Special case of trigger type FIRST

 

With trigger type FIRST, if there is already a message on the application queue when another message arrives, the queue manager does not usually create another trigger message. However, the application serving the queue might not actually open the queue (for example, the application might end, possibly because of a system problem). If an incorrect application name has been put into the process definition object, the application serving the queue will not pick up any of the messages. In these situations, if another message arrives on the application queue, there is no server running to process this message (and any other messages on the queue).

To deal with this, the queue manager creates further trigger messages under the following circumstances:

• If another message arrives on the application queue, but only if a predefined time interval has elapsed since the queue manager created the last trigger message for that queue. This time interval is defined in the queue manager attribute TriggerInterval. Its default value is 999 999 999 milliseconds.

• On WebSphere MQ for z/OS, application queues that name an open initiation queue are scanned periodically. If TriggerInterval milliseconds have passed since the last trigger message was sent and the queue satisfies the conditions for a trigger event and CURDEPTH is greater than zero, a trigger message is generated. This process is called backstop triggering.

Consider the following points when deciding on a value for the trigger interval to use in your application:

• If you set TriggerInterval to a low value, and there is no application serving the application queue, trigger type FIRST might behave like trigger type EVERY. This depends on the rate that messages are being put onto the application queue, which in turn might depend on other system activity. This is because, if the trigger interval is very small, another trigger message is generated each time that a message is put onto the application queue, even though the trigger type is FIRST, not EVERY. (Trigger type FIRST with a trigger interval of zero is equivalent to trigger type EVERY.)

• On WebSphere MQ for z/OS if you set TriggerInterval to a low value, and there is no application serving the trigger type FIRST application queue, backstop triggering will generate a trigger message each time the periodic scan of application queues that name open initiation queues takes place.

• If a unit of work is backed out (see Trigger messages and units of work) and the trigger interval has been set to a high value (or the default value), one trigger message is generated when the unit of work is backed out. However, if you have set the trigger interval to a low value or to zero (causing trigger type FIRST to behave like trigger type EVERY) many trigger messages can be generated. If the unit of work is backed out, all the trigger messages are still made available. The number of trigger messages generated depends on the trigger interval, the maximum number being reached when trigger interval has been set to zero.

 

Parent topic:

Controlling trigger events

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