12.4 The Modeling Languages

What are the consequences of applying the MDA on the modeling languages? To be able to write PIMs that completely specify the system to be generated, including both static and dynamic aspects, we need a different set of modeling languages.

Today's modeling languages provide us with the means to specify the structural part of the functionality in a PIM. For the dynamic part, they depend on ordinary programming languages to fill in the gaps in the model. The action language used in Executable UML (Mellor and Balcer 2002) tries to fill this gap, but as explained in section 3.2.2, the available concepts are at the same abstraction level as the current procedural and object oriented languages. Unless the action language gets to a higher level of abstraction, it will not be able to support the MDA process fully.

A modeling language suited for MDA should offer the following:

• Expressive enough to specify systems completely. This includes both static and dynamic aspects of a system. There should be no need for the developers to fall back to ordinary programming languages.

• A general applicable , non application-specific, language. Application specific languages like 4GLs never really set off. Most programming is still done using generally applicable programming languages.

• Abstract often-used patterns of lower-level constructs into single higher-level constructs.

• Suitable for n-tier application development, including three-tier, two-tier, and single- tier applications. The actual number of tiers should be of no consequence in the model, but should be adjusted in the settings of the transformation tool.

• Suitable for distributed applications. Transformation tools should take care of building the bridges between the various nodes.

• Seamlessness between the model and its implementation.

• Support for managing large models, for instance by supporting an aspect-oriented manner of modeling (Kiczales 1997).

Note that when a modeling language is created that allows full specification of a system, including both static and dynamic aspects, it becomes more than a modeling language. In fact, the modeling languages of tomorrow will have the same status as the high-level programming languages of today.