Idempotency

An operation that produces the same result whether it runs once or many times.

Idempotency is the property of an operation whereby executing it multiple times produces the same state as executing it once. In distributed systems, idempotency is essential: if a message is delivered twice or a request is retried, there must be no duplicate effect — no double charge, for example. Idempotency is usually achieved through unique keys and checked state transitions.