Dead Letter Queue

1. The Incident

Your order fulfillment pipeline runs on a message queue. Checkout publishes an event, a consumer picks it up, reserves inventory, and notifies the warehouse. It processes 200 messages a second, quietly, for months.

At 2:47 AM, a partner integration publishes an order event with a null shipping address. Your consumer reads it, throws an exception, and does not acknowledge. To acknowledge is to tell the queue "done, remove this message." The queue does exactly what at-least-once delivery promises: it redelivers