We present in this paper the essential aspects of the S2 system. This is the internet realization of SEMPER: An active, multi-domain, space-based, object oriented design environment for integrated building performance modeling. We begin with an overview of SEMPER principles. We then present the component based system architecture of S2, which makes use of the Common Object Request Broker Architecture (CORBA) for communication between components including our analysis applications (written in C++) and other S2 components (written in Java). We describe the underlying approach to and implementation of the shared object model. This serves as the shared building representation from which the analysis modules derive their domain object models using a homology-based mapping process.