What is idempotency in APIs and why does it matter?

An operation is **idempotent** if repeating it produces the same result. **Examples:** - GET is idempotent. - PUT/DELETE should be idempotent. - POST is usually not idempotent unless designed with an idempotency key. **Why it matters:** retries happen due to timeouts and network failures. Idempotency prevents double-charging, duplicate orders, and inconsistent state.