Intercommunication

Messages

This section describes the messages put to the SYSTEM.QSG.TRANSMIT.QUEUE.

Message structure

Like all other messages that are put to transmission queues, messages that are put to the SYSTEM.QSG.TRANSMIT.QUEUE are prefixed with the transmission queue header (MQXQH).

Message persistence

In WebSphere MQ Version 5 Release 3, shared queues support both persistent and non-persistent messages.

If the queue manager terminates while the IGQ agent is processing non-persistent messages, or if the IGQ agent terminates abnormally while in the middle of processing messages, non-persistent messages being processed may be lost. Applications must make arrangements for the recovery of non-persistent messages if their recovery is required.

If a put request for a non-persistent message, issued by the IGQ agent, fails unexpectedly, the message being processed is lost.

Delivery of messages

The IGQ agent retrieves and delivers all non-persistent messages outside of syncpoint scope, and all persistent messages within syncpoint scope. In this case, the IGQ agent acts as the syncpoint coordinator.

Batching of messages

The IGQ agent uses a fixed batchsize of 50 messages. Any persistent messages retrieved within a batch will be committed at intervals of 50 messages. The agent commits a batch consisting of persistent messages when there are no more messages available for retrieval on the SYSTEM.QSG.TRANSMIT.QUEUE.

Message size

The maximum size of message that can be put to the SYSTEM.QSG.TRANSMIT.QUEUE is the maximum supported message length for shared queues minus the length of a transmission queue header (MQXQH).

Default message persistence and default message priority

If the SYSTEM.QSG.TRANSMIT.QUEUE is in the queue name resolution path established at open time, then for messages that are put with default persistence and default priority (or with default persistence or default priority), the usual rules are applied in the selection of the queue whose default priority and persistence values are used. (Refer to WebSphere MQ Application Programming Reference for information on the rules of queue selection).

Undelivered/unprocessed messages

If an IGQ agent cannot deliver a message to the destination queue, the IGQ agent:

Honours the MQRO_DISCARD_MSG report option (if the Report options field of the MQMD for the undelivered message indicates that it should) and discards the undelivered message.
Attempts to place the undelivered message on to the dead letter queue for the destination queue manager, if the message has not already been discarded . The IGQ agent prefixes the message with a dead letter queue header (MQDLH).

If a dead letter queue is not defined, or if an undelivered message cannot be put to the dead letter queue, and if the undelivered message is:

persistent, the IGQ agent backs out the current batch of persistent messages that it is processing, and enters a state of retry. For more information, see Retry capability of the intra-group queuing agent.
non-persistent, the IGQ agent discards the message and continues to process the next message.

If a queue manager in a queue-sharing group is terminated before its associated IGQ agent has had time to process all its messages, the unprocessed messages remain on the SYSTEM.QSG.TRANSMIT.QUEUE until the queue manager is next started. The IGQ agent then retrieves and delivers the messages to the destination queues.

If the coupling facility fails before all the messages on the SYSTEM.QSG.TRANSMIT.QUEUE have been processed, any unprocessed non-persistent messages will be lost.

IBM recommends that applications do not put messages directly to transmission queues. If an application does put messages directly to the SYSTEM.QSG.TRANSMIT.QUEUE, the IGQ agent may not be able to process these messages and they will simply remain on the SYSTEM.QSG.TRANSMIT.QUEUE. Users will then have to use their own methods to deal with these unprocessed messages.

Report messages

This section describes report messages.

Confirmation of arrival (COA)/confirmation of delivery (COD) report messages

COA and COD messages are generated by the queue manager, when intra-group queuing is used.

Expiry report messages

Expiry report messages are generated by the queue manager, when intra-group queuing is used.

Exception report messages

Depending on the MQRO_EXCEPTION_* report option specified in the Report options field of the message descriptor for the undelivered message, the IGQ agent generates the required exception report and places it on the specified reply-to queue. (Note that intra-group queuing can be used to deliver the exception report to the destination reply-to queue).

The persistence of the report message is the same as the persistence of the undelivered message. If the IGQ agent fails to resolve the name of the destination reply-to queue, or if it fails to put the reply message to a transmission queue (for subsequent transfer to the destination reply-to queue) it attempts to put the exception report to the dead letter queue of the queue manager on which the report message is generated. If this is not possible, then if the undelivered message is:

persistent, the IGQ agent discards the exception report, backs out the current batch of messages and enters a state of retry. For more information, see Retry capability of the intra-group queuing agent.
non-persistent, the IGQ agent discards the exception report and continues processing the next message on the SYSTEM.QSG.TRANSMIT.QUEUE.