UML Diagram - Sequence Diagram

A UML sequence diagram provides insight and overview of object interactions arranged in a time sequence. It shows the objects and classes that are involved in a scenario and a sequence of messages exchanged between objects needed to carry out functionality.

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When to Draw a Sequence Diagram?

A UML sequence diagram shows the scenario (flow of events) in one single use case.

The flow of messages in the sequence diagram is based on the story told by a use case.

If you want to draw a sequence diagram, you normally need to draw a use case diagram and write down a description of what the use case in question actually does.

The Elements of a Sequence Diagram

A sequence diagram consist of five elements

• Lifeline Notations
• Activation Bars
• Message Arrows

Below you see three examples of these elements

UML Lifeline notation with an Actor symbol

UML Activation bar

UML Message Arrows