What is an idempotency key?

Answer

An idempotency key is a unique client-generated identifier, often a UUID, sent with a request so the server can recognize and safely deduplicate retries of the same operation. On the first request the server processes it and stores the result keyed by that value; if a retry arrives with the same key, the server returns the original result instead of repeating the side effect. This is essential for non-idempotent operations like payments or order creation where network failures may prompt automatic retries. Keys are usually passed in a header such as `Idempotency-Key` and expire after a defined window.