In This Chapter
In every version of the UML, the authors have applied a four-layer metamodel architecture. Although time pressures kept the original specification's implementation from being all it needed to be for the long term, UML 1.4 successfully adheres to the four-layer concept. The authors of 2.0 have taken great pains to expand on the four-layer approach, and their effort results in an improved implementation of it. I'll step through both UML 1.4 and 2.0 versions after I discuss the significance of the four-layer metamodel architecture.
Understanding the four-layer model will be easier if we start with an example using two layers and build up. To do this, though, I need to use Class diagram notation. If you are unfamiliar with this Class diagram notation, you may want to read Chapters 5 and 6 first, and then come back here.