Glossary

Idempotency

An idempotent operation is one that can be executed multiple times and produce the same result as the first execution. Critical for APIs and queues where a request may be retried due to a network error.

In HTTP

Idempotent: GET, PUT, DELETE, HEAD — safe to repeat
Not idempotent: POST — each call creates a new resource

Idempotency Key

For POST requests, the client generates a unique key (Idempotency-Key: uuid) and sends it in a header. The server stores the result under that key: if the same request arrives again, it returns the stored result without reprocessing. Stripe, Twilio, and other payment APIs require this.

In queues

A worker must be prepared to receive the same message twice (at-least-once delivery). Store processed IDs and check before processing.