6.3. Plug-in Components

The discovery component is an implementation written by the plug-in writer that performs the discovery of the actual managed resources. The discovery component's job is to scan the platform (that is, the machine the agent/plug-in container/plug-in is running on) and to report back what it finds. A discovery component is only responsible for finding resources that it directly is in charge of managing. That is to say, a JBoss AS plug-in discovery component is not responsible for discovering all Apache Web Servers - it only needs to find JBoss AS resources (leave the discovery of Apache Web Servers to the Apache plug-in).

A discovery component will be told to go hunt for resources at an appropriate time by the plug-in container. When the plug-in container asks the discovery component to go discover more resources, it will send in a ResourceDiscoveryContext object to the discovery component. This context contains all the information the component needs to perform its duties of finding and creating new resources. The discovery context is also used to inject resources into the discovery component, in the case where the plug-in container was able to discover new resources on behalf of the discovery component. A plug-in container can only auto-discover resources if the appropriate metadata is supplied to it via the plug-in's descriptor.

A resource component is a plug-in abstraction that represents an actual managed resource. A resource component is stateful whose lifecycle is managed by the plug-in container.

The plug-in container will start and stop a resource component at the appropriate times. When a resource component is started, it typically connects to its underlying resource (the managed resource it represents) and maintains that connection until it is stopped by the plug-in container (this is an implementation detail that a plug-in writer is free to change). A resource component implementation provides a way for the plug-in container to ask for the availability of the managed resource ("is this resource up? or has it gone down?") and to access optional functionality facets that a plug-in writer chooses to expose. More on facets below.