Idempotent

An operation that produces the same end result no matter how many times it's performed, so retrying it is always safe.

Mental model: Like pressing an already-lit elevator button — pressing it again doesn't call five elevators.

Example: A payment API where retrying a failed request with the same idempotency key charges the customer only once.

In practice: APIs typically implement this by having the client attach a unique idempotency key, so a retried request with the same key is recognized and safely ignored instead of reapplied.

• often confused with Stateless