Using the Dead Message Queue

The dead message queue, mq.sys.dmq, is a system-created physical destination that holds the dead messages of a broker's physical destinations. The dead message queue is a tool for monitoring, tuning system efficiency, and troubleshooting. For a definition of the term dead message and a more detailed introduction to the dead message queue, see the Message Queue Technical Overview.

The broker automatically creates a dead message queue when it starts. The broker places messages on the queue if it cannot process them or if their time-to-live has expired. In addition, other physical destinations can use the dead message queue to hold discarded messages. This can provide information that is useful for troubleshooting the system.

Managing the Dead Message Queue

The physical destination configuration property useDMQ controls a destination’s use of the dead message queue. Physical destinations are configured to use the dead message queue by default; to disable a destination from using it, set the destination’s useDMQ property to false:

   imqcmd update dst  -t q  -n curlyQueue  -o useDMQ=false

You can enable or disable the use of the dead message queue for all auto-created physical destinations on a broker by setting the broker’s imq.autocreate.destination.useDMQ broker property:

   imqcmd update bkr  -o imq.autocreate.destination.useDMQ=false

You can manage the dead message queue with the Message Queue Command utility (imqcmd) just as you manage other queues, but with some differences. For example, because the dead message queue is system-created, you cannot create, pause, or destroy it. Also, as shown in Table 7–2, default values for the dead message queue’s configuration properties sometimes differ from those of ordinary queues.

Table 7–2 Dead Message Queue Treatment of Physical Destination Properties

Property	Variant Treatment by Dead Message Queue
maxNumMsgs	Default value is 1000, rather than -1 (unlimited) as for ordinary queues.
maxTotalMsgBytes	Default value is 10m (10 megabytes), rather than -1 (unlimited) as for ordinary queues.
limitBehavior	Default value is REMOVE_OLDEST, rather than REJECT_NEWEST as for ordinary queues. FLOW_CONTROL is not supported for the dead message queue.
maxNumProducers	Does not apply to the dead message queue.
isLocalOnly	Permanently set to false in broker clusters; the dead message queue in a cluster is always a global physical destination.
localDeliveryPreferred	Does not apply to the dead message queue.

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Tip –

By default, the dead message queue stores entire messages. If you do not plan to restore dead messages, you can reduce the size of the dead message queue by setting the broker’s imq.destination.DMQ.truncateBody property to true:

   imqcmd update bkr  -o imq.destination.DMQ.truncateBody=true

This will discard the body of all messages and retain only the headers and property data.

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Enabling Dead Message Logging

The broker configuration property logDeadMsgs controls the logging of events related to the dead message queue. When dead message logging is enabled, the broker will log the following events:

• A message is moved to the dead message queue.

• A message is discarded from the dead message queue (or from any physical destination that does not use the dead message queue).

• A physical destination reaches its limits.

Dead message logging is disabled by default. The following command enables it:

   imqcmd update bkr  -o imq.destination.logDeadMsgs=true

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Note –

Dead message logging applies to all physical destinations that use the dead message queue. You cannot enable or disable logging for an individual physical destination.

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