Question

When I read about HA in Websphere MQ I always come to the point, when the best practise is to create two Queue Managers handling the same queue and use out-of-the-box load balancing. Therefore, when one is down, the other takes over his job.

Well, this is great but what about the messages in the queue that belong to the Queue Manager that went down? I mean do these messages reside there (when queue is persistent of course) until QM is up and running again?

Furthermore, is it possible to create a common storage for this doubled Queue Managers? Then no message would wait for the QM to be up. Every message would be delivered in the proper order. Is this correct?

Answer 1

WebSphere MQ provides different capabilities for HA, depending on your requirements. WebSphere MQ clustering uses parallelism to distribute load across multiple instances of a queue. This provides availability of the service but not for in-flight messages.

Hardware clustering and Multi-Instance Queue Manager (MIQM) are both designs using multiple instances of a queue manager that see a single disk image of that queue manager’s state. These provide availability of in-flight messages but the service is briefly unavailable while the cluster fails over.

Using these in combination it is possible to provide recovery of in-flight messages as well as availability of the service across multiple queue instances.

In hardware cluster model the disk is mounted to only one server and the cluster software monitors for failure and swaps the disk, IP address and possibly other resources to the secondary node. This requires a hardware cluster monitor such as PowerHA to manage the cluster.

The Multi-Instance QMgr is implemented entirely within WebSphere MQ and needs no other software. It works by having two running instances of the QMgr pointing to the same NFS 4 shared disk mount. Both instances compete for locks on the files. The first one to acquire a lock becomes the active QMgr. Because there is no hardware cluster monitor to perform IP address takeover this type of cluster will have multiple IP addresses. Any modern version of WMQ allows for this using multi-instance CONNAME where you can supply a comma-separated list of IP or DNS names. Client applications that previously used Client Channel Definition Tables (CCDT) to manage failover across multiple QMgrs will continue to work and CCDT continues to be supported in current versions of WMQ.

Please see the Infocenter topic Using WebSphere MQ with high availability configurations for details of hardware cluster and MIQM support.

Client Channel Definition Table files are discussed in the Infocenter topic Client Channel Definition Table file.