use case diagram is "a diagram that shows the relationships
among actors and use cases within a system."
Use case diagrams are often used to:
an overview of all or part of the usage requirements for a system or
organization in the form of an
essential model or a business model
the scope of a development project
your analysis of your usage requirements in the form of a
system use case
A use case model is comprised of one or more use case diagrams and any
supporting documentation such as use case specifications and actor definitions.
Within most use case models the use case specifications tend to be the primary
artifact with use case diagrams filling a supporting role as the "glue" that
keeps your requirements model together. Use case models should be
developed from the point of view of your project stakeholders and not from the
(often technical) point of view of developers. There are guidelines for:
System Boundary Boxes
A use case describes a sequence of actions that provide a
measurable value to an actor. A use
case is drawn as a horizontal ellipse on a UML use case diagram, as you see in
- Use Case Names Begin With a Strong Verb
- Name Use Cases Using Domain Terminology
- Place Your Primary Use Cases In The Top-Left Corner Of The Diagram
- Imply Timing Considerations By Stacking Use Cases. As you see in
1, the use cases that typically occur first are shown above those that appear
Figure 1. Implying timing considerations between use
An actor is a person, organization, or external system that
plays a role in one or more interactions with your system (actors are typically drawn as
stick figures on UML Use Case diagrams).
Figure 2. Online shopping.
- Place Your Primary Actor(S) In The Top-Left Corner Of The Diagram
- Draw Actors To The Outside Of A Use Case Diagram
- Name Actors With Singular, Business-Relevant Nouns
- Associate Each Actor With One Or More Use Cases
- Actors Model Roles, Not Positions
- Use <<system>> to Indicate System Actors
- Actors Don’t Interact With One Another
- Introduce an Actor Called "Time" to Initiate Scheduled Events
There are several types of relationships that may appear on
a use case diagram:
association between an actor and a use case
association between two use cases
generalization between two actors
generalization between two use cases
Associations are depicted as lines connecting two modeling
elements with an optional open-headed arrowhead on one end of the line
indicating the direction of the initial invocation of the relationship.
Generalizations are depicted as a close-headed arrow with the arrow pointing
towards the more general modeling element.
Figure 3. Enrolling students in a university.
Indicate An Association Between An Actor And A Use Case If The Actor
Appears Within The Use Case Logic
Avoid Arrowheads On Actor-Use Case Relationships
Apply <<include>> When You Know Exactly When To Invoke The
Apply <<extend>> When A Use Case May Be Invoked Across
Several Use Case Steps
Introduce <<extend>> associations sparingly
Generalize Use Cases When a Single Condition Results In Significantly New
Do Not Apply <<uses>>, <<includes>>, or
Avoid More Than Two Levels Of Use Case Associations
Place An Included Use Case To The Right Of The Invoking Use Case
Place An Extending Use Case Below The Parent Use Case
Apply the "Is Like" Rule to Use Case Generalization
Place an Inheriting Use Case Below The Base Use Case
Apply the "Is Like" Rule to Actor Inheritance
Place an Inheriting Actor Below the Parent Actor
The rectangle around the use cases is called the system
boundary box and as the name suggests it indicates the scope of your system -
the use cases inside the rectangle represent the functionality that you intend
- Indicate Release Scope with a System Boundary Box. In
Figure 2 you see that three system boundary boxes are
included, each of which has a label indicating which release the various use
cases have been assigned to.
- Avoid Meaningless System Boundary Boxes.