Use cases

A use case defines a sequence of actions that yields an observable result of value. Use cases provide a structure to express functional requirements within the context of business and system processes. Use cases can be represented graphically in a diagram and in a textual document.

A business use case defines a sequence of actions that a business performs that yields an observable result of value (a work output) to a particular business actor or that shows how the business responds to a business event.

A system use case is a sequence of actions that a system performs that yields an observable result of value to a particular actor (someone or something outside the system that interacts with the system).

Defining use cases and creating diagrams for them is part of the use case-driven development practice, which describes how to capture requirements with a combination of use cases and system-wide requirements, and then drive development and testing from those use cases. For an overview of the practice and links to many resources, see Use case-driven development practice enablement on developerWorks®.

Use cases in Rational® Requirements Composer share the same rich-text behavior that is described in Working with text documents.Use cases also include a spell checker. Misspelled words are marked with a red wavy underline. Right-click a misspelled word to select from suggested spellings or to add it to the project dictionary.

A use-case diagram can include multiple use cases and the relationships between use cases and the persons, groups, or systems that interact to perform the use case. Use-case diagrams include the following elements:

Table 1. Use-case diagram elements
Element Explanation
Use Case A use-case artifact. This is a textual specification that is created independently from a diagram and reused (or shared) in multiple use-case diagrams. It is displayed as a gray symbol in a diagram. To open the artifact, double-click a use case in a diagram.
Use Case Placeholder A symbol that is a placeholder for a use case in a diagram. This use case is created and used in a single use-case diagram. It is displayed as a white symbol in the diagram.
Actor An actor artifact that represents a person, group, or system that interacts with the use case. This is a textual explanation of the actor that is created independently from a diagram and is reused in multiple use-case diagrams. It is displayed as a gray symbol in a diagram. To open the actor artifact, double-click the actor in a diagram.
Actor Placeholder A symbol that is a placeholder for an actor in a diagram. The actor element is created and used in a single use-case diagram. It is displayed as a white symbol in the diagram.
Association A relationship between two elements in the diagram.
System Boundary Defines the extent of a system, which can include one or more use cases. The system boundary separates a system from actors and other systems.

Use-case artifacts provide textual detail for a use case. These use-case specifications often include content that is based on a template or process guidance that is similar to the following outline. See the topic Use-case specification outline for a description of each section in the outline.

  1. Use case name
  2. Brief description
  3. Flow of events
  4. Basic flow
  5. Alternative flows
  6. Special requirements
  7. Preconditions
  8. Post conditions
  9. Extension points

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