Trigger messages and units of work

 

Trigger messages created because of trigger events that are not part of a unit of work are put on the initiation queue, outside any unit of work, with no dependence on any other messages, and are available for retrieval by the trigger monitor immediately.

Trigger messages created because of trigger events that are part of a unit of work are put on the initiation queue as part of the same unit of work. Trigger monitors cannot retrieve these trigger messages until the unit of work completes. This applies whether the unit of work is committed or backed out.

If the queue manager fails to put a trigger message on an initiation queue, it will be put on the dead-letter (undelivered-message) queue.

  1. The queue manager counts both committed and uncommitted messages when it assesses whether the conditions for a trigger event exist.

    With triggering of type FIRST or DEPTH, trigger messages are made available even if the unit of work is backed out so that a trigger message is always available when the required conditions are met. For example, consider a put request within a unit of work for a queue that is triggered with trigger type FIRST. This causes the queue manager to create a trigger message. If another put request occurs, from another unit of work, this does not cause another trigger event because the number of messages on the application queue has now changed from one to two, which does not satisfy the conditions for a trigger event. Now if the first unit of work is backed out, but the second is committed, a trigger message is still created.

    However, this means that trigger messages are sometimes created when the conditions for a trigger event are not satisfied. Applications that use triggering must always be prepared to handle this situation. It is recommended that you use the wait option with the MQGET call, setting the WaitInterval to a suitable value.

  2. For local shared queues (that is, shared queues in a queue-sharing group) the queue manager counts committed messages only.

 

Parent topic:

Designing an application that uses triggered queues


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