System Sequence Diagram Overview
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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System Sequence Diagram Meaning
System sequence diagrams 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.
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 cases are simply another diagram type which represent a user's interaction with the system. For a refresher course on sequence diagrams, you may want to visit this page on “What is a sequence diagram in UML?”. Most elements you see on that page 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.
When to Draw a System Sequence Diagram
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 on resources needed.
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