UML Model – Case Diagram

In UML (Unified Modeling Language), a case diagram can summarise the details of the system users (actors) and their interactions with the system or other users (actors). In other words, the case diagram captures a high-level functionality of a system using notations for users (actors), use cases and relationship among them. Usually, the system architect or senior development manager (team leader) draw the case diagram, which provides the development team with a blueprint.

Key Elements

  • Use Case
    • Use case captures what users (actors) want to do with the system; for example, view an event schedule.
  • System
    • The system provides the solution, software or application; for example, a calendar app.
  • Actors
    • The party which/who use the system, and it can be a person or external service or app.
    • Two categories of Actor: Primary and Secondary.
    • The primary actor triggers the use case in case diagram.
    • The secondary actor is involved in the system in some way and it is often an external system.
  • Association
    • Association is the relationship between actors and the use case, the relationship between use cases and the relationship between actors.

Basic Model

This is a basic model of case diagram. In this diagram, there is a Actor (User), a system and a user case.

The arrow in this diagram means the actor is a primary user in this case.


Case Study

Students and staff need to access the campus event calendar from their smartphones, laptops or student portal. The event calendar intertwines with academic calendar during the semester time.

Model 1: Diagram indicates students and staffs can view the event calendar

In the above diagram, we have student users and staff users. The system is called Unical, and the user case is view event schedule. In this case, students and staff are primary actors (users) and no secondary actor involved.


Model 2: Diagram includes secondary actor

In this diagram, the student is the primary user and External DB (Academic Calendar) is the secondary user.

  • Primary Actor: Arrow travels from user to user case.
  • Secondary Actor: Arrow travels from user case to user.

Model 3: Diagram with associations include multiple use cases

In this diagram, View Event Schedule is the base use case, and it includes a login function.


Model 4: Use cases with extended additional behaviour

In this diagram, there is an external function (Select Date) to the user case.



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