Proxy Pattern and Publish/Subscribe: An Incompatible Combination?

In a ubiquitous computing environment, the state of devices may potentially change over time. Consequently, applications and services should be able to adapt to their environments. Having extensions with additional features is one possibility for adaptation. Developing software using the concepts defined for service oriented architecture promises easily adaptable software by loose coupling of services, which is a clear trend for software in 3G and B3G networks. Services are loosely coupled, in order to allow replacement and adaptation of service ensembles that suit the current situation of users. On top of this, services should optimally not be aware of extensions. Two approaches can be applied: the proxy pattern and the publish/subscribe architecture. The former allows service extension at runtime, while the latter enables communication between loosely coupled software. Implementation of both approaches results in an incompatibility problem. We introduce mutable references to solve this problem. With the proposed pattern one can implement publish/subscribe based services together with service extension at runtime via the proxy pattern.