Introduction to Use Cases

1
Introduction to Use Cases

• One of the many tools within UML
• Supported by Together
• Used at the Feasibility and early Analysis stages
  of a project - to specify WHAT the system should
  do (not How - this comes later)
• Provide a catalyst for discussion with users

2
Use Cases - what for?

• Describe the functions of / processes within a
  system - the Use Cases
• Identify the boundary of a system
• Identify who and/or what uses the system - the
  Actors

3
Use Cases - approach

• Iteration - initially take a broad, top level
  view, then later add lower level detail
• as your investigation reveals more facts
• as you concentrate on specific areas
• Refinement - enable the Use Cases to evolve by
  correcting/improving previous iterations
• Document however little you know, then add to it.
  

4
Example of the Top-down approach

• Pause now and write down what you know about how
  to make Shortbread biscuits..

5
Use Cases - format

• Diagrams are easy to assimilate
• Processes in ellipses
• Actors as stick-people
• Interactions as lines
• Text descriptions provide detail
• much later!

6
Use Case - example
Watsons SalesWeb System
Report Sales
Salesperson
Accounting system
Review Individuals Sales
Inventory system
Review Regional Sales
Regional Sales Manager
(From Understanding UML, Harmon Watson)
7
Actors

• Include users and other systems
• an actor on a diagram a role
• 1 person/system may have 1 or more roles (i.e. in
  different interactions)
• 1 role may comprise 1 or more people
• by convention people are on left, systems on
  right

8
Processes

• Functions (initially the major ones only) within
  the system
• NOT departments in the organisation
• So the name must be a verb
• a system response may be a process with no actor
  involved

9
Interactions

• Where some exchange occurs between an actor and a
  process
• may be in either direction
• the detail of the interaction need not be known
  at this stage
• can be labelled/named

10
Finding the Actors and Use Cases

• Use appropriate fact-finding techniques
  (interviewing, questionnaires, observation,
  document inspection, )
• Identify all users, then the processes they
  initiate or participate in.
• Identify all external events to which the system
  must respond, then relate to actors and processes.

11
Exercise

• Manually draw a Use Case diagram for a video
  rental shop
• start with a top-level view
• add more detail after
• When completed put the diagram into Together