SysML FAQ: What is a Sequence diagram (SD) and how is it used?

What is a SysML Sequence diagram?

Definitions

Message: A Message (notation: arrow) represents communication from one object to another, with the expectation that a useful behavior will ensue. Messages may be synchronous (notation: open arrowhead) or asynchronous (notation: black-triangle arrowhead).

Sequence diagram (seq): A Sequence diagram is a dynamic behavioral diagram that shows interactions (collaborations) among distributed objects or services via sequences of messages exchanged, along with corresponding (optional) events.

• collaborating objects or services are Parts depicted as Lifelines (notation: rectangle with a dashed vertical line below)
• Combined Fragment operators support recursive nesting and Turing Complete semantics (Alternative [alt], Optional [opt], Parallel [par], Loop [loop], etc.)
• compare and contrast: Message Sequence Charts (MSCs).

Purpose

The purpose of Sequence diagrams is to specify dynamic system behaviors as message-passing collaborations among prototypical Blocks (Parts). When properly applied (See Usage Notes below) Activity diagrams are recursively scalable and simulatable.