RHQ: decomposing plugins

I've been talking a lot lately about how to write plugins for RHQ / JBossON.
When you try to manage larger systems like the JBoss Application Server with all its subsystems like Cache, Transactions, JBossWeb etc. your plugin might get relatively large to support all this.
In this posting I will show you how to decompose a large(r) plugin into smaller ones that all together allow you to manage the large(r) system.

This decomposition not only allows you to more easily distribute the development load, but also enables re-use of the parts that have been broken out of the big chunk. The price you have to pay is relatively small and consists mostly of some additional directories and a maven pom.xml file (that I am not going to show here).

The basic trick is to use <depends> and <runs-inside> tags in your plugin descriptor (rhq-plugin.xml) for this new plugin:

  <plugin name="JBossCache" ... >
   <depends plugin="JMX" />
   <depends plugin="JBossAS" useClasses="true"/>

So we need the JMX plugin and the JBossAS plugin (currently part of JBoss ON only) being deployed before our plugin can start. The attribute useClasses means that the classloader of our plugin gets access to the classes of the other plugin (JBossAs here). So we can use those classes too.

    <service name="JBoss Cache" ...>

As you know from previous posts, a service can't just "hang in the air" - it needs another server or service as a container. This is where runs-inside comes into play:

       <runs-inside>
         <parent-resource-type name="JBossAS Server" plugin="JBossAS"/>
      </runs-inside>

So our plugin service "JBoss Cache" will be contained in resources of type "JBossAS Server" that come from the JBossAS plugin (that we declared in the depends element earlier).

Apart from this little magic in the plugin descriptor, there is no more additional work to do.

Voilà, RHQ plugins à la carte ... :-)