Application programming and clusters

You do not need to make any programming changes to take advantage of multiple instances of the same queue. However, some programs do not work correctly unless a sequence of messages is sent to the same instance of a queue.

Applications can open a queue using the MQOPEN call. Applications use the MQPUT call to put messages onto an open queue. Applications can put a single message onto a queue that is not already open, using the MQPUT1 call.

If you set up clusters that have multiple instances of the same queue, there are no specific application programming considerations. However, to benefit from the workload management aspects of clustering, you might need to modify the applications. If you set up a network in which there are multiple definitions of the same queue, review the applications for message affinities.

Suppose for example, you have two applications that rely on a series of messages flowing between them in the form of questions and answers. You probably want answers to go back to the same queue manager that sent a question. It is important that the workload management routine does not send the messages to any queue manager that hosts a copy of the reply queue.

We might have applications that require messages to be processed in sequence (for example, a database replication application that sends batches of messages that must be retrieved in sequence). The use of segmented messages can also cause an affinity problem.

Opening a local or remote version of the target queue

Be aware of how the queue manager chooses whether use a local or remote version of the target queue.

1. The queue manager opens the local version of the target queue to read messages, or to set the attributes of the queue.
2. The queue manager opens any instance of the target queue to write messages to, if at least one of the following conditions is true:
   • A local version of the target queue does not exist.
   • The queue manager specifies CLWLUSEQ(ANY) on ALTER QMGR.
   • The queue on the queue manager specifies CLWLUSEQ(ANY).

• Handling message affinities
  Message affinities are rarely part of good programming design. We need to remove message affinities to use clustering fully. If we cannot remove message affinities, we can force related messages to be delivered using the same channel and to the same queue manager.

Parent topic: Use clusters for workload management

Related concepts

• Example of a cluster with more than one instance of a queue

Related tasks

• Adding a queue manager that hosts a queue locally
• Use two networks in a cluster
• Use a primary and a secondary network in a cluster
• Adding a queue to act as a backup
• Restricting the number of channels used
• Adding a more powerful queue manager that hosts a queue