Size and Complexity
One of the biggest complaints about UML is that it is too large and too complex. Typically a project that uses UML only uses 20% of the specification to help build 80% of the code and other deliverables. This 20% can vary according to the type of project: real-time systems, telecommunications, web applications, business applications, etc. What is considered essential may vary according to the kind of project, but in all cases the unused 80% obscures and complicates the essential.
To address this complaint, UML should be described differently for different user groups. There are ordinary users such as analysts, designers, website builders, database designers, developers, operators, architects, and testers, each bringing a different -- but valid -- perspective that uses different but overlapping subsets of UML. A particular class of users comprises the designers of UML itself and UML tool builders. It goes without saying that if the language is complex, these designers will have a hard time creating a language that is complete, consistent, extensible, and able to integrate with other languages, and the number of specification defects will become high.
Figure 1 depicts the main components of UML 2 and the dependencies between them. Although there is some layering, the overall structure contains a lot of cyclic dependencies, which makes it difficult to define useful subsets of the language. The UML specification does define formal "compliance points" which supposedly describe legal subsets of UML, but UML tool vendors have taken little or no notice of these, because they do not correspond to important use cases for UML users.
A key point with the current UML is that there is no way in a compliant implementation to use the simple version of a concept without having the complicated version; for example, take Class. Most users think of a Class as a simple thing that has attributes, operations, inheritance etc. But a UML 2 class also has Ports, Parts, Connectors, and Receptions -- concepts only useful in specialized domains. There is no way to have just the simple one, so all users are burdened with the understanding required by advanced users. This can be -- and is -- mitigated to some extent by good tools. However, we believe that the simple options should be inherent in the language definition itself. Furthermore, the UML Class differs in detail from the concept of Class found in any particular programming language, which introduces additional conceptual barriers between UML and those of its users who are software developers. Again, flexibility of these definitions should be inherent in the language, so that it can be fine-tuned to match to modern programming technologies.
In summary, there are two major challenges to be addressed: complexity of the UML specification itself, and the need to describe UML in coherent subsets that address the actual needs of users in particular domains.
To address the first challenge, as a direct response to the feedback from the RFI, the OMG has embarked on a program of simplification for the UML specification. By the middle of 2011, a much simplified version of the UML specification should be available in which cyclic dependencies and redundancy have been greatly reduced. This specification will be compatible with existing tools and models, but described in a way that makes it much more amenable to further simplification and integration.