Use Cases

1. Use Cases • Within Requirements discipline/workflow • Verbal descriptions of important functional (behavioral, transactional, block-box view) requirements • Include information for other models such as exceptions, concepts, terminology, constraints, etc. • Simple, easy to show to clients, emphasize client/user’s perspective • Example • Process Sale in POS system • Customer arrives at the cash register. Cashier starts new sale…

2. Use Case Elements • Actor • Entity with behavior, usually external to our system (some are more obviously external others not) • Can be live, or other system, but must be physical • Primary – has goals fulfilled in the course of use case • Supporting – provides supports such as external systems • Offstage – has interest in the system’s behavior • Scenario (use-case instance) • A sequence or actions and interactions between actors and the system in the course of particular story, transaction, or use of the system • Use Case • A collection of related scenarios, some successful some exceptional or failures, describing actors interacting with the system to support a specific goal

3. Use Case Example • Returns in POS • Main scenario: Customer returns an item within 30 days with the original package and cash receipt – pay back the cash • Extensions: • Alternatives: Customer paid by a credit card, or does not have the receipt • Exceptions: Item not purchased in the store

4. Use Case Model • Use Cases • Text • System Sequence Diagrams • Actor-system interfaces and message sequencing • Contracts • Descriptions of change of state in the System • Use-case Diagrams • Relationships among uses cases and actors

5. Use Case model Influence

6. Use Case Formats • Brief • One paragraph summary • Useful during inception and early analysis in general to identify scope and risks • Casual • More detailed but still in early stages • Fully Dressed • All steps, scenarios, exceptions, special requirements, etc. • Needed in Inception for the important use cases to establish glossary, extract concepts, assess risk • Needed in Elaboration/construction when iterating on

7. Use Case Example • Fully dressed Process Sale • See the textbook for a template and the actual use case

8. Use Case Goals and Levels • It is important to identify all stakeholders and assess their goals • The purpose of Use Case is to address these interests • All relevant should be there • Not irrelevant should be there • User-goal level • Elementary Business Process EBP • Fulfillment of primary actor’s goals • Subsystem level • Not complete EBP, such as pay by credit (payment is only a part of sale)

9. More on Use Cases. • Preconditions and Postconditions • Special requirements: related non-functional • Two-column format • More visual and comprehensible Actors does System does • Templates • www.usecases.org • Avoid UI details early on • Essential: item is recognized • Concrete: UPC bar code is scanned with optical scanner • Avoid early on

10. How to Identify Use Cases • Choose system boundary • Is the cashier an actor or system? • Is Credit authorization our responsibility?

11. How to Identify Use Cases • Find all primary actors and their goals • Who does what or needs what? • Rather than asking “what you do” ask “what is your role, goal, interest” • Analyze events • Remember stakeholders and their goals • Cashier wants easy processing while management wants efficient and secure processing • Keep track of these findings separately for reference

12. How to Identify Good Use Cases • Boss test • Can you excuse your time doing THIS? • EBP test • Is it well defined process starting with some external need/event and leaving the state consistent? • Size test • Remember that use case(s) will be processed in your time boxes

13. Use Case Diagrams

14. Use Case Diagrams

15. Use Case Diagrams

16. Activity Diagrams • In requirements analysis it may be useful to have other views: • Dataflow • Process (control flow) • These are expressed with Activity Diagrams • Chapter 28

17. Activity Diagrams