Queue manager aliases and clusters

Use queue manager aliases to hide the name of queue managers when sending messages into or out of a cluster, and to workload balance messages sent to a cluster.

Queue manager aliases, which are created using a remote-queue definition with a blank RNAME, have five uses:
Remapping the queue manager name when sending messages
A queue manager alias can be used to remap the queue manager name specified in an MQOPEN call to another queue manager. It can be a cluster queue manager. For example, a queue manager might have the queue manager alias definition: 
DEFINE QREMOTE(YORK) RNAME(' ') RQMNAME(CLUSQM)

YORK can be used as an alias for the queue manager called CLUSQM. When an application on the queue manager that made this definition puts a message to queue manager YORK, the local queue manager resolves the name to CLUSQM. If the local queue manager is not called CLUSQM, it puts the message on the cluster transmission queue to be moved to CLUSQM. It also changes the transmission header to say CLUSQM instead of YORK.

Note: The definition applies only on the queue manager that makes it. To advertise the alias to the whole cluster, you need to add the CLUSTER attribute to the remote-queue definition. Then messages from other queue managers that were destined for YORK are sent to CLUSQM.
Altering or specifying the transmission queue when sending messages
Aliasing can be used to join a cluster to a non-cluster system. For example, queue managers in the cluster ITALY could communicate with the queue manager called PALERMO, which is outside the cluster. To communicate, one of the queue managers in the cluster must act as a gateway. From the gateway queue manager, issue the command: 
DEFINE QREMOTE(ROME) RNAME(' ') RQMNAME(PALERMO) XMITQ(X) CLUSTER(ITALY)

The command is a queue manager alias definition. It defines and advertises ROME as a queue manager over which messages from any queue manager in the cluster ITALY can multi-hop to reach their destination at PALERMO. Messages put to a queue opened with the queue manager name set to ROME are sent to the gateway queue manager with the queue manager alias definition. Once there, the messages are put on the transmission queue X and moved by non-cluster channels to the queue manager PALERMO.

The choice of the name ROME in this example is not significant. The values for QREMOTE and RQMNAME could both be the same.

Determining the destination when receiving messages

When a queue manager receives a message, it extracts the name of the destination queue and queue manager from the transmission header. It looks for a queue manager alias definition with the same name as the queue manager in the transmission header. If it finds one, it substitutes the RQMNAME from the queue manager alias definition for the queue manager name in the transmission header.

There are two reasons for using a queue manager alias in this way:
  • To direct messages to another queue manager
  • To alter the queue manager name to be the same as the local queue manager
Using queue manager aliases in a gateway queue manager to route messages between queue managers in different clusters.

An application can send a message to a queue in a different cluster using a queue manager alias. The queue does not have to be a cluster queue. The queue is defined in one cluster. The application is connected to a queue manager in a different cluster. A gateway queue manager connects the two clusters. If the queue is not defined as clustered, for the correct routing to take place, the application must open the queue using the queue name and a clustered queue manager alias name. For an example of a configuration, see Creating two overlapping clusters with a gateway queue manager, from which the reply message flow illustrated in figure 1, is taken.

The diagram shows the path taken by the reply message back to a temporary dynamic queue, which is called RQ. The server application, connected to QM3, opens the reply queue using the queue manager name QM2. The queue manager name QM2 is defined as a clustered queue manager alias on gateway queue manager QM1. QM3 routes the reply message to gateway queue manager QM1. QM1 routes the message to QM2.

Figure 1. Using a queue manager alias to return the reply message to a different cluster
Path taken by a reply message from the server to the client application through the gateway queue manager.

The way the routing works is as follows. Every queue manager in each cluster has a queue manager alias definition on gateway queue manager QM1. The aliases are clustered in all the clusters. The grey dashed arrows from each of the aliases to a queue manager show that each queue manager alias is resolved to a real queue manager in at least one of the clusters. In this case, the QM2 alias is clustered in both cluster CL1 and CL2, and is resolved to the real queue manager QM2 in CL1. The server application creates the reply message using the reply to queue name RQ, and reply to queue manager name QM2. The message is routed to gateway queue manager QM1 because the queue manager alias definition QM2 is defined on QM1 in cluster CL2 and queue manager QM2 is not in cluster CL2. As the message cannot be sent to the target queue manager, it is sent to the queue manager that has the alias definition.

Gateway queue manager QM1 places the message on its cluster transmission queue for transfer to QM2. Gateway queue manager QM1 routes the message to QM2 because the queue manager alias definition on QM1 for QM2 defines QM2 as the real target queue manager. The definition is not circular, because alias definitions can refer only to real definitions; the alias cannot point to itself. The real definition is resolved by gateway queue manager QM1, because both QM1 and QM2 are in the same cluster, CL1. Gateway queue manager QM1 finds out the connection information for QM2 from the repository for CL1, and routes the message to QM2. For the message to be rerouted by QM1, the server application must have opened the reply queue with the option DEFBIND set to MQBND_BIND_NOT_FIXED. If the server application had opened the reply queue with the option MQBND_BIND_ON_OPEN, the message is not rerouted and ends up on a dead letter queue.

Using a queue manager as a gateway into the cluster to workload balance messages from coming from outside the cluster.

You define a queue called EDINBURGH on more than one queue manager in the cluster. You want the clustering mechanism to balance the workload for messages coming to that queue from outside the cluster.

A queue manager from outside the cluster needs a transmit queue and sender-channel to one queue manager in the cluster. This queue is called a gateway queue manager. To take advantage of the default workload balancing mechanism, one of the following rules must apply:
  • The gateway queue manager must not contain an instance of the EDINBURGH queue.
  • The gateway queue manager specifies CLWLUSEQ(ANY) on ALTER QMGR.

For an example of workload balancing from outside a cluster, see Configuring workload balancing from outside a cluster