9.1 Overview

In Chapter 8, behavior was modeled in terms of activity diagrams to represent a controlled sequence of actions that transform inputs to outputs. In this chapter, an alternative approach to representing behavior is introduced; it uses sequence diagrams to represent the interaction between structural elements of a block as a sequence of message exchanges. The interaction can be between the system and its environment or between the components of a system at any level of a system hierarchy. A message can represent the invocation of a service on a system component or the sending of a signal.

This representation of behavior is useful when modeling service-oriented concepts, where one part of a system requests services of another part. A service-oriented approach can be useful for representing discrete interactions between software components, where one software component requests a service of another and where the service is specified as a set of operations. However, the sequence diagram is in no way limited to modeling interactions between software components and has found broad application in modeling system-level behaviors as well. An interaction can be written as a specification of how parts of a system should interact, but it can also be used as a record of how the parts of a system did interact.

The structural elements of a block are represented by lifelines on a sequence diagram. The sequence diagram...