Dead letter queue (DLQ)
A separate queue where messages land after failing processing too many times, so they are kept instead of lost or retried forever.
Most queues deliver at least once and retry on failure. A message that can never succeed (bad data, a bug) would block or loop, so after a set number of attempts it moves to the dead letter queue. From there someone can inspect it, fix the cause and replay it. Because delivery is at least once, consumers must be idempotent.
Related terms
- Idempotency (idempotency key): An operation is idempotent if doing it twice has the same effect as doing it once. Retries become safe.
- Webhook: An HTTP request one system sends to another when something happens, instead of the other system asking repeatedly.
- Runbook: A short document that tells someone else how to run a system: where it lives, how to deploy, how to recover.
