Use Case Diagram

Definition

A use case diagram is a mid-level requirement diagram that documents the relationships between use cases and the externals with which they interface.

Objectives

The typical objectives of a use case diagram is to:

• Summarize the operational requirements of a blackbox business enterprise, contact center, data center, application, or component.

Benefits

The typical benefits of a use case diagram are to:

• It summarizes how the use case model is organized in terms of externals and use cases.
• It summarizes the operational requirements.
• It drives the development of the rest of the operational requirements.

Contents

The typical contents of a use case diagram are:

• Use Cases
• Externals:
  • Roles (a.k.a., actors)
  • Teams
  • Organizations
  • Application (e.g., system or software)
  • Databases
• Relationships:
  • Inheritance and referential relationships between externals
  • Referential relationships (uses) between externals and use cases
  • Referential relationships (invokes, precedes) between use cases

Stakeholders

The typical stakeholders of a use case diagram are:

• Producers:
  • Requirements Team
• Evaluators:
  • Project Manager
  • Technical Leader
• Approvers:
  • Project Manager
  • Technical Leader
  • Customer Representative
• Maintainers:
  • Requirements Team
• Users:
  • Architecture Team

Phases

• Business Strategy: Completed
• Business Optimization: Not Applicable
• Initiation: Completed
• Construction: Maintained
• Delivery: Maintained
• Usage: Maintained
• Retirement: Archived

Preconditions

Use case diagrams can typially be started if the following preconditions hold:

• The business strategy phase or initiation phase has started.
• The requirements team is adequately staffed and trained in requirements analysis.

Inputs

The typical inputs to a use case diagram include:

• Work Products:
  • Application Vision Statement
  • Context Diagram
  • Domain Object Diagram
• Stakeholders:
  • TBD

Guidelines

• Place the icons for the use cases in the middle of the diagram, and place the icons for the external actors, hardware, software, systems, etc. around the outside of the diagram.
• Try to have the dependency relationships between actors and use cases flow from top to bottom:
  • Place client externals at the top, peer externals at the sides, and server externals at the bottom.
  • Place client use cases above server use cases when drawing “uses” relationships between them.
• Clarify the dependency relationships of arcs by using one or more arrowheads.
• Try not to have relationship arcs cross.
• Where practical and it does not violate the preceding guidelines, order use cases top-to-bottom and left-to-right in temporal order.
• Use different and intuitive icons for the different kinds of actors:
  • Use stick figures for human actors.
  • Use three dimensional boxes for hardware.
  • Use cylindars for databases.
  • Use rounded rectangles or clouds for software.

Conventions

Use case diagrams are typically constrained by the following conventions:

• Work Flow
• Content and Format Standard
• Inspection Checklist

Examples

Electronic Auction System Use Case Diagram

The following system use case diagram documents the use cases performed by the Buyer actor of an electronic auction system.