Working with alias queues on IBM i

This section contains examples of some of the commands that you can use to manage alias queues. All the commands shown are also available using options from the WRKMQMQ command panel.

An alias queue (sometimes known as a queue alias) provides a method of redirecting MQI calls. An alias queue is not a real queue but a definition that resolves to a real queue. The alias queue definition contains a target queue name, which is specified by the TGTQNAME attribute.

When an application specifies an alias queue in an MQI call, the queue manager resolves the real queue name at run time.

For example, an application has been developed to put messages on a queue called my.alias.queue. It specifies the name of this queue when it makes an MQOPEN request and, indirectly, if it puts a message on this queue. The application is not aware that the queue is an alias queue. For each MQI call using this alias, the queue manager resolves the real queue name, which could be either a local queue or a remote queue defined at this queue manager.

By changing the value of the TGTQNAME attribute, you can redirect MQI calls to another queue, possibly on another queue manager. This is useful for maintenance, migration, and load-balancing.

Defining an alias queue

The following command creates an alias queue:


CRTMQMQ QNAME('my.alias.queue') QTYPE(*ALS) TGTQNAME('yellow.queue')
MQMNAME(MYQUEUEMANAGER)

This command redirects MQI calls that specify my.alias.queue to the queue yellow.queue. The command does not create the target queue; the MQI calls fail if the queue yellow.queue does not exist at run time.

If you change the alias definition, you can redirect the MQI calls to another queue. For example:


CHGMQMQ QNAME('my.alias.queue') TGTQNAME('magenta.queue') MQMNAME(MYQUEUEMANAGER)

This command redirects MQI calls to another queue, magenta.queue.

You can also use alias queues to make a single queue (the target queue) appear to have different attributes for different applications. You do this by defining two aliases, one for each application. Suppose there are two applications:

Application ALPHA can put messages on yellow.queue, but is not allowed to get messages from it.
Application BETA can get messages from yellow.queue, but is not allowed to put messages on it.

You can do this using the following commands:


/* This alias is put enabled and get disabled for application ALPHA */

CRTMQMQ QNAME('alphas.alias.queue') QTYPE(*ALS) 	TGTQNAME('yellow.queue')
PUTENBL(*YES) GETENBL(*NO) MQMNAME(MYQUEUEMANAGER)

/* This alias is put disabled and get enabled for application BETA */

CRTMQMQ QNAME('betas.alias.queue') QTYPE(*ALS) TGTQNAME('yellow.queue')
PUTENBL(*NO) GETENBL(*YES) MQMNAME(MYQUEUEMANAGER)

ALPHA uses the queue name alphas.alias.queue in its MQI calls; BETA uses the queue name betas.alias.queue. They both access the same queue, but in different ways.

You can use the REPLACE *YES attribute when you define alias queues, in the same way that you use these attributes with local queues.

Using other commands with alias queues

You can use the appropriate commands to display or change alias queue attributes. For example:


* Display the alias queue's attributes */

DSPMQMQ QNAME('alphas.alias.queue') MQMNAME(MYQUEUEMANAGER)

/* ALTER the base queue name, to which the alias resolves. */
/* FORCE = Force the change even if the queue is open. */

CHQMQMQ QNAME('alphas.alias.queue') TGTQNAME('orange.local.queue') FORCE(*YES)
MQMNAME(MYQUEUEMANAGER)