Working with alias queues

You can define an alias queue to refer indirectly to another queue or topic.

Before you begin

Note: The steps in this task require you to run MQSC commands. How you do this varies by platform. See Administering IBM® MQ using MQSC commands.

Attention: Distribution lists do not support the use of alias queues that point to topic objects. If an alias queue points to a topic object in a distribution list, IBM MQ returns MQRC_ALIAS_BASE_Q_TYPE_ERROR.

About this task

The queue to which an alias queue refers can be any of the following:

A local queue (see Defining a local queue with DEFINE QLOCAL ).
A local definition of a remote queue (see Working with remote queues ).
A topic.

An alias queue is not a real queue, but a definition that resolves to a real (or target) queue at run time. The alias queue definition specifies the target queue. When an application makes an MQOPEN call to an alias queue, the queue manager resolves the alias to the target queue name.

An alias queue cannot resolve to another locally defined alias queue. However, an alias queue can resolve to alias queues that are defined elsewhere in clusters of which the local queue manager is a member. See Name resolution for further information.

Alias queues are useful for:

Giving different applications different levels of access authorities to the target queue.
Allowing different applications to work with the same queue in different ways. (Perhaps you want to assign different default priorities or different default persistence values.)
Simplifying maintenance, migration, and workload balancing. (Perhaps you want to change the target queue name without having to change your application, which continues to use the alias.)

For example, assume that 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 TARGET attribute, you can redirect MQI calls to another queue, possibly on another queue manager. This is useful for maintenance, migration, and load-balancing.

Procedure

Define an alias queue.
The following MQSC command creates an alias queue:
```
DEFINE QALIAS (MY.ALIAS.QUEUE) TARGET (YELLOW.QUEUE)
```
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:
```
ALTER QALIAS (MY.ALIAS.QUEUE) TARGET (MAGENTA.QUEUE)
```
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.
The following MQSC command defines an alias that is put enabled and get disabled for application ALPHA:
```
DEFINE QALIAS (ALPHAS.ALIAS.QUEUE) +
TARGET (YELLOW.QUEUE) +
PUT (ENABLED) +
GET (DISABLED)
```
The following command defines an alias that is put disabled and get enabled for application BETA:
```
DEFINE QALIAS (BETAS.ALIAS.QUEUE) +
TARGET (YELLOW.QUEUE) +
PUT (DISABLED) +
GET (ENABLED)
```
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 LIKE and REPLACE attributes when you define queue aliases, in the same way that you use these attributes with local queues.
Use other commands with alias queues.
You can use the appropriate MQSC commands to display or alter alias queue attributes, or to delete the alias queue object. For example:
Use the DISPLAY QALIAS command to display the alias queue's attributes:
```
DISPLAY QALIAS (ALPHAS.ALIAS.QUEUE)
```
Use the ALTER QALIAS command to alter the base queue name, to which the alias resolves, where the force option forces the change even if the queue is open:
```
ALTER QALIAS (ALPHAS.ALIAS.QUEUE) TARGET(ORANGE.LOCAL.QUEUE) FORCE
```
Use the DELETE QALIAS command to delete this queue alias:
```
DELETE QALIAS (ALPHAS.ALIAS.QUEUE)
```
You cannot delete an alias queue if an application currently has the queue open.