http://alistair.cockburn.us/

CS499

Use Cases

References

From Alistair Cockburn

Writing Effective Use Cases (Book)

http://alistair.cockburn.us/

- Use Case Fundamentals (Article)

- Structuring Use Cases with Goals (Article)

http://alistair.cockburn.us/

CS499

What is an Actor?

• Basically users of the system

• Actually user groups or categories

• External entities (people or systems)– That interact with the system– In order to achieve a desired goal

http://alistair.cockburn.us/

CS499

What is a Use Case?

• A description of what happens when users interact with the system

• A collection of scenarios about how an Actor uses the system to achieve a particular goal

http://alistair.cockburn.us/

CS499

Use Cases• Hold Functional Requirements in an easy to read, easy to track text format

• Represents the goal of an interaction between an actor and the system. The goal represents a meaningful and measurable objective for the actor.

• Records a set of paths (scenarios) that traverse an actor from a trigger event (start of the use case) to the goal (success scenarios)

• Records a set of scenarios that traverse an actor from a trigger event toward a goal but fall short of the goal (failure scenarios)

• Are multi-level: one use case can include/extend the functionality of another

Use Cases Do Not…

• Specify user interface design. They specify the intent, not the action Detail

• Specify implementation detail (unless it is of particular importance to the actor to be assured that the goal is properly met)

http://alistair.cockburn.us/

CS499

How Are Use Cases Used?• To Capture the Functional Requirements of the system• To act as a springboard for the software design• To validate the software design• For Software Test and Quality Assurance. (Tests are

performed to validate proper and complete implementation of the use cases)

• Potentially as an initial framework for the on line help and user manual

http://alistair.cockburn.us/

CS499

Types of Actors

• Primary Actor– The Actor(s) using the system to achieve a goal.– The Use Case documents the interactions between the

system and the actors to achieve the goal of the primary actor

• Secondary Actor– Actors that the system needs assistance from to achieve the

primary actors goal

http://alistair.cockburn.us/

CS499

Use Case Writing Process

• Suggested in Writing Effective Use Cases• Managing Your Energy

– Start out at a high level and add detail as you go– Too much detail too fast puts you in a corner,

change becomes difficult– It is an iterative, incremental process (use cases,

and OO software development)

http://alistair.cockburn.us/

CS499

Four levels of Use Case “Precision”

• Actors and Goals – List all of the Actors and their goals

• Use Case Brief (or Main Success Scenario) – Write the trigger and the main success scenario

• Failure Conditions – Brainstorm all the failures that could occur.

• Failure Handling – Describe how the system should handle each type of failure

http://alistair.cockburn.us/

CS499

An ExampleAdd Media Copy

Actors: Librarian

Goal: Add a copy of a media item to the library.

Precondition: Media Item exists in library.

Librarian System

1. Searches for the media item

  2. Displays media item information.

3. Issues the command to add a new copy.

  4. Requests copy information (according to media type)

5. Supplies copy information.

  5. Validates the information.

6. Saves the information and informs user.

Exceptions 

1a – Media item is not found (redirect to Add media item)

3a, 5a – Librarian cancels operation.

6a – 1 – Copy is a duplicate

6a – 2 - Required information is missing

6a – 3 – Data does not meet expected format

http://alistair.cockburn.us/

CS499

Trouble Getting Started?

• Cockburn recommends the use of Usage Narratives as a “warm-up”

• A Usage Narrative describes a particular interaction, such as “Joe enters the copy information…”

• User Stories are utilized in eXtreme Programming are similar to Usage Narratives

http://alistair.cockburn.us/

CS499

Validating Use Cases• Since Use Cases are used in so many stages of software

development, it is important to validate them

• Questions to ask:– Is the Use Case complete? Are there any details that need to be added?

– Do I feel confident that the actor’s goal is going to be properly met?

– Are there any procedural or requirement changes I can suggest that would simplify the process depicted in the Use Case?

– Are there any additional Goals of the Actors that are not addressed?

– Are there any additional Actors that are not represented (directly or indirectly)?