Dead letter queue

In message queueing a dead letter queue (DLQ) is a service implementation to store messages that the messaging system cannot or should not deliver.^[1] Although implementation-specific, messages can be routed to the DLQ for the following reasons:

1. The message is sent to a queue that does not exist.^[2][3]
2. The maximum queue length is exceeded.
3. The message exceeds the size limit.
4. The message expires because it reached the TTL (time to live)^[4]
5. The message is rejected by another queue exchange.^[5]
6. The message has been read and rejected too many times.^[6]

Routing these messages to a dead letter queue enables analysis of common fault patterns and potential software problems.^[7] If a message consumer receives a message that it considers invalid, it can instead forward it an Invalid Message Channel,^[8] allowing a separation between application-level faults and delivery failures.

Queueing systems that incorporate dead letter queues include Amazon EventBridge,^[9] Amazon Simple Queue Service,^[7] Apache ActiveMQ, Google Cloud Pub/Sub,^[10] HornetQ, Microsoft Message Queuing,^[2] Microsoft Azure Event Grid and Azure Service Bus,^[11] WebSphere MQ,^[12] Solace PubSub+,^[13] Rabbit MQ,^[5] Confluent Cloud^[14] and Apache Pulsar.^[15][16]

See also

• Poison message
• Dead letter mail