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4.5. Examples: Embedded and Injected Plug-in Dependencies
JBoss ON agent plug-ins have several different ways of defining dependencies between plug-ins: simple depends, embedded, and injected. The way that a dependency is defined has some affect on how the plug-in behaves. This is all explored more in Section 4.2.4, “Plug-in Dependencies: Defining Relationships Between Plug-ins”.
These examples show how each of the dependency types are defined in the plug-in descriptors for agent plug-ins.
4.5.1. Simple Dependency: JBoss AS and JMX Plug-ins
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A required dependency is defined using the <depends> tag. This means that the required plug-in has to be deployed successfully before the plug-in which requires it can be deployed. A simple example of this is JBoss AS, which has a JMX server running inside it. The JBoss AS plug-in for JBoss ON, then, sets a dependency on the JMX plug-in.
The JMX plug-in descriptor simply defines the configuration for the JMX server.
Example 4.16. JMX Plug-in Descriptor
<plugin name="JMX">
<server name="JMX Server" discovery="JMXDiscoveryComponent" class="JMXServerComponent">
...
</server>
</plugin>
The JBoss AS plug-in descriptor lists the JMX plug-in as a dependency. This makes all of the JMX plug-in classes available to the JBoss AS plug-in classloader (since the
useClasses argument is set to true), but the JBoss AS plug-in descriptor does not actually define or use any source types related to or referencing the JMX plug-in.
Example 4.17. JBoss AS Plug-in Descriptor
<plugin name="JBossAS">
<depends plugin="JMX" useClasses="true"/>
<server name="JBossAS Server" discovery="JBossASDiscoveryComponent" class="JBossASServerComponent">
...
</server>
</plugin>IMPORTANT
Plug-ins can require or depend on multiple plug-ins. However, only one of those dependencies can have the value of useClasses=true.
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The JMX plug-in is a generic plug-in and can be used by many other plug-ins. Any application that can be managed by JMX can use the JMX plug-in to pick up all of its dependencies, as well as EMS libraries which extend the JMX plug-in.
4.5.2. Embedded Dependency: JVM MBeanServer and JBoss AS
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Java Virtual Machines have a built-in JMX MBeanServer called the platform MBeanServer. Any JVM with this platform MBeanServer can be monitored for memory, garbage collectors, threading, and other subsystems through MBeans in the MBeanServer.
The JMX plug-in in JBoss ON can define resource types for each platform MBean, allowing the JBoss ON agent to monitor those MBeans as JBoss ON service resources.
The platform MBeanServer can be found in a standalone JVM process or embedded inside a JBoss AS VM process. If the JVM being monitored is embedded in a JBoss server, then the JBoss ON plug-ins are configured with an embedded plug-in dependency. An embedded dependency means that one plug-in is aware that another plug-in is running inside it.
The JMX plug-in descriptor simply defines the JMX server.
Example 4.18. JMX Plug-in Descriptor
<plugin name="JMX">
<server name="JMX Server" discovery="JMXDiscoveryComponent" class="JMXServerComponent">
<service name="VM Memory System"
discovery="MBeanResourceDiscoveryComponent"
class="MBeanResourceComponent"
description="The memory system of the Java virtual machine">
...
</service>
...
</server>
</plugin>
The embedding is done in the JBoss AS plug-in descriptor. Along with setting a required dependency for the JMX plug-in, the JBoss AS plug-in's <server> definition pulls in the
sourcePlugin and sourceType attributes. The reason for this is to run a second JMX discovery scan, this one using the org.rhq.plugins.jmx.EmbeddedJMXServerDiscoveryComponent class to run a special discovery scan looking for a JVM embedded in a JBoss AS instance. The sourcePlugin and sourceType attributes, then, copy the resource type and give it a unique name so that any embedded JVMs are treated as different resource types than standalone JVMs.
Example 4.19. JBoss AS Plug-in Descriptor
<plugin name="JBossAS">
<depends plugin="JMX" useClasses="true"/>
<server name="JBossAS Server" discovery="JBossASDiscoveryComponent" class="JBossASServerComponent">
<server name="JBoss AS JVM"
description="JVM of the JBossAS"
sourcePlugin="JMX"
sourceType="JMX Server"
discovery="org.rhq.plugins.jmx.EmbeddedJMXServerDiscoveryComponent"
class="org.rhq.plugins.jmx.JMXServerComponent">
...
</server>
...
</server>
</plugin>
This type of embedded plug-in also illustrates that an embedded resource type can be discovered using a different discovery component from that of the source plug-in type.
4.5.3. Injected Dependency: Hibernate with JVM and JBoss AS
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An injection dependency is the logical opposite of an embedded dependency; it is an awareness in the plug-in that the resource configured by the plug-in is running inside another resource. The parent resources are listed as dependencies.
One common example of this is Hibernate. Hibernate can run in either a standalone J2SE JVM instance or a JBoss AS server; for this example, it runs inside a JVM in a JBoss AS instance. The plug-ins have chained dependencies, where either the JMX and JBoss AS plug-ins can be parents to the Hibernate plug-in, and both the Hibernate and JBoss AS plug-ins list the JMX plug-in as a required dependency.
Figure 4.4. Hibernate, JMX, and JBoss AS Dependencies
As before, the JMX plug-in descriptor only defines the JMX plug-in, without any dependencies.
Example 4.20. JMX Plug-in Descriptor
<plugin name="JMX">
<server name="JMX Server" discovery="JMXDiscoveryComponent" class="JMXServerComponent">
...
</server>
</plugin>
The JBoss AS plug-in sets a required dependency on the JMX plug-in, but no other dependencies are required to be defined (although they could be).
Example 4.21. JBoss AS Plug-in Descriptor
<plugin name="JBoss AS">
<depends plugin="JMX" useClasses="true"/>
<server name="JBossAS Server" discovery="JBossASDiscoveryComponent" class="JBossASServerComponent">
...
</server>
</plugin>
The most complex definition is for the Hibernate plug-in. This sets an explicit dependency on the JMX plug-in using the <depends> element. The Hibernate plug-in then defines what resource types could operate as its parents by running a discovery scan (specifically for the Hibernate Statistics resource) against potential parent types. The list of parent resource types is contained in the <runs-inside> element, and each potential parent is identified by name and plug-in type in <parent-resource-type> elements.
Example 4.22. Hibernate Plug-in Descriptor
<depends plugin="JMX" useClasses="true"/>
<service name="Hibernate Statistics"
discovery="org.rhq.plugins.jmx.MBeanResourceDiscoveryComponent" class="StatisticsComponent">
<runs-inside>
<parent-resource-type name="JMX Server" plugin="JMX"/>
<parent-resource-type name="JBossAS Server" plugin="JBossAS"/>
</runs-inside>
...
</service>
</plugin>
When a plug-in is dependent on another plug-in, it is implicitly dependent on whatever other plug-ins are required. For example, Hibernate depends on the JBoss AS plug-in. Even if the Hibernate plug-in didn't explicitly state a dependency on the JMX plug-in, it would still be dependent on the JMX Plug-in because the JBoss AS plug-in requires it.
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