Dynamic modeling provides a dynamic (also referred to as behavioral) view of a system in which control and sequencing is considered, either within an object (by means of a state machine) or among objects (by analysis of object interactions). Dynamic state machine modeling is described in Chapter 7. This chapter describes dynamic interaction modeling among objects. However, for state dependent control objects, this chapter also describes how state machines are used to help determine state dependent object interactions. Please note that all references to system in this chapter are to the software system.

Dynamic interaction modeling is based on the realization of the use cases developed during use case modeling. For each use case, it is necessary to determine how the objects that participate in the use case dynamically interact with each other. The object structuring criteria described in Chapter 8 are applied to determine the objects that participate in each use case. This chapter describes how, for each use case, an interaction diagram is developed to depict the objects that participate in the use case and the sequence of messages passed between them. The interaction is depicted on either a sequence diagram or a communication diagram. A textual description of the object interaction is also provided in a message sequence description.

There are two main kinds of dynamic interaction modeling. Stateless dynamic interaction modeling is applied if the interaction sequence does not involve a state dependent control object. State dependent dynamic interaction modeling is applied if at least one of the objects is a state dependent control object, in which case the interaction is state dependent and necessitates the execution of a state machine. State dependent dynamic interaction modeling is particularly important in real-time embedded systems, because object interactions in these systems are frequently state dependent.

For large systems, a preliminary determination of the subsystems is usually necessary – for example, based on geographical distribution, as in distributed component-based systems described in Chapter 12. The analysis is then conducted to determine the object communication in each subsystem. Subsystem structuring is carried out in more depth during the design phase as described in Chapter 10.

Section 9.1 gives an overview of object interaction modeling. Section 9.2 describes message sequence descriptions.