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.