The WWW can be considered as market place where services and products are freely offered and requested. The general idea of a mediator system is to integrate applications and heterogeneous information sources in such a way that they keep their autonomy (cf.\ [Wie92]). A mediator has to ensure that semantic knowledge requests from users or applications can be transformed, distributed and matched by information offers which are stored as concrete resources, having resolved semantic ambiguities. Matching service descriptions have to be transformed back into high-level information, suitable for further processing. The KRAFT project has developed an agent-based architecture for distributed query processing [PHG99]. All information processing units are realised as interacting software agents, using a subset of the Knowledge Query and Manipulation Language (KQML) performatives [FYM97]. The facilitator maintains a data base (``yellow pages'') of all registered service providers and is able to map service types to appropriate providers. A provider first has to export the type of service it wishes to advertise via the facilitator. Information resources are coupled to the mediator, using wrapper modules. Information is exchanged in the KRAFT Constraint Interchange Format (CIF).
Figure: An Architecture for Mediation
With the vast amount of information and hidden knowledge in Web resources, standards as XML and DTDs for structuring, typing and semantic conceptual abstractions as shared ontologies have become evident. Using current data models for WWW information, there seems to be no way of declaring basic types and to constrain admissible values in the same formalism. The presented work establishes means that enable integration, representation and reasoning using Web standards, semantic concepts, and domain-specific knowledge like constraints. Mediation tasks as query processing and integration of heterogeneous information sources can be handled more effectively. Problem solving tools can re-use integrated and transformed knowledge for the automated solution of complex tasks. Knowledge integration and transformation is achieved by adopting the SEAMLESS multi-layer architecture (cf.\ [Eus00]). DTDs and ontology are incorporated into the distributed architecture through an intermediate context-dependent mediator knowledge base that in SEAMLESS terms provides a viewpoint. Figure demonstrates the resulting architecture. The mediator architecture thus employs three essential technologies which will be shortly examined: "XML", "Shared Ontology" and "Viewpoints".