At-Least-Once vs Exactly-Once

Posted Aug 15, 2026 · 1 view

A broker that retries on an un-acknowledged message will sometimes deliver the same message twice — the acknowledgement was lost, not the work. That is at-least-once, and it is what most queues give you.

Exactly-once is what everyone wants and nobody gets for free across a network. What is actually built is at-least-once delivery plus an idempotent consumer: a deduplication key, or an operation that produces the same result however many times it runs.

The animation ends on a counter holding the wrong number — the duplicate was processed. The cause here is redelivery, not concurrency.