- Slides: 30
Use Case Modeling
USE CASE MODELLING • A use case is a scenario that describes the use of a system by an actor to accomplish a specific goal. • An actor is a user playing a role with respect to the system. - people, other systems. • Scenario A sequence of steps that describe the interactions between an actor and the system. • The use case model consists of the collection of all actors and all use cases.
Use cases help. . · Capture the system's functional requirements from the users' perspective · Actively involve users in the requirementsgathering process · Provide the basis for identifying major classes and their relationships · Serve as the foundation for developing system test cases
USE CASE MODELLING Functional reqts. , analysis phase - what a system does: functions represented as use cases - Actor: external agent that interacts with the system, exchanges info. with the system (user, sub-system, etc. ) - a role played by user Note: a use case represents a complete functionality. – – – view of system behavior from an external person’s viewpoint effective tool for validating requirements an effective communication tool basis for a test plan basis for user manual Developing the use cases is not difficult; ensuring that you have them all is very difficult.
• Use Case Diagrams Customer Loses a Tape – Actor – Line: actor communicates with or is associated with use-case
• Use Case relationships: – <<uses>>: one use case always involves the steps of another – <<extends>>: under certain conditions, a use case follows a variant Loses a Tape <<extends>> Customer <<uses>> VIP Loses a Tape Customer Buy a Tape
Goals of use cases • Interactions that provide value to actors • No implementation specific language – No assumptions about how the use case may be realized in code or user-interface – Note: use-cases drive the whole life-cycle, and they get refined – implementation specific use-cases • User-appropriate level of detail – General at requirements gathering stage • User-appropriate volume – Large systems: no more than 70 -80 use cases – Small number of use cases – forces abstraction
Use case scenarios Realization of a use case: • Instance of a use case that effectively tests one path through a use case • To demonstrate whether a use case accurately reflects user needs • useful during testing Example: Use case name: Determine benefits eligibility for enrollee Steps: 1. This use case starts when the social worker enters the enrollee’s name and employment situation. 2. This use case ends when the system responds with a determination of whether the enrollee is eligible for benefits and the financial extent of benefits. Alternative path: In step 1, if the enrollee has applied for benefits previously, based on the enrollee’s own disclosure, the social worker enters the enrollee’s name to search for his or her previous records
Use case scenario - example Example of scenario 1. The social worker asks Edward Trueman if he has applied previously for and/or received benefits. 1. 2. 3. Mr. Trueman replies that he has applied previously. The social worker provides Mr. Trueman’s name as search criterion The system provides Mr. Trueman’s previous records, which state that he applied for benefits on Dec 9 th, 1997, and was determined to be ineligible on Dec 9 th 1997 because of his current part-time employment status at Boeing Aerospace in the capacity of assembly line worker.
Student <<Extends>> Class <<Extends>> registration Registration Clerk Registration for special class Prereq courses not completed Bursar’s office Student Billing Instructor Use-Case Diagram Extends: extension to or variation of a use-case that exists in its own right
Customer Order Food Service Person Applicant Hire Employee <<uses>> Supplier Track sales and inv. data Reorder supplies <<uses>> Produce mgt. reports Uses: factors common behavior amongst multiple use-cases into a generalized use-case. Manager
• Finding the actors - Ask client and domain experts how system will be used - Who will perform the obvious tasks? Secondary tasks of maintenance and administration? Interact with other systems? - Clear and differentiated names (eg. manager vs supervisor)
• Recording the use cases - For each actor, what interactions /”results” they require of the system –each is a use case • Not overly precise yet, just note them down – – What tasks does the actor want the system to perform What information must the actor provide to the system Are there events that the actor must tell system about Does actor need to be informed when something happens – Does actor help initialize or shut down the system
Example: video store system Actor: Customer - What tasks does the actor want the system to perform? - Find movie to rent, rent tape, return tape, reserve tape - What information must the actor provide to the system? - Name, address, membership#, media name - Are there events that the actor must tell system about? - Change of address - Does actor need to be informed when something happens? - Reserved tape is ready to be rented - Does actor help initialize or shut down the system - no
Example: video store system Resulting use cases: • Customer joins and provides contact information including name, address, phone#, credit information, spouse and kids • Customer browses system looking for a tape to rent • Customer comes to store looking for a specific tape to rent • Customer rents a tape • Customer returns a tape • Customer reserves a tape • Customer is contacted when a reserved tape is ready Note: simple phrases, limited details initially.
Example: video store system – Other actors: • Clerk adds a new film to the system • It is time to order more films – manager wants to know which films are popular so as to get extra copies • Store orders 10 new copies of film; when they arrive, clerk adds them to system and puts them on shelf • Manager wants to know if he is making money • thinking of these may lead to some more customer use cases: – Customer wants to be informed when new movies are available.
Example: video store system • Use cases from attributes of “things” – tapes have actors, film name, director, release date – Customer wants to find every movie starring a specific actor – Customer wants to find movies with same director as the last film she rented – Manager wants to put together list of movies rated ‘G’. • Use cases to manage the attributes: – Clerk adds new film to system – Clerk removes obsolete film from system – Clerk updates information about a film
Example: video store system • These use cases can remind us of similar responsibilities for customers: – Clerk adds customer to the system – Clerk removes customer account from system – Clerk updates customer’s account
• Use case name, description Example: Customer Loses a Tape The customer reports to the clerk that he has lost a tape. The clerk prints out the rental record and asks customer to speak with the manager, who will arrange for the customer to pay a fee. The system will be updated to reflect lost tape, and customer’s record is updated as well. The manager may authorize purchase of a replacement tape.
• Scenarios: a use case scenario shows the flow of events in a particular instance of a use case. – focused on the business problem, not the solution - shows most common situation - can have greater or less detail as required to understand flow of interactions and events - also consider exceptional rather than the normal situations. • Scenario guidelines – how does the scenario begin? what causes it to end? - what is the distinction between what the actors do and how the system responds - what feedback will the actor receive? - which activities may repeat, and what causes them to stop? – Is there conditional branching in the flow of the scenario?
• Formal description of use cases: - Use case name Actors, brief description Actions taking place scenario – flow of events Some use cases have multiple scenarios to explore various contingent activities - Preconditions –before the use can begin – Eg. for Lost a Tape use case, customer must be member and must have rented a tape. - Post-conditions: state of system and perhaps for actors, after the use case is completed
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