Sequence Diagram

A Sequence diagram is one of four types of Interaction diagram. (The other three are Timing Diagram, Interaction Overview Diagram and Communication Diagram.)

A Sequence diagram is a structured representation of behavior as a series of sequential steps over time. It is used to depict work flow, message passing and how elements in general cooperate over time to achieve a result.

• Each sequence element is 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 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 configure a Sequence diagram, see the following topics:

• Denote the Lifecycle of an Element
• Layout of Sequence Diagrams
• Sequence Element Activation
• Lifeline Activation Levels
• Message Label Visibility
• Change the Top Margin
• Change the Timing Details.

Also take note of the important information in the Sequence Diagrams and Version Control topic.

Robustness diagrams, used extensively in ICONIX, can be created as Sequence diagrams.

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

Example Diagram

The following example Sequence diagram demonstrates several different elements:

Toolbox Elements and Connectors

Select Sequence diagram elements and connectors from the Interaction pages of the Toolbox.

Enterprise Architect also supports a number of stereotyped elements to represent various entities in business modeling.

OMG UML Specification

The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 503) states:

A sequence diagram describes an Interaction by focusing on the sequence of Messages that are exchanged, along with their corresponding OccurrenceSpecifications on the Lifelines.