The interaction view describes sequences of message exchanges among the parts of a system. An interaction is based on a structured classifier or a collaboration. A role is a slot that may be filled by objects in a particular use of an interaction. This view provides a holistic view of behavior in a system—that is, it shows the flow of control across many objects. The interaction view is displayed in two diagrams focused on different aspects: sequence diagrams (Figure 3-9) and communication diagrams (Figure 3-10).