System Scenarios and Use Cases for Effective Development

1. Chapter 4,Requirements Elicitation:Functional Modeling

2. Can you develop this?

3. Types of Scenarios • As-is scenario: • Used in describing a current situation. Usually used during re-engineering. The user describes the system. • Visionary scenario: • Used to describe a future system. • Can often not be done by the user or developer alone • Evaluation scenario: • User tasks against which the system is to be evaluated • Training scenario: • Step by step instructions designed to guide a novice user through a system

4. What is This?

5. Cave lighting entrance enter() leave() Shoe Size Color Type Wear() Coat Size Color Type Wear() Possible Object Model: Eskimo Eskimo Size Dress() Smile() Sleep() livesIn *

6. Ear Size listen() Alternative: Head Head Hair Dress() Smile() Sleep() Mouth Teeth Size open() speak() Face Nose smile() close_eye() *

7. Nose Eyes Mouth The Artist’s View Picture View 1 View 2 Picture of Eskimo Picture of Sculpture Legs Hands Jacket

8. System & Object identification • System development is not just taking a picture of a domain • 2 important problems during requirements analysis: • Identify objects • Define the system’s purpose • Depending on the system’s purpose, different objects are found • What object is inside, what object is outside? • How can we identify the purpose of a system? • Scenarios • Use cases: Abstractions of scenarios

9. Why Scenarios & Use Cases? • Comprehensible by the user • Use cases [should] model a system from the users’ point of view • Define every possible event flow through the system • Description of interaction between objects • Use cases form basis for whole development process • User manual • System design & object design • Implementation • Test specification • Client acceptance test • An excellent basis for incremental & iterative development

10. How do we find scenarios? • Don’t expect the client to be verbal if the system does not exist • Don’t wait for information even if the system exists • Engage in a dialogue (evolutionary, incremental) • You help the client to formulate the requirements • The client helps you to understand the requirements • The requirements evolve while the scenarios are being developed

11. Example: Accident Management System • What needs to be done to report a “Cat in a Tree” incident? • What needs to be done if a person reports “Warehouse on Fire?” • Who is involved in reporting an incident? • What does the system do if: • No police cars are available? • The police car has an accident on the way to the “cat in a tree” incident? • What needs to be done if the “Cat in the Tree” turns into a “Grandma has fallen from the Ladder”? • Can the system cope with a simultaneous incident report “Warehouse on Fire?”

12. Scenario Example: Warehouse on Fire • Bob, driving down main street in his patrol car notices smoke coming out of a warehouse. His partner, Alice, reports the emergency from her car. • Alice enters the address of the building, a brief description of its location (i.e., north west corner), & an emergency level. In addition to a fire unit, she requests several paramedic units on the scene given that area appear to be relatively busy. She confirms her input & waits for an acknowledgment. • John, the Dispatcher, is alerted to the emergency by a beep of his workstation. He reviews the information submitted by Alice & acks the report. He allocates a fire unit & 2 paramedic units to the Incident site & sends their estimated arrival time (ETA) to Alice. • Alice received the ack & the ETA.

13. Observations about Warehouse on Fire Scenario • Concrete scenario • Describes 1 instance of reporting a fire incident. • Does not describe all possible situations in which a fire can be reported. • Participating actors • Bob, Alice and John

14. Next goal, after the scenarios are formulated: • Find a use case in the scenario that specifies all possible instances of how to report a fire • Example: “Report Emergency “ in the first paragraph of the scenario is a candidate for a use case • Describe this use case in more detail • Describe the entry condition • Describe the flow of events • Describe the exit condition • Describe exceptions • Describe special requirements (constraints, nonfunctional requirements)

15. Example of steps in formulating a use case • Name the use case • Use case name: ReportEmergency • Find the actors • Generalize the concrete names (“Bob”) to actors (“Field officer”) • Participating Actors: • Field Officer (Bob and Alice in the Scenario) • Dispatcher (John in the Scenario) • Identify the flow of events • Use informal natural language

16. Example of steps in formulating a use case • Formulate the Flow of Events: • The FieldOfficer activates the “Report Emergency” function on her terminal. FRIEND responds by presenting a form to the officer. • The FieldOfficer fills the form, by selecting the emergency level, type, location, & brief description of the situation. The FieldOfficer also describes possible responses to the emergency situation. Once the form is completed, the FieldOfficer submits the form, at which point, the Dispatcher is notified. • The Dispatcher reviews the submitted information & creates an Incident in the database by invoking the OpenIncident use case. The Dispatcher selects a response & acks the emergency report. • The FieldOfficer receives the ack & the selected response.

17. Example of steps in formulating a use case • Write down the exceptions: • The FieldOfficer is notified immediately if the connection between her terminal & the central is lost. • The Dispatcher is notified immediately if the connection between any logged in FieldOfficer & the central is lost. • Identify & write down any special requirements: • The FieldOfficer’s report is ack’d within 30 seconds. • The selected response arrives no later than 30 seconds after it is sent by the Dispatcher.

18. How to Specify a Use Case (Summary) • Name the Use Case • Actors • Describe the actors involved • Entry condition • Flow of Events • Free form, informal natural language • Exit condition • Exceptions • Describe what happens if things go wrong • Special Requirements • List nonfunctional requirements & constraints

19. Use Case Model for Incident Management Dispatcher FieldOf f icer OpenIncident ReportEmergency AllocateResources

20. Use Case Associations • Use case association relates use cases • Types: • Extends • A use case extends another use case • Include • A use case uses another use case(“functional decomposition”) • Generalization • An abstract use case has different specializations

21. <<Include>>: Functional Decomposition • Problem: • A function in the original problem is too complex • Solution: • Describe the function as the aggregation of simpler functions. • Decompose the use case CreateDocument <<include>> <<include>> <<include>> Check OCR Scan

22. <<Include>>: Reuse of Existing Functionality • Problem: • How can we reuse existing functions? • Example Solution: • The use case “ViewMap” describes behavior that can be used by the use case “OpenIncident” (“ViewMap” is factored out) <<include>> OpenIncident Base Use Case ViewMap <<include>> Supplier Use Case AllocateResources

23. <Extend>> Association for Use Cases • Problem: • The functionality of the problem needs to be extended. • Solution: • An extend association from a use case A to a use case B indicates that use case B is an extension of use case A. • Example: • The use case “ReportEmergency” is complete by itself • But can be extended by the use case “Help” for a specific scenario in which the user requires help Help FieldOfficer <<extend>> ReportEmergency

24. Generalization association in use cases • Problem: • There is common behavior among use cases. • Solution: • Generalization among use cases factors out common behavior. • The child use cases inherit the behavior & meaning of the parent use case & adds or overrides some behavior. • Example: • In the use case “ValidateUser”, customer requires 2 realizations: “CheckPassword” & “CheckFingerprint” CheckPassword Parent Case Child Use Case ValidateUser CheckFingerprint

25. How do I find use cases? • Select a narrow vertical slice of the system: 1 scenario • Discuss it in detail with the user. • Understand the user’s preferred style of interaction • Select a horizontal slice (many scenarios) to define the scope of the system. • Discuss the scope with the user • Use mock-ups as visual support • Find out what the user does • Task observation (Good) • Questionnaires (Bad)

26. From Use Cases to Objects Le v el 1 Top Level Use Case Level 2 Use Cases Le v Le v el 2 el 2 Level 3 Use Cases Le v el 3 Le v Le v el 3 el 3 Operations Le v Le v el 4 el 4 A B Participating Objects

27. Finding Participating Objects in Use Cases • For each use case identify • Terms that developers/users use to clarify the flow of events • Always start with the user’s terms, then negotiate: • FieldOfficer, StationBoundary, or FieldOfficerStation? • IncidentBoundary or IncidentForm? • EOPControl or EOP? • Real world entities that the system tracks. • Examples: FieldOfficer, Dispatcher, Resource • Real world procedures that the system tracks. • Example: EmergencyOperationsPlan • Data sources or sinks. • Example: Printer • Interface artifacts. • Example: PoliceStation • Nouns that are used a lot in general.

28. Summary The requirements elicitation activities aimed at defining the boundary of the system: • Identify Scenarios • Identify Use Cases • Refine Use Cases • Identify participating objects Elicit requirements is to build a functional model of the system This is used in analysis to build an object model & a dynamic model.