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.

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