Different message delivery semantics.

At-most-once delivery: each message is delivered zero or one time, implies that messages can be lost, but are never duplicated. This is the default behavior which is cheap and produces high performance.

At-least-once delivery: multiple attempts may be made to deliver each message, until at least one succeeds; messages can be duplicated but are never lost. Overhead of keeping the state at the sending end and having an acknowledgment mechanism at the receiving end

Exactly-once delivery: each message is delivered exactly once to the recipient; the message can neither be lost nor be duplicated. This may exhibit worst performance.

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