Package Aspects

If you think about Figure 7.2, you'll realize that the diagram has two kinds of structures. One is a structure of layers in the application: presentation, domain, data mapper, and database. The other is a structure of subject areas: leasing and assets.

You can make this more apparent by separating the two aspects, as in Figure 7.3. With this diagram, you can clearly see each aspect. However, these two aspects aren't true packages, because you can't assign classes to a single package. (You would have to pick one from each aspect.) This problem mirrors the problem in the hierarchic namespaces in programming languages. Although diagrams like Figure 7.3 are nonstandard UML, they are often very helpful in explaining the structure of a complex application.