Composite structure diagrams are an addition to UML 2, although one style used to be referred to as an instance collaboration diagram. Composite structure diagrams are used to

1. ■ depict the internal structure of a classifier (such as a class, component, or use case), including the interaction points of the classifier to other parts of the system;

2. ■ explore how a collection of cooperating instances achieves a specific task or set of tasks;

3. ■ describe a design or architectural pattern or strategy.

There are two basic styles of UML composite structure diagram, the “collaboration style” depicted in Figure 53 from UML 1 and the new “detailed style” of Figure 54. Both diagrams model the same concept, the Persist Object Via Framework architectural strategy (Ambler 2003). Each box represents a lifeline and the dashed oval a collaboration. In the case of Figure 53 the focus is on the overall collaboration required to persist business objects, whereas in Figure 54 the focus is on the individual collaborations required to do so. The diagrams could in fact be combined—the detailed style of diagram could be encircled by a collaboration bubble.

Focus on Object Roles in Collaboration-Style Composite Structure Diagrams

In collaboration-style composite structure diagrams, such as Figure 53, most of the value of the diagram comes in identifying the various roles that lifelines play in the collaboration.

Focus on Collaborations in Detailed-Style Composite Structure Diagrams

In detailed-style composite structure diagrams, such as the one depicted in Figure 54, the value comes from identifying the collaborations between lifelines.