[hibernate-dev] Concept of "service availability"

[mailing at bibbernet.org]

I had a similar problem before:

In an osgi project we needed the ability to extend a main system with 
functionality provided in addon bundles
We developed a main system which could be extended and complemented with 
extenders of several kinds.

The main system provided (exported) a tree of marker interfaces like

public interface ExtenderMarker {};
public interface TypeAExtender extends ExtenderMarker
public interface TypeBExtender extends ExtenderMarker
....

and had a "helper" which scans services, tracks services and initially 
scans the framework for interesting services (started from the bundle 
activator of the main system).

The extender bundles only implemented the extenders and registered OSGI 
services with the ExtenderMarker interface and maybe also the concrete 
interface. They had no more tasks to do.

Now the main system could "use" the extenders which got registered by 
the "helper" and the "helper" was handling the live cycle events, etc.

Simple approach based on the whiteboard pattern.

- Martin