Idempotency Handling

This handles the tricky problem of ensuring operations don't run twice when requests get retried, which matters a lot for payments, order creation, or anything where duplicates cause real problems. The reference guides cover Redis-based storage, database implementations, and patterns modeled after Stripe's approach. You get middleware examples for Express and message queue handlers. The best practices are sensible: store the request body with the idempotency key so you can validate duplicates match, set TTLs so you're not storing this data forever, and use database constraints to handle concurrent requests atomically. If you're building any API that handles money or irreversible actions, you need something like this.