https://nalar.dev/tags/idempotency/ Recent content in Idempotency on Nalar Hugo en-us Tue, 01 Sep 2026 00:00:00 +0700 https://nalar.dev/idempotent-event-consumers-at-least-once-delivery/ Tue, 01 Sep 2026 00:00:00 +0700 https://nalar.dev/idempotent-event-consumers-at-least-once-delivery/ <p>Many queues and event brokers provide at-least-once delivery: a message that has been accepted can be delivered again when acknowledgements are lost, consumers crash, visibility timeouts expire, or the broker retries after uncertain outcomes.</p> <p>Duplicates are therefore not exceptional. A robust consumer should assume that the same logical event can arrive more than once.</p> <h2 id="why-duplicates-happen">Why duplicates happen</h2> <p>Consider this sequence:</p> <ol> <li>a consumer receives an event;</li> <li>it updates the database successfully;</li> <li>the process crashes before acknowledging the message;</li> <li>the broker makes the message visible again;</li> <li>another consumer receives it.</li> </ol> <p>The broker cannot know that the database update happened. Redelivery is the safer choice.</p>