RabbitMQ DLX

This post analyzes RabbitMQ’s DLX (Dead Letter Exchange).

1. RabbitMQ DLX (Dead Letter Exchange)

RabbitMQ DLX (Dead Letter Exchange) is a feature that sends Messages that have become Dead Letters to a designated Exchange. There are the following 3 conditions under which a Message becomes a Dead Letter.

• A Message rejected through a reject/nack response by a Consumer configured with Requeue False.
  • A Message rejected through a reject/nack response by a Consumer configured with Requeue True is requeued to the Queue where the Message existed, and the DLX feature does not operate.
• A Message whose Per-message TTL (Time to Leave) has expired.
• A Message dropped because the Queue is full.

[Figure 1] shows the processing flow of RabbitMQ’s DLX feature when a Consumer rejects a Message. Exchange A and Exchange B are bound to Queue A. Queue A is configured with Exchange B as its DLX. Dead Letter Routing Key is an option that sets the Routing Key of the Message sent to the DLX. The Dead Letter Routing Key of Queue A is set to the string ssup2. The Dead Letter Routing Key does not necessarily have to be set on a Queue and can be set as needed. The Message processing flow is as follows.

• The Message sent by the Producer to the Consumer passes through Exchange A and Queue A and is delivered to the Consumer.
• The Consumer rejects the Message received from RabbitMQ through a reject or nack response without the Requeue setting.
• The Message is sent to Exchange B, which is designated as the DLX, along with the string ssup2 as the Routing Key.
• Exchange B sends the Message to Queue A, which is bound to it.
• The Message is delivered to the Consumer again.

The x-death Header of a Message that has become a Dead Letter stores the reason the Message became a Dead Letter and related information. The main information stored in the x-death Header is as follows.

• reason : The reason the Message became a Dead Letter.
• time : The time the Message became a Dead Letter.
• count : The number of times the Message became a Dead Letter with the same reason and the same queue.
• queue : The Queue where the Message existed before it became a Dead Letter.
• exchange : The Exchange that last processed the Message that became a Dead Letter. For a Message processed multiple times by a DLX, DLX information may be stored.

2. References

• Dead Letter Exchanges : https://www.rabbitmq.com/dlx.html