How Message Queues Work: The Architecture Your Signup Endpoint Needs

A message queue is a buffer between a producer and a consumer, managed by a broker. You drop slow work off the request path — email sending, image processing, webhook delivery — and a separate worker picks it up later. Your API responds in milliseconds instead of seconds.

The mechanics that make this reliable: visibility timeouts hide a message while it's being processed, and if the worker crashes, the message reappears for retry. A dead letter queue catches poison messages after N failed attempts so they don't loop forever. Delivery is at-least-once, so consumers need to be idempotent.

The single most important metric is queue depth — messages waiting to be processed. If it's growing, your consumers can't keep up. Alert on it, auto-scale on it, and investigate when it spikes.