9a Use Case Diagrams
Use Case diagrams serve two purposes: As a form of graphical table of contents
listing the individual use-cases, and also to define the boundary of what is included as
part of the proposed system and what is not included.
A use case diagram identifies the boundaries between the users (actors) and the
product. You arrive at the product boundary by inspecting each business use case and
determining, in conjunction with the appropriate stakeholders, which part of the
business use case should be automated (or satisfied by some sort of product) and what
part should be done by the user. This task must take into account the abilities of the
actors (section 3), the constraints (section 4), the goals of the project (section 1), and
your knowledge of both the work and the technology that can make the best
contribution to the work.
The use case diagram shows the actors outside the product boundary (the rectangle).
The product use cases are the ellipses inside the boundary. The lines denote usage.
Note that actors can be either automated or human.
Depending on the complexity of the product it may be necessary to use more than one
diagram to list all of the use cases. When more than one diagram is required the usecases
can be divided up several ways: Normal operations versus exceptional cases, or
daily tasks versus monthly tasks, or user tasks versus administration tasks, etc.