At-most-once: The call executes at most once - either it does not execute at all or it executes exactly once depending on whether the server machine goes down. Unlike the previous semantics, these semantics require the detection of duplicate packets, but work for non-idempotent operations.

What is the meaning of at most once?

As the name suggests, At-most-once means the message will be delivered at-most once. Once delivered, there is no chance of delivering again. If the consumer is unable to handle the message due to some exception, the message is lost.

What is at least once semantics?

at-least-once delivery means that for each message handed to the mechanism potentially multiple attempts are made at delivering it, such that at least one succeeds; again, in more casual terms this means that messages may be duplicated but not lost.