You can have "at most once", "exactly once", or "at least once" [0]. Each has a different tradeoff in terms of implementation complexity, speed, and other avail

You can have "at most once", "exactly once", or "at least once" [0]. Each has a different tradeoff in terms of implementation complexity, speed, and other availability/consistency concerns. For different applications differently delivery QoSes are tolerable. For instance, if your messages are idempotent (either by design or nature) then "at least once" is equivalent to "exactly once" and may be much faster/easier. Generally, "exactly once" demands the least of the application and the most of the broker. [0] I suppose you could have "any number of times" as well, but that's no guarantee at all.