Use-Case Diagram – Actors, System Boundary, Include, Extend & Generalization

This article is a concept guide to use-case diagrams, complete with exam questions and tags.

In a Nutshell

A use-case diagram illustrates what functions a system offers from the perspective of its users (actors) and how they interact with it.

Core Definition

Use-case diagrams are part of UML (Unified Modeling Language) and visualize the functional requirements of a software system. They show which actors interact with which use cases. Each use case describes a complete, self-contained piece of system functionality. These diagrams are particularly valuable early in software development for requirements analysis and stakeholder communication. Common relationships like include, extend, and generalization clarify dependencies between use cases. Actors can be real users or external systems.

Exam Essentials

• Part of UML for requirements analysis
• Visualizes user and system interactions
• Composed of actors, system boundaries, and use cases
• Critical for requirements specifications and system design
• Facilitates communication with clients and end users
• Supports risk analysis through clear functional overview
• Can be used early on for effort estimation
• Must be properly documented and versioned

Core Components

1. Actor (e.g., user, external service)
2. System boundary
3. Use case
4. Relationships: include
5. Relationships: extend
6. Generalization between actors or use cases
7. System context definition
8. Interaction scenarios
9. Access control through role assignment
10. Validation against requirements documents

Practical Example

// Example: Online shop
Actor: Customer
Use cases: Create account, log in, place order
Relationship: "Place order" includes "log in"

Explanation: The customer can only place an order after logging in. Therefore, "log in" is a mandatory subprocess of the "place order" use case.

Strengths and Limitations

Strengths

• Builds shared understanding
• Clearly defines system responsibilities
• Easy to visualize
• Can be introduced early in modeling

Limitations

• Lacks technical detail
• Limited for modeling complex workflows
• Risk of over-abstraction

Common Exam Questions (with Brief Answers)

1. What does a use-case diagram represent? Shows which actors interact with which use cases of a system.

2. Include versus extend? Include forces a use case to execute; extend is optional and augments functionality as needed.

3. Elements in a use-case diagram? Actors, use cases, system boundary, and relationships (include, extend, generalization).

4. What purpose does the system boundary serve? Separates the system from its environment and clarifies what belongs to the system.

5. Primary versus secondary actor? Primary actors initiate the use case; secondary actors are required by the system.

6. Valuable early in a project? Helps gather requirements and verify them with stakeholders.

7. How does it support quality assurance? Clear functional separation makes testing more straightforward.

8. How does it contribute to security analysis? Identifies critical functions and access points for role-based controls.

Key References

1. https://www.omg.org/spec/UML/
2. https://www.visual-paradigm.com/guide/uml-unified-modeling-language/what-is-use-case-diagram/
3. https://www.ihk-aka.de/