onsdag, juni 25, 2008

Specifying the Default ESB Design Time Instance

This week I'm running a SOA HA class, and one question that usually comes up during these sessions is the one about the Oracle ESB design time instance. As you probably are aware of, this instance can only run in an Active-Passive setup (I will not dwell about the details of that in this post), so the obvious follow-up question, which isn't answered in the Enterprise Deployment Guide, is how do you specify which one of the instance you have configured and deployed the ESB design time on is going to be the default one?

Well, if you follow the instructions in the EDG 3.1.16, where you set:

<process-type id="OC4J_ESBDT" module-id="OC4J" service-failover="1" status="enabled">

you cannot really tell. If you start the instances at approximately the same time, you cannot really say which instance that will be started, for example, suppose that you have configured the instances to be brought up at boot time, and you start the machines at the same time. This might be fine, but under most circumstances you probably would like to select which one that will be the default running ESB design time instance. Well, you obviously always have the option to manually start the selected default instance first, and this problem will then go away.

Using the "service-weight" parameter can help you a bit with this task. The details of this parameter are described in the Process Manager and Notification Server Administrator's Guide. Basically, the higher value of this parameter, the higher priority to use this instance.

So, suppose that you configure the instance on machine Apa as:

<process-type id="OC4J_ESBDT" module-id="OC4J" service-failover="1" service-weight="200" status="enabled">

and the instance on machine Bepa as:

<process-type id="OC4J_ESBDT" module-id="OC4J" service-failover="1" service-weight="100" status="enabled">

OPMN will start the instance on machine Apa, given that they are started approximately at the same time. However, if the instance on machine Bepa has already been started and is up & running, and you then start the instance on machine Apa, OPMN will not bring down the instance on machine Bepa. So, using this solution will only work if the instances are started at approximately the same time.

If you really want to be sure that a specific instance is primary used, like the instance on machine Apa in the example above, you will have to create an event-script in the pre-start section for the instance running on Apa that brings down the instance running on Bepa. This way the instance on Apa will always will be used, regardless if Bepa is already running. There are some drawbacks with this solution that are documented in the OPMN documentation that you should be aware of if considering this solution.

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