If you are interested in building decoupled distributed systems,
the Inquirer e-zine has a special offer for Java developers at

http://www.theinquirer.net/?article=15197

The software in question is an enhanced implementation of Sun's
JavaSpaces.  AutevoSpaces 3.0 conforms totally to the JavaSpaces API
whilst offering these additional features:

* Single System Image Clustering. This allows a single virtual
AutevoSpace to be distributed across multiple servers with obvious
scalability and failover benefits, whilst requiring no code change to
the application.

* Evolution allows different simultaneous views of the same Java
object from different clients. Multiple versions of the same service,
or multiple services can make updates to the same instance of an
object without any loss of data. This facilitates non-disruptive,
system transition with non-stop operation, as well as concurrent
operation of different application/data model versions. It also allows
disparate systems to interact with a common information model in a
truly collaborative, bi-directional manner, updating the model as they
need to, and seeing what they need to. In the real world this means
that existing systems can remain in place while they themselves are
able to participate in the incremental evolution of the information
model.

Gervas