System Design Patterns: From Fundamentals to Real Systems

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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 doesn't acknowledge. The queue does exactly what at-least-once delivery promises: it redelivers. The consumer throws again

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