CS499 Use Cases References From Alistair Cockburn Writing Effective Use Cases (Book) - Use Case Fundamentals (Article) - Structuring Use Cases with Goals (Article)

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

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

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)

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

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

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)

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

CS499 An Example Add Media Copy Actors: Librarian Goal: Add a copy of a media item to the library. Precondition: Media Item exists in library. LibrarianSystem 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

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

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)?