The Essence of Multilevel Metamodeling

As the UML attempts to make the transition from a single, albeit extensible, language to a framework for a family of languages, the nature and form of the underlying meta-modeling architecture will assume growing importance. It is generally recognized that without a simple, clean and intuitive theory of how metamodel levels are created and related to one another, the UML 2.0 vision of a coherent family of languages with a common core set of concepts will remain elusive. However, no entirely satisfactory metamodeling approach has yet been found. Current (meta-)modeling theories used or proposed for the UML all have at least one fundamental problem that makes them unsuitable in their present form. In this paper we bring these problems into focus, and present some fundamental principles for overcoming them. We believe that these principles need to be embodied within the metamodeling framework ultimately adopted for the UML 2.0 standard.