Dead Letter Queues

1. The Poison Message Problem

The resilience story so far has covered synchronous calls, where the caller waits for an answer. Queues fail in a different shape. The trouble starts with a feature that is working exactly as designed.

Brokers give us at-least-once delivery. A broker is the service that holds the queue and hands messages to consumers. An acknowledgement (ack) is the consumer telling the broker "I finished this one, you can delete it." If no ack arrives, the broker sends the message again.

That promise is why a consumer crash and restart loses nothing.