Shared queues and queue sharing groups

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.

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 MQ supports the offloading of messages to Db2® or a shared message data set (SMDS). The offloading of messages of any size is configurable.

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.

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.

What is a queue sharing group?

A group of queue managers that can access the same shared queues is called a queue sharing group. Each member of the queue sharing group has access to the same set of shared queues.

Queue sharing groups have a name of up to four characters. The name must be unique in your network, and must be different from any queue manager names.

Figure 2 illustrates a queue sharing group that contains two queue managers. Each queue manager has a channel initiator and its own local page sets and log data sets.

Each member of the queue sharing group must also connect to a Db2 system. The Db2 systems must all be in the same Db2 data-sharing group so that the queue managers can access the Db2 shared repository used to hold shared object definitions. These are definitions of any type of IBM MQ object (for example, queues and channels) that are defined only once and then any queue manager in the group can use them. These are called global definitions and are described in Private and global definitions.

More than one queue sharing group can reference a particular data-sharing group. You specify the name of the Db2 subsystem and which data-sharing group a queue manager uses in the IBM MQ system parameters at startup.

When a queue manager has joined a queue sharing group, it has access to the shared objects defined for that group, and you can use that queue manager to define new shared objects within the group. If shared queues are defined within the group, you can use this queue manager to put messages to and get messages from those shared queues. Any queue manager in the group can retrieve the messages held on a shared queue.

You can enter an MQSC command once, and have it executed on all queue managers within the queue sharing group as if it had been entered at each queue manager individually. The command scope attribute is used for this. This attribute is described in Directing commands to different queue managers.

When a queue manager runs as a member of a queue sharing group it must be possible to distinguish between IBM MQ objects defined privately to that queue manager and IBM MQ objects defined globally that are available to all queue managers in the queue sharing group. The queue sharing group disposition attribute is used for this. This attribute is described in Private and global definitions.

You can define a single set of security profiles that control access to IBM MQ objects anywhere within the group. This means that the number of profiles you have to define is greatly reduced.

A queue manager can belong to only one queue sharing group, and all queue managers in the group must be in the same sysplex. You specify which queue sharing group the queue manager belongs to in the system parameters at startup.