Ontology concerns itself with the representation of the objects in the universe and the web of their various connections. The traditional task of ontologists has been to extract from this tangle a single ordered structure, in the form of a tree or lattice. This structure consists of the terms that represent the objects, and the relationships that represent connections between objects. Recent work in ontology goes so far as to consider several distinct, superimposed structures, which each represent a classification of the universe according to a particular criterion. Our purpose is to defer the task of globally classifying terms and relationships. Instead, we focus on composing them for use as we need them. We define contexts to be our unit of encapsulation for ontologies, and use a rule-based algebra to compose novel ontological structures within them. We separate context from concept, the unit of ontological abstraction. Also, we distinguish composition from subsumption, or containment, the relationships that commonly provide structure to ontologies. Adding a formal notation of encapsulation and composition to ontologies leads to more dynamic and maintainable structures, and, we believe, greater computational efficiency for knowledge bases.

Many information-processing tasks can be part of multiple customer applications, as summarizing stock prices, integrating catolog information from several companies in the same line of business, predicting the weather, and checking on transportation resources. We assign such sharable services to an active middleware layer, interposed between clients and servers. We define domain-specific mediator modules to populate this layer. Such mediating services must be of value to the customers, so that it will benefit their client applications to access mediators rather than the server sources directly. Several categories of value can be considered: improvement in access and coverage, improvement of content, and delegation of maintenance. We will define criteria for mediating modules: ownership by party who assumes responsibility for the rseults of the services, domain-specificity to delimit the scope of such a responsibility, and, of course, conformance with interface standards that ...