Use Case Diagrams

No system exists in isolation. Every system interacts with human or automated actors that use the system to accomplish some goal.

Definition: A use case specifies the behavior of the boundaries of the system and is a description of a set of sequences of actions that a system performs to yield an observable result to an actor.

One applies use cases to capture intended responsibilities of a system in development, without having to specify how that behavior is implemented. i.e. Use cases specify the what, not the how. Well structured use cases specify essential system behaviors only, and are neither overly general or dwell in esoteric minutiae.

Use case diagrams are one of the basic five diagrams in UML for modeling systems. Use case diagrams are generally used for the following reasons:

Helps visualize the related responsibilities of a system.
Help document the behavior of an element of the system.
Help codify the actors and their interactions with the system.
Help guide test plans and test cases for evaluation of the quality of the developed system.
Help define the scope of a system's responsibilities.

When you create use case diagrams in the UML, remember that every use case diagram is just a presentation of the static use case view of the system. No single use case diagram can capture the complete essence of a system.

Hints and tips for well-structured use case diagrams:

Focus on communicating one aspect of a system's responsibilities.
Limit the actors and use cases to only those essential for understanding a single aspect of the system.
Not overly minimalist as to leave out important semantic details.
Give each diagram a title that communicates its purpose.
Avoid crossing lines whenever possible.
Organize so that like responsibilities of the system are spatially close together.