The Knowledge Model of Protégé-2000: Combining Interoperability and Flexibility

Knowledge-based systems have become ubiquitous in recent years. The World-Wide Web consortium is developing the Resource Description Framework (RDF)—a system for annotating even Web pages with knowledge elements. Knowledge-base developers need to be able to share and reuse knowledge bases that they build. Therefore, interoperability among different knowledge-representation systems is essential. The Open Knowledge-Base Connectivity protocol (OKBC) is a common query and construction interface for frame-based systems that facilitates this interoperability. Protégé-2000 is an OKBC-compatible knowledge-base–editing environment developed in our laboratory. Protégé-2000 has an easy-to-use and configurable interface. We describe its OKBCcompatible knowledge model that makes the import and export of knowledge bases from and to other knowledge-base servers easy. We discuss how the requirements of being usable and configurable knowledge-acquisition tool affected our decisions in the knowledge-model design. Protégé-2000 also has a flexible metaclass architecture which provides configurable templates for new classes in the knowledge base. The use of metaclasses makes Protégé-2000 easily extensible and enables its use with other knowledge models. For example, we demonstrate that we can resolve many of the differences between the knowledge models of Protégé-2000 and RDF by defining a new metaclass set. Resolving the differences between the knowledge models in declarative way enables easy adaptation of Protégé-2000 as an editor for other knowledgerepresentation systems. 1 The trade-off between interoperability and usability of knowledgebased systems In recent years, knowledge sharing and reuse has become one of the primary goals of the knowledge-based systems research community [12]. Enabling interoperability among knowledge-representation systems is a crucial step in achieving this goal. The Open Knowledge-Base Connectivity (OKBC) protocol [2, 7] facilitates this interoperability by providing an application-programming interface (API) that serves as a common query and construction interface for frame-based systems. A number of OKBC-compatible knowledge-representation systems is currently available including Ontolingua [5], Loom [11], and Protégé-2000, which was developed in our laboratory. Protégé-2000 is the latest component-based and platform-independent generation of the Protégé toolset [6]. Two goals have driven the design and development of Protégé-2000: (1) achieving interoperability with other knowledge-representation systems, and (2) being