How to Stop Charging Customers Twice: A Practical Idempotency Guide

Idempotency prevents duplicate operations like double-charging customers when retries occur. The guide covers HTTP method idempotency (GET/PUT/DELETE are safe, POST is not), idempotency keys for APIs, and message consumer patterns. The critical insight: storing the idempotency key in the same database transaction as the work — backed by a unique constraint — is what guarantees exactly-once effects. Redis can serve as a fast cache in front of that constraint, but the database is the source of truth. Common pitfalls covered include caching failures, key reuse with different payloads, TTL expiry windows, and state-dependent operations that can't be replayed.