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Table 2-1 Advantages and disadvantages of clones and tailored execution groups Cloned execution groups Tailored execution groups Advantages Advantages Provides for the highest availability level because all brokers can process any message. Gives a wide distribution of work evenly across the system, reducing the potential for bottlenecks. Disadvantages Disadvantages Does not provide granular control over resource allocation. Does not provide support for flows which may be required to turnaround messages more quickly than others. 2.3 WebSphere MQ options in supporting HA with Message Broker There are advantages and disadvantages you should consider before embarking on a particular method of providing HA with Message Broker. However, once you have determined the best method for your configuration, there are a number of ways you can use WebSphere MQ to support your configuration, using either WebSphere MQ clustering or WebSphere MQ shared queues. This section discusses the WebSphere MQ options available in supporting a highly available Message Broker configuration. 2.3.1 WebSphere MQ clustering Allows for the best use of the resources available. Allows more flows which are used more or have a more demanding message processing need. May introduce affinities to particular brokers and LPARS. Provides a lower availability because fewer brokers can process all messages. To understand how clustering works, consider this example. A sending application that sends a message to a receiving application uses the queue manager to send the message. If the queue manager of the receiving application is offline, the clustering functionality would automatically reroute the message to another queue manager. This queue manager acts as a clone of the receiving application. This function is totally transparent to the applications. Additional information about clustering can be found in WebSphere MQ Queue Manager Clusters, SC34-6061. 12 High Availability z/OS Solutions for WebSphere Business Integration Message Broker V5
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
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