Sequence Diagram

A Sequence diagram is a structured representation of behavior as a series of sequential steps over time. You can use it to:

  • Depict workflow, Message passing and how elements in general cooperate over time to achieve a result
  • Capture the flow of information and responsibility throughout the system, early in analysis; Messages between elements eventually become method calls in the Class model
  • Make explanatory models for Use Case scenarios; by creating a Sequence diagram with an Actor and elements involved in the Use Case, you can model the sequence of steps the user and the system undertake to complete the required tasks

Construction

  • Sequence elements are arranged in a horizontal sequence, with Messages passing back and forward between elements
  • Messages on a Sequence diagram can be of several types; the Messages can also be configured to reflect the operations and properties of the source and target elements (see the Notes in the Message Help topic)
  • An Actor element can be used to represent the user initiating the flow of events
  • Stereotyped elements, such as Boundary, Control and Entity, can be used to illustrate screens, controllers and database items, respectively
  • Each element has a dashed stem called a Lifeline, where that element exists and potentially takes part in the interactions

To toggle the numbering of messages on a Sequence diagram, select or deselect the 'Show Sequence Numbering' checkbox on the 'Options' dialog.

You generate Sequence diagram elements and connectors from the 'Interaction' pages of the Toolbox.

Example Diagram

This example Sequence diagram demonstrates several different elements.

Sequence Diagram Element Toolbox Items

Icon

Description

See also

An Actor is a user of the system; user can mean a human user, a machine, or even another system or subsystem in the model.

Actor

A Lifeline represents a distinct connectable element and is an individual participant in an interaction.

Lifeline

Boundary elements are used in analysis to capture user interactions, screen flows and element interactions.

Boundary

A Control organizes and schedules other activities and elements.

Control

An Entity is a stereotyped Object that models a store or persistence mechanism that captures the information or knowledge in a system.

Entity

A Fragment element can represents iterations or alternative processes in a sequence diagram.

Fragment

An Endpoint is used in Interaction diagrams to reflect a lost or found Message in sequence.

Endpoint

A Diagram Gate is a simple graphical way to indicate the point at which messages can be transmitted into and out of interaction fragments.

Diagram Gate

The State/Continuation element serves two different purposes for Sequence diagrams, as State Invariants and Continuations.

State/Continuation

You can use an Interaction element to insert an Interaction diagram as a child of a Class element.

Interaction

Sequence Diagram Connector Toolbox Items

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Description

See also

A Message indicates a flow of information or transition of control between elements.

Message

A Self-Message reflects a new process or method invoked within the calling lifeline's operation.

Self Message

A Recursion is a type of Message used in Sequence diagrams to indicate a recursive function.

Recursion

A Call is a type of Message connector that extends the level of activation from the previous Message.

Call

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