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The UML metamodel has been architected with the following design principles in mind:
• Modularity — This principle of strong cohesion and loose coupling is applied to group constructs into packages and organize features into metaclasses.
• Layering — Layering is applied in two ways to the UML metamodel. First, the package structure is layered to separate the metalanguage core constructs from the higher-level constructs that use them. Second, a 4-layer metamodel architectural pattern is consistently applied to separate concerns (especially regarding instantiation) across layers of abstraction.
• Partitioning — Partitioning is used to organize conceptual areas within the same layer. In the case of the InfrastructureLibrary, fine-grained partitioning is used to provide the flexibility required by current and future metamodeling standards. In the case of the UML metamodel, the partitioning is coarser-grained in order to increase the cohesion within packages and loosening the coupling across packages.
• Extensibility — The UML can be extended in two ways: 1) a new dialect of UML can be defined by using Profiles to customize the language for particular platforms (e.g., J2EE/EJB, .NET/COM+) and domains (e.g., finance, telecommunications, aerospace); 2) a new language related to UML can be specified by reusing part of the InfrastructureLibrary package and augmenting with appropriate metaclasses and metarelationships. The former case defines a new dialect of UML, while the latter case defines a new member of the UML family of languages.
• Reuse — A fine-grained, flexible metamodel library is provided that is reused to define the UML metamodel, as well as other architecturally related metamodels, such as the Meta Object Facility (MOF) and the Common Warehouse Metamodel (CWM).