Modeling Message-Based Behavior with Interactions

This chapter discusses the use of sequence diagrams to model how parts of a block interact by exchanging messages. Messages are exchanged between lifelines and typically represent an invocation of an operation or a sending of a signal. Messages do not represent data flows, but the flow of data (or other items such as matter or energy) can be captured via arguments of the message. Messages are sent and received by behaviors executing on the lifelines and can be either asynchronous (sender continues executing), or synchronous (sender waits for a response). Messages are sent by behaviors that are executing on the lifeline, or, more precisely, invocation actions, such as send signal or call operation actions, within those behaviors. Receipt of a message by a lifeline often triggers the execution of a behavior but may simply be accepted by a currently executing behavior.