System Sequence Diagrams in UML

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A system sequence diagram is, as the name suggests, a type of sequence diagram in UML. These charts show the details of events that are generated by actors from outside the system.

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What are system sequence diagrams in UML?

System sequence diagrams, also known as SSD, are actually a sub-type of sequence diagrams, whose style and notation is dictated by the Unified Modeling Language. This language provides a toolkit for diagram creators to make and read diagrams that are comprehensible regardless of location or industry.

Standard sequence diagrams show the progression of events over a certain amount of time, while system sequence diagrams go a step further and present sequences for specific use cases. Use case diagrams are simply another diagram type which represents a user's interaction with the system. Most elements we cover in use case diagrams remain in use throughout a system sequence diagram, including:

  • Objects

    - this box shape with an underlined title represents a class, or object, in UML. Within a SSD, this shape models the system as a black box (a system with inner workings that are not immediately visible).

  • Actors

    - shown by stick figures, actors are entities that interact with the system, and yet are external to it.

  • Events

    - the system events that the actors generate in the sequence. A dashed line, known as a lifeline, represents events in an SSD. Lifelines may begin with a labeled rectangle shape or an actor symbol.

Benefits of system sequence diagrams

SSDs are ideal for demonstrating when and how tasks are completed in a system, especially as those tasks relate to use cases. Here are a few specific examples of when to use SSDs:

  • In a step-wise fashion, modeling operations of the system in response to events.

  • Building a blueprint for the main success scenario of a given use case, then creating alternative paths.

  • Identifying major system events and operations in order to come up with realistic estimates of resources needed.

system sequence diagram uml

Additional Resources

Creating system sequence diagrams in Lucidchart is simple and intuitive. Simply drag shapes onto the canvas and use editor tools to personalize the chart. Get started by signing up today!

Do you want to create your own UML diagram? Try Lucidchart. It's fast, easy, and totally free.

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