Idempotency
Idempotency means an operation produces the same effect whether executed once or many times, guaranteeing no duplicated side effects across retries and network failures.
Purpose
Make operations safely retryable, eliminating duplicate-execution risk in distributed systems.
Mechanism
f(f(x))=f(x). HTTP idempotent methods: GET/PUT/DELETE are naturally idempotent; POST is not. Idempotency keys (unique request IDs) make POST safely retryable. Payments/transfers must be idempotent.
Use cases
- Retry safety
- Payment systems
- Distributed consistency