What belongs into a DLQ / Invalid Message Queue?

Question

Is there a good best practice about what kind of messages an application is allowed to reject?

My understanding is that all messages which can't be handled should be rejected to the dead letter queue - no matter if the problem is a syntax error or a semantic error in the message or if the application is temporarily not able to handle the message (for instance because the db just went down).

Of course - if the app already knows upfront that it will not be able to handle a message (DB down), it should stop accepting messages.

So what's the common understanding / best practice?

Answer 1

My response is with respect to WebSphere MQ:

A Dead Letter Queue (DLQ for short) is a place where messages that could not be delivered to their destination are put. Messages can be put on the DLQ by queue managers, message channel agents (MCAs), and applications. All messages on the DLQ must be prefixed with a dead-letter header structure, MQDLH. The MQDLH header is automatically fixed when queue manager or MCAs put messages whereas applications must prefix the MQDLH explicitly.

As far applications are concerned, if they are unable to handle the message, say for example the message format is not understood, they can put the message to a BACKOUT queue instead of a DLQ. A BACKOUT queue is just like any normal queue where messages rejected by applications can be put. The advantage of BACKOUT queue is that you can specify a BACKOUT queue on a per queue basis and the messages put there need not have MQDLH header prefixed.

An application can be written to read the messages from BACKOUT and route them back to the target queue as it is. However the messages in a DLQ require additional processing to remove the MQDLH before they are put onto a target queue.