Conventional object-oriented data models are "closed". Although they allow users to define application-specific classes, they usually come with a fixed set of modeling primitives. This constitutes a major problem as different application domains have different requirements on a data model. At a first glance there are two common solutions to the problem: (1) Use different special data models for different application domains. (2) Use a common data model covering all specific application needs to more or less extent. The first solution leads to disintegrated and isolated applications. The second solution does not lead to really usable models as they become either very simple and general or overloaded and very complex to manage. A solution to this problem is to provide an "open" object-oriented data model, which is simple but which can be extended in itself by additional modeling concepts for specific application needs. Such models have recently been termed "RISC models" in analogy to reduced instruction set computers and found very relevant in providing meta object protocols for object system interoperability. The approach towards an "open" object-oriented data model also goes beyond the approaches of static, closed models as, e.g., taken in the international ODMG standardization effort. In this book, we present a solution to develop a RISC model using an extended metaclass concept. We show, how the extended metaclass concept can be integrated homogeneously into object-oriented data models. The extended metaclass concept leads to an open data model that provides mechanisms to tailor it for particular application needs. The work reported in this book served as a basis for the open object-oriented database management systems project VODAK and for many other projects at GMD-IPSI.

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