Conditional Writes
Writers appending to the same stream sometimes need coordination. Ursula currently supports one form of optimistic concurrency control via request headers, without locks or transactions.
The Durable Streams Protocol also specifies an If-Match ETag-based precondition for multi-writer guards. Ursula does not yet implement If-Match. For current deployments use Stream-Seq or producer epoch/seq headers (exactly-once writes) for ordering and dedup.
Stream-Seq
Stream-Seq is a client-supplied monotonic sequence token. The server tracks the last accepted value per stream and rejects any append whose Stream-Seq is not lexicographically greater than the previous one.
This is useful when a single logical writer wants to enforce ordering without relying on server-side ETags. For instance, an agent that numbers its steps and wants the server to reject out-of-order delivery.
When to use
- Stream-Seq: single-writer ordering. "Reject this if my writes arrive out of order."
- Producer-Id / Producer-Epoch / Producer-Seq (exactly-once writes): deduplicate retries from a producer that may resend the same logical append after a network hiccup or restart.
- Neither: append-only workloads where every write is independent (e.g. event logging). Just POST.