idempotency

Learn how message queues and pub/sub decouple services, why at-least-once delivery demands idempotent consumers, how backpressure and dead-letter queues protect the system, and when microservices are worth the complexity — with a sequenceDiagram of async order processing.

Why "the same request twice should produce the same result" is one of the most useful properties you can give an API, the standard patterns for implementing it (keys, dedupe tables, natural idempotency), and what goes wrong when you don't.