Idempotency

Summary

A request is idempotent if making it multiple times has the same effect as making it once — a critical property for safely retrying failed requests, e.g. after a network timeout.

• 1Identify which HTTP methods are idempotent by spec (GET, PUT, DELETE) and which aren't (POST).
• 2Design an idempotency-key system for a payment endpoint to prevent double-charging.
• 3Understand why retries without idempotency can cause duplicate data or charges.