What is a shared queue?
A shared queue is a type of local queue. The messages on that queue can be accessed by one or more queue managers that are in a sysplex.
A queue sharing group
The queue managers that can access the same set of shared queues form a group called a queue-sharing group.
Any queue manager can access messages
Any queue manager in the queue-sharing group can access a shared queue. This means that you can put a message on to a shared queue on one queue manager, and get the same message from the queue from a different queue manager. This provides a rapid mechanism for communication within a queue-sharing group that does not require channels to be active between queue managers.
IBM® WebSphere® MQ 7.1 and later supports the offloading of messages to Db2® or a shared message data set (SMDS). The offloading of messages of any size is configurable.
In earlier versions of IBM MQ, large messages (> 63 KB) have a placeholder stored in the coupling facility (4 K), and their message data stored in Db2.
Figure 1 shows three queue managers and a coupling facility, forming a queue-sharing group. All three queue managers can access the shared queue in the coupling facility.
An application can connect to any of the queue managers within the queue-sharing group. Because all the queue managers in the queue-sharing group can access all the shared queues, the application does not depend on the availability of a specific queue manager; any queue manager in the queue-sharing group can service the queue.
This gives greater availability because all the other queue managers in the queue-sharing group can continue processing the queue if one of the queue managers has a problem.
Queue definition is shared by all queue managers
Shared queue definitions are stored in the Db2 database table OBJ_B_QUEUE. Because of this, you need to define the queue only once and then it can be accessed by all the queue managers in the queue-sharing group. This means that there are fewer definitions to make.
By contrast, the definition of a non-shared queue is stored on page set zero of the queue manager that owns the queue (as described in Page sets ).
You cannot define a shared queue if a queue with that name has already been defined on the page sets of the defining queue manager. Likewise, you cannot define a local version of a queue on the queue manager page sets if a shared queue with the same name exists.