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10.23At-most-once, at-least-once, exactly-once
This terminology often appears in messaging interviews.
At-most-once
message processed 0 or 1 times
No duplicate processing, but message may be lost.
At-least-once
message processed 1 or more times
No intended loss, but duplicates possible.
Exactly-once
processed exactly once
Very desirable, but much harder across distributed systems.