Redpaper - IBM Redbooks

The ability to define these clustered queues on several of the queue managers in
the cluster leads to increased system availability. Each queue manager runs
equivalent instances of the applications that process the messages. If one of the
queue managers fails, or the communication to it is suspended, that queue
manager is temporarily excluded from the choice of destinations for the
messages. This functionality provides a number of benefits, the main one being
HA.
There are a few points to consider when approaching WebSphere MQ clustering
as the preferred choice for implementing HA. For example, if a queue manager
fails, then although any subsequent messages sent to the cluster are not routed
to the failed queue manager, any persistent messages already sent to the queue
manager, but not yet processed by the broker, are marooned on this failed queue
manager. Also, for WebSphere MQ clustering to effectively balance loads
between queues on different queue managers, messages sent from outside the
cluster need to be sent to a gateway queue manager. A gateway queue manager
creates a single point of failure for the whole logical hub. Therefore, the gateway
queue manager would need to have very high availability.
Also, remember that in the event that the Message Broker falls over but the
clustered queue manager is still operational, messages under a cluster queue
environment are still sent to the queue manager. These messages are not
processed until the Message Broker is once again functional. Unless there is a
monitoring tool which registers that the Message Broker is inoperative,
messages can build up unnoticed.
A way of safeguarding against this situation is to employ a monitoring tool, which
registers both the status of the Message Broker and the subsequent build up of
messages on the broker input queue. If, for example, the normal buildup on the
broker input queue is 50 messages, it might be wise to set an alert on the queue.
If messages build to a number greater than 50, then an alert is sent to the
monitoring tool, and an operator can determine if there is a problem.
Table 2-2 shows the advantages and disadvantages to using WebSphere MQ
clustering.
Table 2-2 Advantages and disadvantages to using WebSphere MQ clustering
Advantages Disadvantages
Provides resilient failover because
WebSphere MQ can redirect messages
away from a failed queue manager to be
picked up by a functional one.
Persistent messages awaiting processing
while on the queue of a failed queue
manager remain there until the queue
manager is operational.
Chapter 2. Design decisions that affect high availability 13