The dead-letter queue (or undelivered-message queue) is the queue to which messages are sent if they cannot be routed to their correct destination.
Messages are put on this queue when they cannot be put on the destination queue. For example, because the queue does not exist, or because it is full. Dead-letter queues are also used at the sending end of a channel, for data-conversion errors.
Consider defining a dead-letter queue for each queue manager. If you do not, and the MCA is unable to put a message, it is left on the transmission queue and the channel is stopped.
Also, if fast, non-persistent messages (see Fast, nonpersistent messages) cannot be delivered, and no dead-letter queue exists on the target system, these messages are discarded.
However, using dead-letter queues can affect the sequence in which messages are delivered, and so you might choose not to use them.
You can use the USEDLQ channel attribute to determine whether the dead-letter queue is used when messages cannot be delivered. This attribute can be configured so that some functions of the queue manager use the dead-letter queue, while other functions do not. For more information about the use of the USEDLQ channel attribute on different MQSC commands, see DEFINE CHANNEL, DISPLAY CHANNEL, ALTER CHANNEL, and DISPLAY CLUSQMGR.
Use Dead-Letter Queue (USEDLQ)
This attribute determines whether the dead-letter queue (or undelivered message queue) is used when messages cannot be delivered by channels.
Possible values are:
Messages that cannot be delivered by a channel are treated as a failure. The channel either discards these messages, or the channel ends, in accordance with the setting of NPMSPEED.
If the queue manager DEADQ attribute provides the name of a dead-letter queue, then it is used, otherwise the behaviour is as for NO.
Handling poison messages in WebSphere MQ classes for JMS
A poison message is one which cannot be processed by a receiving MDB application. If a poison message is encountered, the JMS MessageConsumer and ConnectionConsumer objects can requeue it according to two queue properties, BOQNAME, and BOTHRESH.
Sometimes, a badly formatted message arrives on a queue, the receiving application cannot process the message correctly. Such a message can cause the receiving application to fail and to back out this badly formatted message. The message can then be repeatedly delivered to the input queue and repeatedly backed out by the application. These messages are known as poison messages. The JMS MessageConsumer object detects poison messages and reroutes them to an alternative destination.
The WebSphere® MQ queue manager keeps a record of the number of times that each message has been backed out. When this number reaches a configurable threshold value, the message consumer requeues the message to a named backout queue. If this requeuing fails for any reason, the message is removed from the input queue and either requeued to the dead-letter queue, or discarded.
The threshold value and the name of the back out queue are attributes of a WebSphere MQ queue. The names of the attributes are BackoutThreshold and BackoutRequeueQName.
To set the BackoutThreshold and BackoutRequeueQName attributes, issue the following MQSC command:
ALTER QLOCAL(your.queue.name) BOTHRESH(threshold value) BOQNAME(your.backout.queue.name)
If the BackoutThreshold attribute is set to a value other than zero, to avoid unexpected behavior set the BackoutRequeueQName attribute to a valid queue name.
For publish/subscribe messaging, if your system creates a dynamic queue for each subscription, these attribute values are obtained from the WebSphere MQ classes for JMS model queue, SYSTEM.JMS.MODEL.QUEUE. To alter these settings, use:
ALTER QMODEL(SYSTEM.JMS.MODEL.QUEUE) BOTHRESH(threshold value) BOQNAME(your.backout.queue.name)
If the backout threshold value is zero, poison message handling is disabled, and poison messages remain on the input queue. Otherwise, when the backout count reaches the threshold value, the message is sent to the named backout queue. If the backout count reaches the threshold value, but the message cannot go to the backout queue, the message is sent to the dead-letter queue or it is discarded. This situation occurs if the backout queue is not defined, or if the MessageConsumer object cannot send the message to the backout queue
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