Idempotency

Idempotency is the property of an operation where performing it multiple times produces the same result as performing it once. In API design, GET requests are inherently idempotent - fetching a product listing twice gives the same result as fetching it once.

Idempotency matters most when building retry logic. If a request times out, you cannot know whether the server received and processed it. For idempotent operations, retrying is safe because the duplicate request will not cause a duplicate side effect.

APIs that support state-changing operations often provide idempotency keys - a client-generated unique identifier in the request header that the server uses to detect and reject duplicate submissions, making inherently non-idempotent operations safe to retry.