Intra-group queuing 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 IBM® WebSphere MQ Version 5.3 and above, 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 might 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 nonpersistent messages outside of sync point scope, and all persistent messages within sync point scope. In this case, the IGQ agent acts as the sync point coordinator. The IGQ agent therefore processes nonpersistent messages like the way fast, nonpersistent messages are processed on a message channel. See Fast, nonpersistent messages.

Batching of messages

The IGQ agent uses a fixed batch size of 50 messages. Any persistent messages retrieved within a batch are 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 normal rules are applied in the selection of the queue that has default priority and persistence values that are used. (See the IBM MQ messages section for more information about the rules of queue selection).