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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

Metamodeling is one of the core foundations of computer-automated multiparadigm modeling. However, there is currently little agreement about what form the required metamodeling approach should take and precisely what role metamodels should play. This article addresses the problem by first describing some fundamental problems in the industry's leading metamodeling technology, the UML framework, and then explaining how this framework could be rearchitected to overcome these problems. Three main issues are identified in the current framework: the dual classification problem arising from the need to capture both the logical and physical classification of model elements, the class/object duality problem arising from the need to capture both the classlike and objectlike facets of some model elements, and the replication of concepts problem arising from the need to define certain concepts multiple times. Three main proposals for rearchitecting the UML framework to overcome these problems are then presented: the separation of logical and physical classification dimensions, the unification of the class and object facets of model elements, and the enhancement of the instantiation mechanism to allow definitions to transcend multiple levels. The article concludes with a discussion of other practical issues involved in rearchitecting the UML modeling framework in the proposed way.

A fundamental principle in engineering, including software engineering, is to minimize the amount of accidental complexity which is introduced into engineering solutions due to mismatches between a problem and the technology used to represent the problem. As model-driven development moves to the center stage of software engineering, it is particularly important that this principle be applied to the technologies used to create and manipulate models, especially models that are intended to be free of solution decisions. At present, however, there is a significant mismatch between the "two level" modeling paradigm used to construct mainstream domain models and the conceptual information such models are required to represent-a mismatch that makes such models more complex than they need be. In this paper, we identify the precise nature of the mismatch, discuss a number of more or less satisfactory workarounds, and show how it can be avoided.

The product line and component-based approaches to software engineering both hold the potential to significantly increase the level of reuse in industrial software development and maintenance. They also have complementary strengths, since they address the problem of reuse at opposite ends of the granularity spectrum; product line development essentially supports "reuse in the large" while component based development supports "reuse in the small." This paper describes a method, KobrA, that cleanly integrates the two paradigms into a systematic, unified approach to software development and maintenance. Key synergies resulting from this integration include support for the rapid and flexible instantiation of system variants, and the provision of methodological support for componentbased framework development.

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