The plugin descriptor is a fundamental piece of an agent-side plugin. This XML file is used to provide metadata about the plugin. The types of metadata include the following:

• A series of resource types supported by the plugin. This is what allows the plugin to use the abstraction in the platform towards a specific purpose. These resource types are written to best model the specific domain the plugin is addressing. Resource types can be nested to further help model the domain by providing the ability to indicate parent/child relationships between resource types (for more information, see Hierarchy).

• For each resource type, the descriptor indicates which aspects of the platform's features will be supported and provides specific details on how they should function. This includes defining what metrics will be collected, the types of packages that may be discovered and deployed, and the definition of what properties are used to configure resources of that type.

• The descriptor provides information to the plugin container on the location of which Java classes to instantiate to handle the discovery and control for each resource type (for more information, see Components).

Each agent-side plugin must have this plugin descriptor at META-INF/rhq-plugin.xml. The plugin container will look for and parse this descriptor for its metadata when starting the plugin. The descriptor must adhere to the rhq-plugin.xsd XML Schema schema, which is found in the client-api module.