Use Cases (Architectural Blue Print)

This document synthesizes the principles and practices of using Use Cases for software requirements engineering. Use cases are defined as text-based stories that discover and record system requirements, primarily focusing on functional aspects from a user's perspective. They serve as a central mechanism in modern methodologies for defining what a system must do, emphasizing user goals and interactions over traditional feature lists. The core components of the use case approach include actors (entities with behavior, such as people or other systems), scenarios (sequences of actions), and use case diagrams that visually map the relationships between them. Actors are categorized into three types: Primary Actors, who initiate an interaction to achieve a goal; Supporting Actors, who provide services to the system; and Offstage Actors, who have an interest in the use case's outcome. Use cases themselves can be written in varying levels of detail from a Brief one-paragraph summary to a Casual multi-paragraph description, and culminating in a Fully Dressed format. The Fully Dressed use case is a comprehensive specification that details all steps, variations, pre- and post-conditions, and non-functional requirements, providing a exhaustive blueprint for development and testing.