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Sequence Diagram

A Sequence diagram is a type of Interaction diagram that shows the time ordered interaction between objects. The diagram has two axes; the vertical axis represents time and the horizontal axis represents the objects that take part in the interaction, typically ordered in a way that best illuminates the interaction. These diagrams have their origin in the modeling of software interactions, but they can be used with systems engineering to be prescriptive of how elements (such as Blocks) should interact, or descriptive in showing how they do interact, in practice.

This Sequence diagram shows the interactions and sequence of message flows between a driver and a vehicle. The diagram expresses the necessary interactions for the 'Drive the Vehicle' Use Case. The interaction is owned by the 'AutomotiveDomain' Block.


The main elements that can appear in Parametric diagrams are:

  • Sequence
  • Fragment
  • Endpoint
  • Diagram Gate
  • State/Continuation

The main connectors that can appear in Parametric diagrams are:

  • Message
  • Self Message
  • Recursion
  • Dependency


A variety of tools can be used with behavioral modeling and Activity diagrams, including:

  • Diagram Filters - which allows a user to filter elements out of the diagram to achieve a more specific focus,
  • Pan and Zoom - which allows a modeler or viewer to easily move around large diagrams,
  • Spreadsheet (CSV) Import and Export - which allows content in spreadsheets to be imported or exported from the model,
  • Documentation - which allows formal or informal documentation to be generated from the model in a variety of formats,
  • Traceability - which provides a hierarchical view of an element's relationships to other model elements,
  • Responsibilities Window - which provides a composite view of the important responsibilities of an element, including Constraints, Requirements and Scenarios

Relationship Matrix - which allows the connections between Activities (or other elements) and other elements such as Blocks, Requirements and Use Cases to be visualized in a matrix.


The very fact that we are modeling a system implies that it has a degree of complexity that can not be managed without the use of tools. Sequence diagrams can be used to represent system scenarios showing how parts of a system interact together to achieve some specified outcome. Messages are said to be exchanged between lifelines representing the lifetime of the object, the messages represent operations or signals sent and received by the objects.