1.2 The Model Driven Architecture

The Model Driven Architecture (MDA) is a framework for software development defined by the Object Management Group (OMG). Key to MDA is the importance of models in the software development process. Within MDA the software development process is driven by the activity of modeling your software system.

In this section we first explain the basic MDA development life cycle, and next illustrate how MDA can help to solve (at least part of) the problems mentioned in the previous sections.

1.2.1 The MDA Development Life Cycle

The MDA development life cycle, which is shown in Figure 1-2, does not look very different from the traditional life cycle. The same phases are identified. One of the major differences lies in the nature of the artifacts that are created during the development process. The artifacts are formal models, i.e., models that can be understood by computers. The following three models are at the core of the MDA.

Figure 1-2. MDA software development life cycle

Platform Independent Model

The first model that MDA defines is a model with a high level of abstraction that is independent of any implementation technology. This is called a Platform Independent Model (PIM).

A PIM describes a software system that supports some business. Within a PIM, the system is modeled from the viewpoint of how it best supports the business. Whether a system will be implemented on a mainframe with a relational database or on an EJB application server plays no role in a PIM.

Platform Specific Model

In the next step, the PIM is transformed into one or more Platform Specific Models (PSMs). A PSM is tailored to specify your system in terms of the implementation constructs that are available in one specific implementation technology. For example, an EJB PSM is a model of the system in terms of EJB structures. It typically contains EJB-specific terms like "home interface," "entity bean," "session bean," and so on. A relational database PSM includes terms like "table," "column," "foreign key," and so on. It is clear that a PSM will only make sense to a developer who has knowledge about the specific platform.

A PIM is transformed into one or more PSMs. For each specific technology platform a separate PSM is generated. Most of the systems today span several technologies, therefore it is common to have many PSMs with one PIM.

Code

The final step in the development is the transformation of each PSM to code. Because a PSM fits its technology rather closely, this transformation is relatively straightforward.

The MDA defines the PIM, PSM, and code, and also defines how these relate to each other. A PIM should be created, then transformed into one or more PSMs, which then are transformed into code. The most complex step in the MDA development process is the one in which a PIM is transformed into one or more PSMs.

Raising the Level of Abstraction

The PIM, PSM, and code are shown as artifacts of different steps in the development life cycle. More importantly, they represent different abstraction levels in the system specification. The ability to transform a high level PIM into a PSM raises the level of abstraction at which a developer can work. This allows a developer to cope with more complex systems with less effort.

1.2.2 Automation of the Transformation Steps

The MDA process may look suspiciously much like traditional development. However, there is a crucial difference. Traditionally, the transformations from model to model, or from model to code, are done mainly by hand. Many tools can generate some code from a model, but that usually goes no further than the generation of some template code, where most of the work still has to be filled in by hand.

In contrast, MDA transformations are always executed by tools as shown in Figure 1-3. Many tools are able to transform a PSM into code; there is nothing new to that. Given the fact that the PSM is already very close to the code, this transformation isn't that exciting. What's new in MDA is that the transformation from PIM to PSM is automated as well. This is where the obvious benefits of MDA come in. How much effort has been spent in your projects with the painstaking task of building a database model from a high-level design? How much (precious) time was used by building a COM component model, or an EJB component model from that same design? It is indeed about time that the burden of IT-workers is eased by automating this part of their job.

Figure 1-3. The three major steps in the MDA development process

At the time of writing, the MDA approach is very new. As a result of this, current tools are not sophisticated enough to provide the transformations from PIM to PSM and from PSM to code for one hundred percent. The developer needs to manually enhance the transformed PSM and/or code models. However, current tools are able to generate a running application from a PIM that provides basic functionality, like creating and changing objects in the system. This does allow a developer to have immediate feedback on the PIM that is under development, because a basic prototype of the resulting system can be generated on the fly.