Local queues

A queue is known to a program as local if it is owned by the queue manager to which the program is connected. You can get messages from, and put messages on, local queues.

The queue definition object holds the definition information of the queue as well as the physical messages put on the queue.

Each queue manager can have some local queues that it uses for special purposes:

Transmission queues

When an application sends a message to a remote queue, the local queue manager stores the message in a special local queue, called a transmission queue.

A message channel agent is a channel program is associated with the transmission queue and it delivers the message to its next destination. The next destination is the queue manager to which the message channel is connected. It is not necessarily the same queue manager as the final destination of the message. When the message is delivered to its next destination, it is deleted from the transmission queue. The message might have to pass through many queue managers on its journey to its final destination. You must define a transmission queue at each queue manager along the route, each holding messages waiting to be transmitted to the next destination. A normal transmission queue holds messages for the next destination, although the messages might have different eventual destinations. A cluster transmission queue holds messages for multiple destinations. The correlID of each message identifies the channel that the message is placed on to transfer it to its next destination.

You can define several transmission queues at a queue manager. You might define several transmission queues for the same destination, with each one being used for a different class of service. For example, you might want to create different transmission queues for small messages and large messages going to the same destination. You can then transfer the messages using different messages channels, so that the large messages do not hold up the smaller messages. All messages to cluster queues or cluster topics are placed on the single cluster transmission queue SYSTEM.CLUSTER.TRANSMIT.QUEUE, by default. As an option, you can change the default, and separate the message traffic going to different cluster queue managers onto different cluster transmission queues. If you set the queue manager attribute DEFCLXQ to CHANNEL, each cluster-sender channel creates a separate cluster transmission queue. As an alternative, you can manually define cluster transmission queues for cluster-sender channels to use.

Transmission queues can trigger a message channel agent to send messages onward; see Starting IBM® MQ applications using triggers.

On IBM MQ for z/OS®, if you are using intra-group queuing, the transmission queue is serviced by an intra-group queuing agent. A shared transmission queue is used when using intra-group queuing on IBM MQ for z/OS.

Initiation queues

An initiation queue is a local queue on which the queue manager puts a trigger message when a trigger event occurs on an application queue.

A trigger event is an event that is intended to cause a program to start processing a queue. For example, an event might be more than 10 messages arriving. For more information about how triggering works, see Starting IBM MQ applications using triggers.

Dead-letter (undelivered message) queue

A dead-letter (undelivered message) queue is a local queue on which the queue manager puts messages that it cannot deliver.

When the queue manager puts a message on the dead-letter queue, it adds a header to the message. The header information includes the reason that the queue manager put the message on the dead-letter queue. It also contains the destination of the original message, the date, and the time that the queue manager put the message on the dead-letter queue.

Applications can also use the queue for messages that they cannot deliver. For more information, see Using the dead-letter (undelivered message) queue.

System command queue

The system command queue is a queue to which suitably authorized applications can send IBM MQ commands. These queues receive the PCF, MQSC, and CL commands, as supported on your platform, in readiness for the queue manager to action them.

On IBM MQ for z/OS the queue is called SYSTEM.COMMAND.INPUT ; on other platforms it is called SYSTEM.ADMIN.COMMAND.QUEUE. The commands accepted vary by platform. See Programmable command formats reference for details.

System default queues

The system default queues contain the initial definitions of the queues for your system. When you create a queue definition, the queue manager copies the definition from the appropriate system default queue. Creating a queue definition is different from creating a dynamic queue. The definition of the dynamic queue is based upon the model queue you choose as the template for the dynamic queue.

Event queues

Event queues hold event messages. These messages are reported by the queue manager or a channel.