At-most-once, at-least-once, and exactly-once are message delivery semantics that describe reliability guarantees in distributed systems: at-most-once means a message might be delivered zero or one times (it may be lost but never duplicated), at-least-once means each message is delivered at least one time (never lost but possibly delivered multiple times), and exactly-once means every message is delivered exactly one time with no loss or duplication.

Understanding Message Delivery Semantics

In [distributed systems](https://www.designgurus