduction)
2. [Terminology](#2-terminology)
3. [Protocol Overview](#3-protocol-overview)
4. [Stream Model](#4-stream-model)
- 4.1. [Stream Closure](#41-stream-closure)
5. [HTTP Operations](#5-http-operations)
- 5.1. [Create Stream](#51-create-stream)
- 5.2. [Append to Stream](#52-append-to-stream)
- 5.2.1. [Idempotent Producers](#521-idempotent-producers)
- 5.3. [Close Stream](#53-close-stream)
- 5.4. [Delete Stream](#54-delete-stream)
- 5.5. [Stream Metadata](#55-stream-metadata)
- 5.6. [Read Stream - Catch-up](#56-read-stream---catch-up)
- 5.7. [Read Stream - Live (Long-poll)](#57-read-stream---live-long-poll)
- 5.8. [Read Stream - Live (SSE)](#58-read-stream---live-sse)
6. [Offsets](#6-offsets)
7. [Content Types](#7-content-types)
8. [Caching and Collapsing](#8-caching-and-collapsing)
9. [Extensibility](#9-extensibility)
10. [Security Considerations](#10-security-considerations)
11. [IANA Considerations](#11-iana-considerations)
12. [References](#12-references)
---
## 1. Introduction
Modern web and cloud applications frequently require ordered, durable sequences of data that can be replayed from arbitrary points and tailed in real time. Common use cases include:
- Database synchronization and change feeds
- Event-sourced architectures
- Collaborative editing and CRDTs
- AI conversation histories and token streaming
- Workflow execution histories
- Real-time application state updates
- Finite response streaming (proxied HTTP responses, job outputs, file transfers)
While these patterns are widespread, the web platform lacks a simple, first-class primitive for durable streams. Applications typically implement ad-hoc solutions using combinations of databases, queues, and polling mechanisms, each reinventing similar offset-based replay semantics.
The Durable Streams Protocol provides a minimal HTTP-based interface for durable, append-only byte streams. It is intentionally low-level and byte-oriented, allowing higher-level abstractions to be built on top without protocol changes.
## 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
**Stream**: A URL-addressable, append-only byte stream that can be read and written to. A stream is simply a URL. The protocol defines how to interact with that URL using HTTP methods, query parameters, and headers. Streams are durable and immutable by position. New data can only be appended.
**Offset**: An opaque, lexicographically sortable token that identifies a position within a stream. Clients use offsets to resume reading from a specific previously reached point.
**Content Type**: A MIME type set on stream creation that describes the format of the stream's bytes. The content type is returned on reads and may be used by clients to interpret message boundaries.
**Tail Offset**: The offset immediately after the last byte in the stream. This is the position where new appends will be written.
**Closed Stream**: A stream that has been explicitly closed by a writer. Once closed, a stream is in a terminal state: no further appends are permitted, and readers can observe the closure as an end-of-stream (EOF) signal. Closure is durable and monotonic - once closed, a stream remains closed.
## 3. Protocol Overview
The Durable Streams Protocol is an HTTP-based protocol that operates on URLs. A stream is simply a URL. The protocol defines how to interact with that URL using standard HTTP methods, query parameters, and custom headers.
The protocol defines operations to create, append to, read, close, delete, and query metadata for streams. Reads have three modes: catch-up, long-poll, and Server-Sent Events (SSE). The primary operations are:
1. **Create**: Establish a new stream at a URL with optional initial content (PUT)
2. **Append**: Add bytes to the end of an existing stream (POST)
3. **Close**: Transition a stream to closed state, optionally with a final append (POST with `Stream-Closed: true`)
4. **Read**: Retrieve bytes starting from a given offset, with support for catch-up and live modes (GET)
5. **Delete**: Remove a stream (DELETE)
6. **Head**: Query stream metadata without transferring data (HEAD)
The protocol does not prescribe a specific URL structure. Servers may organize streams using any URL scheme they choose (e.g., `/v1/stream/{path}`, `/{id}`, or domain-specific paths). The protocol is defined by the HTTP methods, query parameters, and headers applied to any stream URL.
Streams support arbitrary content types. The protocol operates at the byte level, leaving message framing and schema interpretation to clients.
**Independent Read/Write Implementation**: Servers **MAY** implement the read and write paths independently. For example, a database synchronization server may only implement the read path and use its own injection system for writes, while a collaborative editing service might implement both paths.
## 4. Stream Model
A stream is an append-only sequence of bytes with the following properties:
- **Durability**: Once written and acknowledged, bytes persist until the stream is deleted or expired
- **Immutability by Position**: Bytes at a given offset never change. New data is only appended
- **Ordering**: Bytes are strictly ordered by offset
- **Content Type**: Each stream has a MIME content type set at creation
- **TTL/Expiry**: Streams may have a sliding time-to-live window (resets on each read or write) or an absolute expiry time
- **Retention**: Servers **MAY** implement retention policies that drop data older than a certain age while the stream continues. If a stream is deleted a new stream **SHOULD NOT** be created at the same URL.
- **Stream State**: A stream is either **open** (accepts appends) or **closed** (no further appends permitted). Streams start in the open state and transition to closed via an explicit close operation. This transition is **durable** (persisted) and **monotonic** (once closed, a stream cannot be reopened).
Clients track their position in a stream using offsets. Offsets are opaque to clients but are lexicographically sortable, allowing clients to determine ordering and resume from any point.
### 4.1. Stream Closure
Stream closure provides an explicit end-of-stream (EOF) signal that allows readers to distinguish between "no data yet" and "no more data ever." This is essential for finite streams where writers need to signal completion, such as:
- Proxied HTTP responses that have finished streaming
- Completed job outputs or workflow executions
- Finalized conversation histories or document streams
**Properties of stream closure:**
- **Durable**: The closed state is persisted and survives server restarts
- **Monotonic**: Once closed, a stream cannot be reopened
- **Idempotent**: Closing an already-closed stream succeeds (or returns a stable "already closed" response)
- **Observable**: Readers can detect closure and treat it as EOF
- **Atomic with final append**: Writers can atomically append a final message and close in a single operation
After closure, the stream's data remains fully readable. Only new appends are rejected.
**Stream-Closed Header Value:**
The `Stream-Closed` header uses the value `true` (case-insensitive) to indicate closure. Servers **MUST** treat the header as present only when its value is exactly `true` (case-insensitive comparison). Other values such as `false`, `yes`, `1`, or empty string **MUST** be treated as if the header were absent. Servers **SHOULD NOT** return error responses for non-`true` values. They simply ignore the header.
## 5. HTTP Operations
The protocol defines operations that are applied to a stream URL. The examples in this section use `{stream-url}` to represent any stream URL. Servers may implement any URL structure they choose. The protocol is defined by the HTTP methods, query parameters, and headers.
### 5.1. Create Stream
#### Request
```
PUT {stream-url}
```
Where `{stream-url}` is any URL that identifies the stream to be created.
Creates a new stream. If the stream already exists at `{stream-url}`, the server **MUST** either:
- return `200 OK` if the existing stream's configuration (content type, TTL/expiry, and closure status) matches the request, or
- return `409 Conflict` if it does not.
This provides idempotent "create or ensure exists" semantics aligned with HTTP PUT expectations.
**Closure status matching:** When checking for idempotent success (200 OK), servers **MUST** compare the `Stream-Closed` header in the request against the stream's current closure status. For example:
- `PUT /stream` (no `Stream-Closed`) to an **open** stream with matching config -> `200 OK`
- `PUT /stream` (no `Stream-Closed`) to a **closed** stream -> `409 Conflict` (closure status mismatch)
- `PUT /stream + Stream-Closed: true` to a **closed** stream with matching config -> `200 OK`
- `PUT /stream + Stream-Closed: true` to an **open** stream -> `409 Conflict` (closure status mismatch)
#### Request Headers (Optional)
- `Content-Type: `
- Sets the stream's content type. If omitted, the server **MAY** default to `application/octet-stream`.
- `Stream-TTL: `
- Sets a sliding time-to-live window in seconds. The stream expires after being idle (no reads or writes) for this duration. Each read or write operation resets the expiry countdown to this value. `HEAD` requests do **not** reset the countdown. The value **MUST** be a non-negative integer in decimal notation without leading zeros, plus signs, decimal points, or scientific notation (e.g., `3600` is valid; `+3600`, `03600`, `3600.0`, and `3.6e3` are not).
- TTL resets are a server-side concern: only requests that reach the origin server reset the countdown. Reads served from intermediate caches (CDN catch-up reads with `Cache-Control: public, max-age=60`) do not reach the server and do not reset the TTL. For live read modes (long-poll, SSE), the TTL resets when the server begins processing the request, not when data is delivered or the response completes. This means a stream with active live readers will not expire, even if no new data is being produced.
- `Stream-Expires-At: `
- Sets an absolute expiry time as an RFC 3339 timestamp.
- If both `Stream-TTL` and `Stream-Expires-At` are supplied, servers **SHOULD** reject the request with `400 Bad Request`. Implementations **MAY** define a deterministic precedence rule, but **MUST** document it.
- `Stream-Closed: true` (optional)
- When present, the stream is created in the **closed** state. Any body provided becomes the complete and final content of the stream.
- This enables atomic "create and close" semantics for single-message or empty streams that are immediately complete (e.g., cached responses, placeholder errors, pre-computed results).
- **Examples:**
- `PUT /stream + Stream-Closed: true` (empty body): Creates an empty, immediately-closed stream (useful for "completed with no output" or error placeholders).
- `PUT /stream + Stream-Closed: true + body`: Creates a single-shot stream with the body as its complete content (useful for cached responses, pre-computed results).
#### Request Body (Optional)
- Initial stream bytes. If provided, these bytes form the first content of the stream.
#### Response Codes
- `201 Created`: Stream created successfully
- `200 OK`: Stream already exists with matching configuration (idempotent success)
- `409 Conflict`: Stream already exists with different configuration
- `400 Bad Request`: Invalid headers or parameters (including conflicting TTL/expiry)
- `429 Too Many Requests`: Rate limit exceeded
#### Response Headers (on 201 or 200)
- `Location: {stream-url}` (on 201): Servers **SHOULD** include a `Location` header equal to `{stream-url}` in `201 Created` responses.
- `Content-Type: `: The stream's content type
- `Stream-Next-Offset: `: The tail offset after any initial content
- `Stream-Closed: true`: Present when the stream was created in the closed state
### 5.2. Append to Stream
#### Request
```
POST {stream-url}
```
Where `{stream-url}` is the URL of an existing stream.
Appends bytes to the end of an existing stream. Supports both full-body and streaming (chunked) append operations. Optionally closes the stream atomically with the append.
Servers that do not support appends for a given stream **SHOULD** return `405 Method Not Allowed` or `501 Not Implemented` to `POST` requests on that URL.
#### Request Headers
- `Content-Type: `
- **MUST** match the stream's existing content type when a body is provided. Servers **MUST** return `409 Conflict` when the content type is valid but does not match the stream's configured type.
- **MAY** be omitted when the request body is empty (i.e., close-only requests with `Stream-Closed: true`). When the request body is empty, servers **MUST NOT** reject based on `Content-Type` and **MAY** ignore it entirely. This ensures close-only requests remain robust even when clients/libraries attach default `Content-Type` headers.
- `Transfer-Encoding: chunked` (optional)
- Indicates a streaming body. Servers **SHOULD** support HTTP/1.1 chunked encoding and HTTP/2 streaming semantics.
- `Stream-Seq: ` (optional)
- A monotonic, lexicographic writer sequence number for coordination.
- `Stream-Seq` values are opaque strings that **MUST** compare using simple byte-wise lexicographic ordering. Sequence numbers are scoped per authenticated writer identity (or per stream, depending on implementation). Servers **MUST** document the scope they enforce.
- If provided and less than or equal to the last appended sequence (as determined by lexicographic comparison), the server **MUST** return `409 Conflict`. Sequence numbers **MUST** be strictly increasing.
- `Stream-Closed: true` (optional)
- When present with value `true`, the stream is **closed** after the append completes. This is an atomic operation: the body (if any) is appended as the final data, and the stream transitions to the closed state in the same step.
- If the request body is empty (Content-Length: 0 or no body), the stream is closed without appending any data. This is the only case where an empty POST body is valid.
- Once closed, the stream rejects all subsequent appends with `409 Conflict` (see below).
- **Close-only requests are idempotent**: if the stream is already closed and the request includes `Stream-Closed: true` with an empty body, servers **SHOULD** return `204 No Content` with `Stream-Closed: true`.
- **Append-and-close requests are NOT idempotent** (without idempotent producer headers): if the stream is already closed and the request includes a body but no idempotent producer headers, servers **MUST** return `409 Conflict` with `Stream-Closed: true`, since the body cannot be appended. However, if idempotent producer semantics apply and the request matches the `(producerId, epoch, seq)` tuple that performed the closing append, servers treat it as a deduplicated success (see Section 5.2.1).
#### Request Body
- Bytes to append to the stream. Servers **MUST** reject POST requests with an empty body (Content-Length: 0 or no body) with `400 Bad Request`, **unless** the `Stream-Closed: true` header is present (which allows closing without appending data).
#### Response Codes
- `204 No Content`: Append successful (or stream already closed when closing idempotently)
- `400 Bad Request`: Malformed request (invalid header syntax, missing Content-Type, empty body without `Stream-Closed: true`)
- `404 Not Found`: Stream does not exist
- `405 Method Not Allowed` or `501 Not Implemented`: Append not supported for this stream
- `409 Conflict`: Content type mismatch with stream's configured type, sequence regression (if `Stream-Seq` provided), or **stream is closed** (when attempting to append without `Stream-Closed: true`)
- `413 Payload Too Large`: Request body exceeds server limits
- `429 Too Many Requests`: Rate limit exceeded
#### Response Headers (on success)
- `Stream-Next-Offset: `: The new tail offset after the append
- `Stream-Closed: true`: Present when the stream is now closed (either by this request or previously)
#### Response Headers (on 409 Conflict due to closed stream)
When a client attempts to append to a closed stream (without `Stream-Closed: true`), servers **MUST** return:
- `409 Conflict` status code
- `Stream-Closed: true` header
- `Stream-Next-Offset: `: The final offset of the closed stream (useful for clients to know the stream's final position)
This allows clients to detect and handle the "stream already closed" condition programmatically without parsing the response body. Servers **SHOULD** keep the response body empty or use a standardized error format. Clients **SHOULD NOT** rely on parsing the body to determine the reason for rejection.
**Error Precedence:** When an append request would trigger multiple conflict conditions (e.g., stream is closed AND content type mismatches), servers **SHOULD** check the stream's closed status first. This ensures clients receive the `Stream-Closed: true` header, enabling correct error handling. The recommended precedence is:
1. Stream closed -> `409 Conflict` with `Stream-Closed: true`
2. Content type mismatch -> `409 Conflict`
3. Sequence regression -> `409 Conflict`
### 5.2.1. Idempotent Producers
Durable Streams supports Kafka-style idempotent producers for exactly-once write semantics. This enables fire-and-forget writes with server-side deduplication, eliminating duplicates from client retries.
#### Design
- **Client-provided producer IDs**: Zero RTT overhead, no handshake required
- **Client-declared epochs, server-validated fencing**: Client increments epoch on restart. Server validates monotonicity and fences stale epochs
- **Per-batch sequence numbers**: Separate from `Stream-Seq`, used for retry safety
- **Two-layer sequence design**:
- Transport layer: `Producer-Id` + `Producer-Epoch` + `Producer-Seq` (retry safety)
- Application layer: `Stream-Seq` (cross-restart ordering, lexicographic)
#### Request Headers
All three producer headers **MUST** be provided together or none at all. If only some headers are provided, servers **MUST** return `400 Bad Request`.
- `Producer-Id: `
- Client-supplied stable identifier (e.g., "order-service-1", UUID)
- **MUST** be a non-empty string. Empty values result in `400 Bad Request`
- Identifies the logical producer across restarts
- `Producer-Epoch: `
- Client-declared epoch, starting at 0
- Increment on producer restart to establish a new session
- Server validates that epoch is monotonically non-decreasing
- **MUST** be a non-negative integer ≤ 2^53-1 (for JavaScript interoperability)
- `Producer-Seq: `
- Monotonically increasing sequence number per epoch
- Starts at 0 for each new epoch
- Applies per-batch (per HTTP request), not per-message
- **MUST** be a non-negative integer ≤ 2^53-1 (for JavaScript interoperability)
#### Response Headers
- `Producer-Epoch: `: Echoed back on success (200/204), or current server epoch on stale epoch (403)
- `Producer-Seq: `: On success (200/204), the highest accepted sequence number for this `(stream, producerId, epoch)` tuple. Enables clients to confirm pipelined requests and recover state after crashes.
- `Producer-Expected-Seq: `: On 409 Conflict (sequence gap), the expected sequence
- `Producer-Received-Seq: `: On 409 Conflict (sequence gap), the received sequence
#### Validation Logic
```
# Epoch validation (client-declared, server-validated)
if epoch < state.epoch:
-> 403 Forbidden
-> Headers: Producer-Epoch:
if epoch > state.epoch:
if seq != 0:
-> 400 Bad Request (new epoch must start at seq=0)
-> Accept: update state.epoch = epoch, state.lastSeq = 0
-> 200 OK (new epoch established)
# Same epoch: sequence validation
if seq <= state.lastSeq:
-> 204 No Content (duplicate, idempotent success)
-> Return the exact original byte range for this seq when its receipt is
retained (the stream's receipt window, and always the newest seq);
beyond the window, return no byte range
if seq == state.lastSeq + 1:
-> Accept, update state.lastSeq = seq
-> 200 OK
if seq > state.lastSeq + 1:
-> 409 Conflict
-> Headers: Producer-Expected-Seq: , Producer-Received-Seq:
```
#### Response Codes (with Producer Headers)
- `200 OK`: Append successful (new data)
- `204 No Content`: Duplicate append (idempotent success, data already exists)
- `400 Bad Request`: Invalid producer headers (e.g., non-integer values, epoch increase with seq != 0)
- `403 Forbidden`: Stale producer epoch (zombie fencing). Response includes `Producer-Epoch` header with current server epoch.
- `409 Conflict`: Sequence gap detected. Response includes `Producer-Expected-Seq` and `Producer-Received-Seq` headers.
#### Bootstrap and Restart Flow
1. **Initial start (epoch=0)**:
- Producer sends `(epoch=0, seq=0)`
- Server accepts, establishes producer state
2. **Producer restart**:
- Producer increments local epoch (0 -> 1), resets seq to 0
- Sends `(epoch=1, seq=0)`
- Server sees epoch > state.epoch, accepts, updates state
3. **Zombie fencing**:
- Old producer (zombie) still sending `(epoch=0, seq=N)` gets 403 Forbidden
- Response includes `Producer-Epoch: 1` header
#### Auto-claim Flow (for ephemeral producers)
For serverless or ephemeral producers without persisted epoch:
1. Producer starts fresh with `(epoch=0, seq=0)`
2. If server has `state.epoch=5`, returns 403 with `Producer-Epoch: 5`
3. Client can retry with `(epoch=6, seq=0)` to claim the producer ID
This is opt-in client behavior and should be used with caution.
#### Concurrency Requirements
Servers **MUST** serialize validation + append operations per `(stream, producerId)` pair. HTTP requests can arrive out-of-order. Without serialization, seq=1 arriving before seq=0 would cause false sequence gaps.
#### Atomicity Requirements
For persistent storage, servers **SHOULD** commit producer state updates and log appends atomically (e.g., in a single database transaction). Non-atomic implementations have a crash window where:
1. Data is appended to the log
2. Crash occurs before producer state is updated
3. On recovery, a retry may be re-accepted, causing duplicate data
**Recovery for non-atomic stores**: Clients can bump their epoch after a crash to establish a clean session. This trades "exactly once within epoch" for "at least once across crashes" which is acceptable for many use cases. Stores **SHOULD** document their atomicity guarantees clearly.
#### Producer State Cleanup
Servers **MAY** implement TTL-based cleanup for producer state:
- **In-memory stores**: 7 days TTL recommended, clean up on stream access
- **Persistent stores**: Retain as long as stream data exists (stronger guarantee)
After state expiry, the producer is treated as new. A zombie alive past TTL expiry can write again, which is acceptable for testing but persistent stores should use longer retention.
#### Stream Closure with Idempotent Producers
Idempotent producers can close streams using the `Stream-Closed: true` header. The behavior is:
- **Close with final append**: Include body, producer headers, and `Stream-Closed: true`. The append is deduplicated normally, and the stream closes atomically with the final append.
- **Close without append**: Include `Stream-Closed: true` with empty body. Producer headers are optional but if provided, the close operation is still idempotent.
- **Duplicate close**: If the stream was already closed by the same `(producerId, epoch, seq)` tuple, servers **SHOULD** return `204 No Content` with `Stream-Closed: true`.
When a closed stream receives an append from an idempotent producer:
- If the `(producerId, epoch, seq)` matches the request that closed the stream, return `204 No Content` (duplicate/idempotent success) with `Stream-Closed: true`
- Otherwise, return `409 Conflict` with `Stream-Closed: true` (stream is closed, no further appends allowed)
### 5.3. Close Stream
To close a stream without appending data, send a POST request with `Stream-Closed: true` and an empty body:
#### Request
```
POST {stream-url}
Stream-Closed: true
```
#### Response Codes
- `204 No Content`: Stream closed successfully (or already closed - idempotent)
- `404 Not Found`: Stream does not exist
- `405 Method Not Allowed` or `501 Not Implemented`: Append/close not supported for this stream
#### Response Headers
- `Stream-Next-Offset: `: The tail offset (unchanged, since no data was appended)
- `Stream-Closed: true`: Confirms the stream is now closed
This is the canonical "close-only" operation. For atomic "append final message and close", include a request body as described in Section 5.2.
### 5.4. Delete Stream
#### Request
```
DELETE {stream-url}
```
Where `{stream-url}` is the URL of the stream to delete.
Deletes the stream and all its data. In-flight reads may terminate with a `404 Not Found` on subsequent requests after deletion.
#### Response Codes
- `204 No Content`: Stream deleted successfully
- `404 Not Found`: Stream does not exist
- `405 Method Not Allowed` or `501 Not Implemented`: Delete not supported for this stream
### 5.5. Stream Metadata
#### Request
```
HEAD {stream-url}
```
Where `{stream-url}` is the URL of the stream. Checks stream existence and returns metadata without transferring a body. This is the canonical way to find the tail offset, TTL, expiry information, and **closure status**.
#### Response Codes
- `200 OK`: Stream exists
- `404 Not Found`: Stream does not exist
- `429 Too Many Requests`: Rate limit exceeded
#### Response Headers (on 200)
- `Content-Type: `: The stream's content type
- `Stream-Next-Offset: `: The tail offset (next offset after the current end)
- `Stream-TTL: ` (optional): The stream's time-to-live window. Each read or write resets the expiry countdown to this value.
- `Stream-Expires-At: ` (optional): Absolute expiry time, if applicable
- `Stream-Closed: true` (optional): Present when the stream has been closed. Absence indicates the stream is still open.
- `Cache-Control`: See Section 8
#### Caching Guidance
Servers **SHOULD** make `HEAD` responses effectively non-cacheable, for example by returning `Cache-Control: no-store`. Servers **MAY** use `Cache-Control: private, max-age=0, must-revalidate` as an alternative, but `no-store` is recommended to avoid stale tail offsets and closure status.
### 5.6. Read Stream - Catch-up
#### Request
```
GET {stream-url}?offset=
```
Where `{stream-url}` is the URL of the stream. Returns bytes starting from the specified offset. This is used for catch-up reads when a client needs to replay stream content from a known position.
#### Query Parameters
- `offset` (optional)
- Start offset token. If omitted, defaults to the stream start (offset -1).
- `consistency` (optional)
- `local` (default) permits immutable catch-up bytes from a follower's applied
state. `leader` routes the read to the current Raft leader and is intended
for recovery that must observe an acknowledged write before proceeding.
`leader` cannot be combined with a live read mode.
#### Response Codes
- `200 OK`: Data available (or empty body if offset equals tail)
- `400 Bad Request`: Malformed offset or invalid parameters
- `404 Not Found`: Stream does not exist
- `410 Gone`: Offset is before the earliest retained position (retention/compaction)
- `429 Too Many Requests`: Rate limit exceeded
For non-live reads without data beyond the requested offset, servers **SHOULD** return `200 OK` with an empty body and `Stream-Next-Offset` equal to the requested offset. If the stream is closed, this response **MUST** also include `Stream-Closed: true` to signal EOF.
#### Response Headers (on 200)
- `Cache-Control`: Derived from TTL/expiry (see Section 8)
- `ETag: {internal_stream_id}:{start_offset}:{end_offset}`
- Entity tag for cache validation
- `Stream-Cursor: ` (optional for catch-up, required for live modes)
- Cursor to echo on subsequent long-poll requests to improve CDN collapsing. Servers **MAY** include this on catch-up reads. It is **required** for live modes when the stream is open (see Sections 5.7, 5.8). Servers **MAY** omit it when `Stream-Closed` is true. Clients **MUST** tolerate its absence when `Stream-Closed` is present.
- `Stream-Next-Offset: `
- The next offset to read from (for subsequent requests)
- `Stream-Up-To-Date: true`
- **MUST** be present and set to `true` when the response includes all data available in the stream at the time the response was generated (i.e., when the requested offset has reached the tail and no more data exists).
- **SHOULD NOT** be present when returning partial data due to server-defined chunk size limits (when more data exists beyond what was returned).
- Clients **MAY** use this header to determine when they have caught up and can transition to live tailing mode.
- **Important:** `Stream-Up-To-Date: true` does **NOT** imply EOF. More data may be appended in the future. Only `Stream-Closed: true` indicates that no more data will ever arrive.
- `Stream-Closed: true`
- **MUST** be present when the stream is closed **and** the client has reached the final offset **at the time the response is generated**. This includes:
- Responses that return the final chunk of data, when the stream is already closed at response generation time, or
- Responses with an empty body when the requested offset equals the tail offset of a closed stream (the canonical EOF signal).
- When present, clients can conclude that no more data will ever be appended and treat this as EOF.
- **SHOULD NOT** be present when returning partial data from a closed stream (when more data exists between the response and the final offset). In this case, `Stream-Closed: true` will be returned on a subsequent request that reaches the final offset.
- **Timing note:** If a stream is closed **after** the final chunk was served (or cached), that chunk will not include `Stream-Closed: true`. Clients discover closure by requesting the next offset (`Stream-Next-Offset` from the previous response), which returns an empty body with `Stream-Closed: true`. This is the expected flow when closure occurs between chunk responses or when serving cached chunks.
- Clients that need to know closure status before reaching the tail **SHOULD** use `HEAD` (see Section 5.5).
#### Response Body
- Bytes from the stream starting at the specified offset, up to a server-defined maximum chunk size.
### 5.7. Read Stream - Live (Long-poll)
#### Request
```
GET {stream-url}?offset=&live=long-poll[&cursor=]
```
Where `{stream-url}` is the URL of the stream. If no data is available at the specified offset, the server waits up to a timeout for new data to arrive. This enables efficient live tailing without constant polling.
#### Query Parameters
- `offset` (required)
- The offset to read from. **MUST** be provided.
- `live=long-poll` (required)
- Indicates long-polling mode.
- `cursor` (optional)
- Echo of the last `Stream-Cursor` header value from a previous response.
- Used for collapsing keys in CDN/proxy configurations.
#### Response Codes
- `200 OK`: Data became available within the timeout
- `204 No Content`: Timeout expired with no new data
- `400 Bad Request`: Invalid parameters
- `404 Not Found`: Stream does not exist
- `429 Too Many Requests`: Rate limit exceeded
#### Response Headers (on 200)
- Same as catch-up reads (Section 5.6), plus:
- `Stream-Cursor: `: Servers **MUST** include this header. See Section 8.1.
#### Response Headers (on 204)
- `Stream-Next-Offset: `: Servers **MUST** include a `Stream-Next-Offset` header indicating the current tail offset.
- `Stream-Up-To-Date: true`: Servers **MUST** include this header to indicate the client is caught up with all available data.
- `Stream-Cursor: `: Servers **MUST** include this header when the stream is open. Servers **MAY** omit this header when `Stream-Closed` is true (cursor is unnecessary when no further polling is expected). Clients **MUST** tolerate its absence when `Stream-Closed` is present. See Section 8.1.
- `Stream-Closed: true`: **MUST** be present when the stream is closed (see Section 5.6 for semantics). A `204 No Content` with `Stream-Closed: true` indicates EOF.
**EOF Signaling Across Modes:**
Clients should treat **either** of the following as EOF, depending on the mode used:
- **Catch-up mode**: `200 OK` with empty body and `Stream-Closed: true`
- **Long-poll mode**: `204 No Content` with `Stream-Closed: true`
- **SSE mode**: `control` event with `streamClosed: true`
In all cases, `Stream-Closed` / `streamClosed` is the definitive EOF signal. The presence of `Stream-Up-To-Date` / `upToDate` alone does **not** indicate EOF - it only means the client has caught up with currently available data, but more may arrive.
#### Stream Closure Behavior in Long-poll Mode
When the stream is closed and the client is already at the tail offset:
- Servers **MUST NOT** wait for the long-poll timeout
- Servers **MUST** immediately return `204 No Content` with `Stream-Closed: true` and `Stream-Up-To-Date: true`
This ensures clients observing a closed stream do not have hanging connections waiting for data that will never arrive.
#### Response Body (on 200)
- New bytes that arrived during the long-poll period.
#### Timeout Behavior
The timeout for long-polling is implementation-defined. Servers **MAY** accept a `timeout` query parameter (in seconds) as a future extension, but this is not required by the base protocol.
### 5.8. Read Stream - Live (SSE)
#### Request
```
GET {stream-url}?offset=&live=sse
```
Where `{stream-url}` is the URL of the stream. Returns data as a Server-Sent Events (SSE) stream.
SSE mode supports all content types. Servers **MUST** include `stream-data-content-type` to identify the decoded `event: data` payload type. For `application/json` streams, this value is `application/x-ndjson`. For all other streams, this value is the stream's configured content type. For streams with `content-type: text/*` or `application/json`, data events carry UTF-8 text directly. For streams with any other `content-type` (binary streams), servers **MUST** automatically base64-encode data events and include the response header `stream-sse-data-encoding: base64`.
SSE responses **MUST** use `Content-Type: text/event-stream` in the HTTP response headers.
Clients **MUST** check `stream-sse-data-encoding` before interpreting `event: data`. If it is `base64`, clients decode the event data first, then interpret the decoded bytes according to `stream-data-content-type`. For `application/x-ndjson` SSE payloads, `data:` lines are transport lines, not record boundaries; clients MUST buffer until newline before parsing records.
#### Query Parameters
- `offset` (required)
- The offset to start reading from.
- `live=sse` (required)
- Indicates SSE streaming mode.
- `max_bytes` (optional)
- Maximum bytes to read in one data batch before SSE framing. For non-base64 UTF-8 data, servers **MUST NOT** emit invalid UTF-8; if a read window ends inside a code point, servers shorten the emitted payload and set `streamNextOffset` to the offset just past the emitted data. For `application/x-ndjson` SSE payloads, servers **SHOULD** prefer ending at the last complete newline-delimited record when one is available, but **MAY** emit a partial record to make progress.
#### Response Codes
- `200 OK`: Streaming body (SSE format)
- `400 Bad Request`: Invalid parameters
- `404 Not Found`: Stream does not exist
- `429 Too Many Requests`: Rate limit exceeded
#### Response Format
Data is emitted in [Server-Sent Events format](https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#event_stream_format).
**Events:**
- `data`: Emitted for each batch of data
- Each line prefixed with `data:`
- The decoded data payload type is identified by the response header `stream-data-content-type`.
- `data:` lines are transport framing and are not guaranteed to align with application message boundaries.
- For binary streams (where `stream-sse-data-encoding: base64` is present), the `data` event payload represents bytes encoded using standard base64 per [RFC 4648](https://www.rfc-editor.org/rfc/rfc4648) (alphabet: A-Z, a-z, 0-9, +, /).
- Servers **MAY** split the base64 text across multiple `data:` lines within the same SSE `data` event.
- Clients **MUST** concatenate the `data:` lines for the event (per SSE rules) and **MUST** remove all `\n` and `\r` characters inserted between lines before base64-decoding.
- The resulting string (after removing `\n` and `\r`) **MUST** be valid base64 text with length that is a multiple of 4 (or empty).
- If a `data` event's byte payload length is 0, the base64 text **MUST** be the empty string.
- Base64 encoding affects only `event: data` payloads. `event: control` events remain JSON as specified and are not encoded.
- When the stream content type is `application/json`, implementations **MAY** batch multiple logical messages into a single SSE `data` event by streaming newline-delimited JSON across multiple `data:` lines, as in the example below.
- `control`: Emitted after every data event
- **MUST** include `streamNextOffset`. See Section 8.1.
- **MUST** include `streamCursor` when the stream is open. Servers **MAY** omit `streamCursor` when `streamClosed` is true (cursor is unnecessary when no reconnection is expected).
- **MUST** include `upToDate: true` when the client is caught up with all available data. Note: `streamClosed: true` implies `upToDate: true` (a closed stream at the final offset is by definition up-to-date), so `upToDate` **MAY** be omitted when `streamClosed` is true.
- **MUST** include `streamClosed: true` when the stream is closed and all data up to the final offset has been sent.
- Format: JSON object with offset, cursor (when applicable), up-to-date status, and optionally closed status. Field names use camelCase: `streamNextOffset`, `streamCursor`, `upToDate`, and `streamClosed`.
**Example (normal data):**
```
event: data
data: {"k":"v"}
data: {"k":"w"}
event: control
data: {"streamNextOffset":"123456_789","streamCursor":"abc"}
```
**Example (final data with stream closure):**
```
event: data
data: {"k":"final"}
event: control
data: {"streamNextOffset":"123456_999","streamClosed":true}
```
Note: `streamCursor` is omitted when `streamClosed` is true, since clients must not reconnect after receiving a closed signal.
**Client Compatibility:** Clients **MUST** tolerate the absence of `streamCursor` (in SSE) and `Stream-Cursor` (in HTTP headers) when `streamClosed` / `Stream-Closed` is present. Implementations that assume cursor is always present will break when processing closed stream responses.
#### Stream Closure Behavior in SSE Mode
When the stream is closed:
- The final `control` event **MUST** include `streamClosed: true`
- After emitting the final control event, servers **MUST** close the SSE connection
- Clients receiving `streamClosed: true` **MUST NOT** attempt to reconnect, as no more data will arrive
If the stream is already closed when an SSE connection is established and the client's offset is at the tail:
- Servers **MUST** immediately emit a `control` event with `streamClosed: true` and `upToDate: true`
- Servers **MUST** then close the connection
**Example (binary stream with automatic base64 encoding):**
```
event: data
data: AQIDBAUG
data: BwgJCg==
event: control
data: {"streamNextOffset":"123456_789","streamCursor":"abc"}
```
#### Connection Lifecycle
- Server **SHOULD** close connections roughly every ~60 seconds to enable CDN collapsing
- Client **MUST** reconnect using the last received `streamNextOffset` value from the control event
- Client **MUST NOT** reconnect if the last control event included `streamClosed: true`
## 6. Offsets
Offsets are opaque tokens that identify positions within a stream. They have the following properties:
1. **Opaque**: Clients **MUST NOT** interpret offset structure or meaning
2. **Lexicographically Sortable**: For any two valid offsets for the same stream, a lexicographic comparison determines their relative position in the stream. Clients **MAY** compare offsets lexicographically to determine ordering.
3. **Persistent**: Offsets remain valid for the lifetime of the stream (until deletion or expiration)
4. **Unique**: Each offset identifies exactly one position in the stream. No two positions **MAY** share the same offset.
5. **Strictly Increasing**: Offsets assigned to appended data **MUST** be lexicographically greater than all previously assigned offsets. Server implementations **MUST NOT** use schemes (such as raw UTC timestamps) that can produce duplicate or non-monotonic offsets. Time-based identifiers like ULIDs, which combine timestamps with random components to guarantee uniqueness and monotonicity, are acceptable.
**Format**: Offset tokens are opaque, case-sensitive strings. Their internal structure is implementation-defined. Offsets are single tokens and **MUST NOT** contain `,`, `&`, `=`, `?`, or `/` (to avoid conflict with URL query parameter syntax). Servers **SHOULD** use URL-safe characters to avoid encoding issues, but clients **MUST** properly URL-encode offset values when including them in query parameters. Servers **SHOULD** keep offsets reasonably short (under 256 characters) since they appear in every request URL.
**Sentinel Values**: The protocol defines two special offset sentinel values:
- **`-1` (Stream Beginning)**: The special offset value `-1` represents the beginning of the stream. Clients **MAY** use `offset=-1` as an explicit way to request data from the start. This is semantically equivalent to omitting the offset parameter. Servers **MUST** recognize `-1` as a valid offset that returns data from the beginning of the stream.
- **`now` (Current Tail Position)**: The special offset value `now` allows clients to skip all existing data and begin reading from the current tail position. This is useful for applications that only care about future data (e.g., presence tracking, live monitoring, late joiners to a conversation). The behavior varies by read mode:
**Catch-up mode** (`offset=now` without `live` parameter):
- Servers **MUST** return `200 OK` with an empty response body.
- Servers **MUST** include a `Stream-Next-Offset` header set to the current tail position
- Servers **MUST** include `Stream-Up-To-Date: true` header
- Servers **SHOULD** return `Cache-Control: no-store` to prevent caching of the tail offset
- The response **MUST** contain no data messages, regardless of stream content
**Long-poll mode** (`offset=now&live=long-poll`):
- Servers **MUST** immediately begin waiting for new data (no initial empty response)
- This eliminates a round-trip: clients can subscribe to future data in a single request
- If new data arrives during the wait, servers return `200 OK` with the new data
- If the timeout expires, servers return `204 No Content` with `Stream-Up-To-Date: true`
- The `Stream-Next-Offset` header **MUST** be set to the tail position
**SSE mode** (`offset=now&live=sse`):
- Servers **MUST** immediately begin the SSE stream from the tail position
- The first control event **MUST** include the tail offset in `streamNextOffset`
- If no data has arrived, the first control event **MUST** include `upToDate: true`
- If data arrives before the first control event, `upToDate` reflects the current state
- No historical data is sent. Only future data events are streamed
**Closed streams** (`offset=now` on a closed stream):
- Regardless of the `live` parameter, servers **MUST** return immediately with the closure signal
- The response **MUST** include `Stream-Closed: true` and `Stream-Up-To-Date: true` headers
- The `Stream-Next-Offset` header **MUST** be set to the stream's final (tail) offset
- For catch-up mode: `200 OK` with empty body
- For long-poll mode: `204 No Content` (no waiting, immediate return)
- For SSE mode: The first (and only) control event includes `streamClosed: true` and `upToDate: true`, then the connection closes
- This ensures clients using `offset=now` can immediately discover that a stream has no future data
**Reserved Values**: The sentinel values `-1` and `now` are reserved by the protocol. Server implementations **MUST NOT** generate these strings as actual stream offsets (in `Stream-Next-Offset` headers or SSE control events). This ensures clients can always distinguish between sentinel requests and real offset values.
The opaque nature of offsets enables important server-side optimizations. For example, offsets may encode chunk file identifiers, allowing catch-up requests to be served directly from object storage without touching the main database.
Clients **MUST** use the `Stream-Next-Offset` value returned in responses for subsequent read requests. They **SHOULD** persist offsets locally (e.g., in browser local storage or a database) to enable resumability after disconnection or restart.
## 7. Content Types
The protocol supports arbitrary MIME content types. Most content types operate at the byte level, leaving message framing and interpretation to clients. The `application/json` content type has special semantics defined below.
**SSE Encoding:**
- SSE mode (Section 5.8) supports all content types. For streams with `content-type: text/*` or `application/json`, data events carry UTF-8 text natively. For all other content types, servers automatically base64-encode data events (see Section 5.8).
Clients **MAY** use any content type for their streams, including:
- `application/json` for JSON mode with message boundary preservation
- `application/ndjson` for newline-delimited JSON
- `application/x-protobuf` for Protocol Buffer messages
- `text/plain` for plain text
- Custom types for application-specific formats
### 7.1. JSON Mode
Streams created with `Content-Type: application/json` have special semantics for message boundaries and batch operations.
#### Message Boundaries
For `application/json` streams, servers **MUST** preserve message boundaries by storing each normalized message as one newline-delimited JSON record. `HEAD` responses report the configured stream content type (`application/json`). GET responses **MUST** return those records as newline-delimited JSON with `Content-Type: application/x-ndjson`, because that is the read representation.
#### Array Flattening for Batch Operations
When a POST request body contains a JSON array, servers **MUST** flatten exactly one level of the array, treating each element as a separate message. This enables clients to batch multiple messages in a single HTTP request while preserving individual message semantics.
**Examples (direct POST to server):**
- POST body `{"event": "created"}` stores one message: `{"event": "created"}`
- POST body `[{"event": "a"}, {"event": "b"}]` stores two messages: `{"event": "a"}`, `{"event": "b"}`
- POST body `[[1,2], [3,4]]` stores two messages: `[1,2]`, `[3,4]`
- POST body `[[[1,2,3]]]` stores one message: `[[1,2,3]]`
**Note:** Client libraries **MAY** automatically wrap individual values in arrays for batching. For example, a client calling `append({"x": 1})` might send POST body `[{"x": 1}]` to the server, which flattens it to store one message: `{"x": 1}`.
#### Empty Arrays
Servers **MUST** reject POST requests containing empty JSON arrays (`[]`) with `400 Bad Request`. Empty arrays in append operations represent no-op operations with no semantic meaning and likely indicate a client bug.
PUT requests with an empty array body (`[]`) are valid and create an empty stream. The empty array simply means no initial messages are being written.
#### JSON Validation
Servers **MUST** validate that appended data is valid JSON. If validation fails, servers **MUST** return `400 Bad Request` with an appropriate error message.
#### Response Format
GET responses for `application/json` streams **MUST** return `Content-Type: application/x-ndjson` with a body containing newline-delimited JSON messages from the requested offset range. `max_bytes` applies to the encoded byte stream and MAY end a response mid-record; clients MUST resume from `Stream-Next-Offset` and buffer incomplete trailing records before parsing.
```http
HTTP/1.1 200 OK
Content-Type: application/x-ndjson
{"event":"created"}
{"event":"updated"}
```
If no messages exist in the range, servers **MUST** return an empty body.
## 8. Caching and Collapsing
### 8.1. Catch-up and Long-poll Reads
For **shared, non-user-specific streams**, servers **SHOULD** return:
```
Cache-Control: public, max-age=60, stale-while-revalidate=300
```
For **streams that may contain user-specific or confidential data**, servers **SHOULD** use `private` instead of `public` and rely on CDN configurations that respect `Authorization` or other cache keys:
```
Cache-Control: private, max-age=60, stale-while-revalidate=300
```
This enables CDN/proxy caching while allowing stale content to be served during revalidation.
**Caching and Stream Closure:**
Catch-up chunks remain fully cacheable, including chunks at the tail of the stream. When a chunk is returned, it may or may not be the final chunk - this is unknown until the client requests the next offset.
The closure signal is discovered when the client requests the offset **after** the final data:
1. Client reads data and receives `Stream-Next-Offset: X` (the tail offset)
2. Client requests offset `X`
3. If stream is closed: server returns `200 OK` with **empty body** and `Stream-Closed: true`
4. If stream is open: server returns `200 OK` with empty body and `Stream-Up-To-Date: true` (or long-poll/SSE waits for data)
This design ensures:
- All data chunks are cacheable (a chunk that later becomes "final" was still valid data)
- The closure signal is a distinct request/response at the tail offset
- Cached chunks never become "stale" due to closure - clients simply make one more request to discover EOF
**ETag Usage:**
Servers **MUST** generate `ETag` headers for GET responses, except for `offset=now` responses. Clients **MAY** use `If-None-Match` with the `ETag` value on repeat catch-up requests. When a client provides a valid `If-None-Match` header that matches the current ETag, servers **MUST** respond with `304 Not Modified` (with no body) instead of re-sending the same data. This is essential for fast loading and efficient bandwidth usage.
**ETag and Stream Closure:** ETags **MUST** vary with the stream's closure status. When a stream is closed (without new data being appended), the ETag **MUST** change to ensure clients do not receive `304 Not Modified` responses that would hide the closure signal. Implementations **SHOULD** include a closure indicator in the ETag format (e.g., appending `:c` to the ETag when the stream is closed).
**Query Parameter Ordering:**
For optimal cache behavior, clients **SHOULD** order query parameters lexicographically by key name. This ensures consistent URL serialization across implementations and improves CDN cache hit rates.
**Collapsing:**
Clients **SHOULD** echo the `Stream-Cursor` value as `cursor=` in subsequent long-poll requests. This, along with the appropriate `Cache-Control` header, enables CDNs and proxies to collapse multiple clients waiting for the same data into a single upstream request.
**Server-Generated Cursors:**
To prevent infinite CDN cache loops (where clients receive the same cached empty response indefinitely), servers **MUST** generate cursors on all live mode responses:
- **Long-poll**: `Stream-Cursor` response header
- **SSE**: `streamCursor` field in `control` events
The cursor mechanism works as follows:
1. **Interval-based Calculation**: Servers divide time into fixed intervals (default: 20 seconds) counted from an epoch (default: October 9, 2024 00:00:00 UTC). The cursor value is the interval number as a decimal string.
2. **Cursor Generation**: For each live response, the server calculates the current interval number and returns it as the cursor value.
3. **Monotonic Progression**: Servers **MUST** ensure cursors never go backwards. When a client provides a `cursor` query parameter that is greater than or equal to the current interval number, the server **MUST** return a cursor strictly greater than the client's cursor (by adding random jitter of 1-3600 seconds). This guarantees monotonic progression and prevents cache cycles.
4. **Client Behavior**: Clients **MUST** include the received cursor value as the `cursor` query parameter in subsequent requests. This creates different cache keys as time progresses, ensuring CDN caches eventually expire.
**Example Cursor Flow:**
```
# Client makes initial long-poll request
GET /stream?offset=123&live=long-poll
# Server returns cursor based on current interval (e.g., interval 1000)
< Stream-Cursor: 1000
# Client echoes cursor on next request
GET /stream?offset=123&live=long-poll&cursor=1000
# If still in same interval, server adds jitter and returns advanced cursor
< Stream-Cursor: 1050
```
**Long-poll Caching:**
CDNs and proxies **SHOULD NOT** cache `204 No Content` responses from long-poll requests in most cases. Long-poll `200 OK` responses are safe to cache when keyed by `offset`, `cursor`, and authentication credentials.
### 8.2. SSE
SSE connections **SHOULD** be closed by the server approximately every 60 seconds. This enables new clients to collapse onto edge requests rather than maintaining long-lived connections to origin servers.
## 9. Extensibility
The Durable Streams Protocol is designed to be extended for specific use cases and implementations. Extensions **SHOULD** be pure supersets of the base protocol, ensuring compatibility with any client that implements the base protocol.
### 9.1. Protocol Extensions
Implementations **MAY** extend the protocol with additional query parameters, headers, or response fields to support domain-specific semantics. For example, a database synchronization implementation might add query parameters to filter by table or schema, or include additional metadata in response headers.
Extensions **SHOULD** follow these principles:
- **Backward Compatibility**: Extensions **MUST NOT** break base protocol semantics. Clients that do not understand extension parameters or headers **MUST** be able to operate using only base protocol features.
- **Pure Superset**: Extensions **SHOULD** be additive only. New parameters and headers **SHOULD** be optional, and servers **SHOULD** provide sensible defaults or fallback behavior when extensions are not used.
- **Version Independence**: Extensions **SHOULD** work with any version of a client that implements the base protocol. Extension negotiation **MAY** be handled through headers or query parameters, but base protocol operations **MUST** remain functional without extension support.
### 9.2. Authentication Extensions
See Section 10.1 for authentication and authorization details. Implementations **MAY** extend the protocol with authentication-related query parameters or headers (e.g., API keys, OAuth tokens, custom authentication headers).
## 10. Security Considerations
### 10.1. Authentication and Authorization
Authentication and authorization are explicitly out of scope for this protocol specification. Clients **SHOULD** implement all standard HTTP authentication primitives (e.g., Basic Authentication [RFC7617], Bearer tokens [RFC6750], Digest Authentication [RFC7616]). Implementations **MUST** provide appropriate access controls to prevent unauthorized stream creation, modification, or deletion, but may do so using any mechanism they choose, including extending the protocol with authentication-related parameters or headers as described in Section 9.2.
### 10.2. Multi-tenant Safety
If stream URLs are guessable, servers **MUST** enforce access controls even when using shared caches. Servers **SHOULD** validate and sanitize stream URLs to prevent path traversal attacks and ensure URL components are within acceptable limits.
### 10.3. Untrusted Content
Clients **MUST** treat stream contents as untrusted input and **MUST NOT** evaluate or execute stream data without appropriate validation. This is particularly important for append-only streams used as logs, where log injection attacks are a concern.
### 10.4. Content Type Validation
Servers **MUST** validate that appended content types match the stream's declared content type to prevent type confusion attacks.
### 10.5. Rate Limiting
Servers **SHOULD** implement rate limiting to prevent abuse. The `429 Too Many Requests` response code indicates rate limit exhaustion.
### 10.6. Sequence Validation
The optional `Stream-Seq` header provides protection against out-of-order writes in multi-writer scenarios. Servers **MUST** reject sequence regressions to maintain stream integrity.
### 10.7. Browser Security Headers
When serving streams to browser clients, servers **SHOULD** include the following headers to prevent MIME-sniffing attacks, cross-origin embedding exploits, and cache-related vulnerabilities:
- `X-Content-Type-Options: nosniff`
- Servers **SHOULD** include this header on all responses. This prevents browsers from MIME-sniffing the response content and potentially executing it as a different content type (e.g., interpreting binary data as HTML/JavaScript).
- `Cross-Origin-Resource-Policy: cross-origin` (or `same-origin`/`same-site`)
- Servers **SHOULD** include this header to explicitly control cross-origin embedding. Use `cross-origin` to allow cross-origin access via `fetch()`, `same-site` to restrict to the same registrable domain, or `same-origin` for strict same-origin only. This prevents Cross-Origin Read Blocking (CORB) issues and protects against Spectre-like side-channel attacks.
- `Cache-Control: no-store`
- Servers **SHOULD** include this header on HEAD responses and on responses containing sensitive or user-specific stream data. This prevents intermediate proxies and CDNs from caching potentially sensitive content. For public, non-sensitive historical reads, servers **MAY** use `Cache-Control: public, max-age=60, stale-while-revalidate=300` as described in Section 8.
- `Content-Disposition: attachment` (optional)
- Servers **MAY** include this header for `application/octet-stream` responses to prevent inline rendering if a user navigates directly to the stream URL.
These headers provide defense-in-depth for scenarios where stream URLs might be accessed outside the intended programmatic fetch context (e.g., direct navigation, malicious cross-origin embedding via ` 
Tonbo.
Ursula is a self-hosted, distributed server for the replayable, append-only event timelines behind document edits, agent runs, workflows, and chat. It speaks the [Durable Streams Protocol](/docs/0.7/specs/durable-stream) over plain HTTP and SSE.
Quorum-replicated in-memory front-ends give you single-digit-millisecond appends and live tail on top of S3 durability, with no separate broker, no batched 250 ms commits, and no S3 Express bill.
- **HTTP-native.** `PUT` creates, `POST` appends, `GET` replays or tails with long-poll/SSE. Any HTTP client is a valid client.
- **One timeline per resource.** A stream per document, session, workflow, room, or agent run, instead of a few high-throughput pipelines.
- **Thread-per-core × multi-Raft.** Each stream hashes to one Raft group and one owner core. Hot-path requests touch that core only, with no cross-core synchronization. Hundreds to thousands of small groups co-exist per node, so a slow follower for one group never stalls another.
- **Hot ring + S3 cold tier.** Writes commit at Raft quorum in an in-memory ring. Older segments flush to S3 in the background. A single `GET` transparently spans both tiers.
## Try it
Start a single throwaway node with Docker (or build from source, see [Install](/docs/0.7/install)):
```bash
docker run --rm -p 4437:4437 ghcr.io/tonbo-io/ursula:0.7.6
```
Create a stream, append bytes, and read them back:
```bash
curl -X PUT http://127.0.0.1:4437/demo
curl -X PUT http://127.0.0.1:4437/demo/hello
curl -X POST http://127.0.0.1:4437/demo/hello \
-H 'Content-Type: application/octet-stream' \
--data-binary 'hello world'
curl 'http://127.0.0.1:4437/demo/hello?offset=-1'
```
Continue with the [Quick Start](/docs/0.7/quick-start) to replay and tail streams live, and [Deploy a Cluster](/docs/0.7/deploy-cluster) when you're ready for the production three-voter shape.
## Learn more
- **[Why Ursula](/docs/0.7/why-ursula):** the four properties Ursula keeps that other servers force you to trade
- **[Architecture](/docs/0.7/architecture/overview):** thread-per-core, multi-Raft, hot/cold tiers
- **[Protocol Spec](/docs/0.7/specs/durable-stream):** Durable Streams plus [Ursula's extensions](/docs/0.7/specs/extensions)
- **[ursulactl](/docs/0.7/cli):** the operator CLI for a running cluster (leadership drain, status, readiness gates)
## Credits
- **[ElectricSQL](https://electric-sql.com/)** for the original Durable Streams Protocol that Ursula implements.
- **[Loro](https://loro.dev/)** for the snapshot and replay extension design that Ursula adopted on top of the base protocol.
---
# Why Ursula
## What Ursula keeps
A new generation of event streams lives outside the broker network. Document editors, agents, and durable workflows need timelines that browsers, mobile apps, and serverless functions can read, write, and tail over the public internet. That asks for HTTP-native, distributed, S3-backed infrastructure, not the SDK-locked, single-network shape Kafka-style brokers were built for.
The [Durable Streams Protocol](/docs/0.7/specs/durable-stream) nails that wire format, but its reference server is a single process: a node loss is data loss. The other servers we evaluated each force you to give up one of four things this primitive deserves to keep:
- **Open-source self-hosting.**
- **Low write latency** (sub-50 ms appends, no batching window required).
- **Plain S3 economics** (cold tier on standard S3, no S3 Express tier, no per-GB SaaS markup).
- **Quorum-replicated durability** (acknowledged writes survive a single-node failure).
Ursula keeps all four. For the head-to-head numbers, see [How Ursula compares](/docs/0.7/competitive-comparison).
## One timeline per entity
Most event systems multiplex many entities into shared channels and rebuild per-entity state downstream. Ursula inverts that: each document, session, task, room, or agent run gets its own durable timeline. Writers append directly to it and readers resume from its offsets.
Treat it as a per-entity durable log runtime, purpose-built for that pattern rather than a general event backbone.
## What you get from one stream per entity
- **Replayable recovery.** When a worker, sandbox, or agent restarts, replay its log and continue.
- **Live tails with simple clients.** Catch-up reads, long-poll, or SSE, all over plain HTTP.
- **Lifecycle in the same primitive.** Snapshots, bootstrap, and TTL ride the timeline instead of being separate infrastructure.
## The tradeoff
Ursula makes one timeline per entity cheap. In exchange, it isn't the most general primitive for cross-system event distribution or arbitrary stream processing. Keeping the per-entity model simple is what makes recovery, inspection, and operation tractable for long-running application state.
For latency and durability numbers, see [Architecture](/docs/0.7/architecture/overview) and [Competitive comparison](/docs/0.7/competitive-comparison).
---
# Install
This page gets a **single Ursula node** running on your machine so you can try the API. There are two ways to do it:
| Pick | When |
| ---- | ---- |
| **Docker** | You just want to try Ursula. One command, no toolchain. |
| **Build from source** | You want local binaries, plan to hack on Ursula, or can't run Docker. |
Both start a single, non-replicated node on port `4437` that keeps nothing across restarts. The source build runs the in-memory engine. The Docker image runs a single Raft node whose WAL lives in a temporary directory inside the container. For a production cluster, see [Deploy a Cluster](/docs/0.7/deploy-cluster).
## Docker
Every release publishes a multi-arch image to GHCR:
```bash
docker run --rm -p 4437:4437 ghcr.io/tonbo-io/ursula:0.7.6
```
The image runs as a non-root user and contains both `ursula` (the server) and `ursulactl` (the operator CLI). Images are published only by the reviewed stable-release workflow, so pin the version you want to run.
## Build from source
You need rustup, a C compiler, and `pkg-config`. The repository pins the toolchain in `rust-toolchain.toml`, so rustup selects the right nightly automatically.
### Installing the prerequisites (macOS, Debian/Ubuntu)
On macOS:
```bash
brew install rustup pkg-config
rustup-init -y
```
On Debian or Ubuntu:
```bash
sudo apt-get update
sudo apt-get install -y build-essential pkg-config curl
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
```
Clone, build, and start the server:
```bash
git clone https://github.com/tonbo-io/ursula.git
cd ursula
cargo build --release -p ursula -p ursula-ctl
./target/release/ursula server
```
The server is now listening on `127.0.0.1:4437`. The build also produces `target/release/ursulactl`, the operator CLI you will use once a cluster is up.
## Verify
With the node running, from another terminal:
```bash
curl http://127.0.0.1:4437/__ursula/metrics
```
A JSON snapshot of runtime state means the node is up.
## Next
- [Quick Start](/docs/0.7/quick-start): create streams, append, replay, and tail live over SSE
- [Deploy a Cluster](/docs/0.7/deploy-cluster): the production three-voter shape with Helm
- [Configuration](/docs/0.7/configuration): persistence, presets, and S3 cold storage
---
# Quick Start
This page assumes a single node is already running on `127.0.0.1:4437`. Starting one is a single Docker or Cargo command, see [Install](/docs/0.7/install). The default node keeps nothing across restarts. For disk-backed persistence, set `raft.wal.path` (see [Configuration](/docs/0.7/configuration)).
Drive the HTTP API with `curl` from another terminal.
## Validate the bucket ID (optional)
Buckets are implicit: a bucket exists once a stream is created in it, so this call is optional. It only validates the bucket ID (`201`, or `400` for an invalid one):
```bash
curl -X PUT http://127.0.0.1:4437/demo
```
## Create a stream
```bash
curl -X PUT http://127.0.0.1:4437/demo/hello
```
## Append data
```bash
curl -X POST http://127.0.0.1:4437/demo/hello \
-H 'Content-Type: application/octet-stream' \
--data-binary 'first message'
curl -X POST http://127.0.0.1:4437/demo/hello \
-H 'Content-Type: application/octet-stream' \
--data-binary 'second message'
```
Each successful append returns `204 No Content` with a `Stream-Next-Offset` header. Add `Producer-Id` / `Producer-Epoch` / `Producer-Seq` if you need [exactly-once retries](/docs/0.7/concepts/exactly-once-writes).
## Read everything from the beginning
```bash
curl -i 'http://127.0.0.1:4437/demo/hello?offset=-1'
```
The body contains the appended bytes. Response headers include `Stream-Next-Offset`, `Stream-Up-To-Date`, and an `ETag`.
## Subscribe for live updates
Open a second terminal:
```bash
curl -N 'http://127.0.0.1:4437/demo/hello?offset=-1&live=sse'
```
`-N` keeps `curl` from line-buffering the SSE stream. Append more data from the first terminal and you'll see it arrive immediately as `event: data` lines. Binary streams are delivered as raw base64 text in `event: data` payloads (`Stream-Sse-Data-Encoding: base64`). See [binary SSE](/docs/0.7/concepts/binary-sse) for details.
## Inspect runtime state
For a multi-node cluster the canonical day-2 tool is [`ursulactl`](/docs/0.7/cli). It speaks to every node, summarises leadership, and wraps restarts in safe drain and catch-up steps. For a single local node you can either point it at a one-line manifest:
```bash
cat > /tmp/local.json <<'JSON'
{"nodes": [{"id": 1, "http_url": "http://127.0.0.1:4437", "host": "127.0.0.1"}]}
JSON
ursulactl status --config /tmp/local.json
```
Or hit the underlying JSON endpoint directly:
```bash
curl http://127.0.0.1:4437/__ursula/metrics
```
The raw endpoint is also what `ursulactl` consumes. Use it directly when you want the full snapshot or are building custom tooling.
## Next steps
- [Deploy a Cluster](/docs/0.7/deploy-cluster): the production three-voter shape with Helm
- [Configuration](/docs/0.7/configuration): persistence, presets, and S3 cold storage in one place
- [ursulactl](/docs/0.7/cli): the operator CLI you'll use once a cluster is up
- [API overview](/docs/0.7/api/overview): the HTTP surface Ursula currently exposes
- [Streams](/docs/0.7/concepts/streams): the core stream abstraction and lifecycle
- [Architecture](/docs/0.7/architecture/overview): thread-per-core, multi-Raft internals
---
# Clients
Ursula speaks plain HTTP and Server-Sent Events. There is no required client library. Any HTTP client in any language works.
The examples elsewhere in these docs use `curl` because it's universal. The same routes, headers, and query parameters apply to every other client.
## Minimal examples