Idempotent operation
Also: retry-safe operation
An operation whose repeated application has the same intended effect as applying it once.
Idempotency simplifies retries when acknowledgments can be lost; sequence numbers may still be needed for ordering and freshness.
Example “Set output to zero” is idempotent; “increase output by one” is not.
Common misconception Idempotent does not mean side-effect free or immune to stale commands.