With Use Case Diagrams - Drexel CCI

Specifying Requirementswith Use Case Diagrams

OUTLINEIntroductionUse Case DiagramsWriting Use CasesGuidelines for Effective Use Cases

SOURCE OF REQUIREMENTSInitial requirements come from the customer, by: Documents, Meetings, reportsAdvanced requirements come from the analysts, afterstudying: Scope and price Feasibility (technological, organizational etc) PrototypesFinal requirements are stabilized in an iterative process.Introduction Diagrams Writing Guidelines

REQUIREMENTS VS. DESIGNRequirements: What the system shoulddo More abstractDesign: How the system shoulddo it More detailedIntroduction Diagrams Writing Guidelines

TYPES OF REQUIREMENTSVisible Functional Requirements “The system will deliver cash to thecustomer” “Cash will be delivered after card wastaken out”Qualitative Requirements “The authorization process will take nomore than 1 sec”Hidden Requirements “Database maintenance processes willoccur every night”Introduction Diagrams Writing ionalRequirementsQualitativeRequirements

USE CASESRegister UseradminUse case in diagramUse Case in scriptIllustrationA use case is a contract of an interaction between the system andan actor.A full use-case model comprise of: A diagram, describing relations between use-cases and actors. A document describing the use case in detailsIntroduction Diagrams Writing Guidelines

USE CASE DIAGRAM OBJECTIVE1.Create a semi-formal model of the functionalrequirements2.Analyze and define: ScopeExternal interfacesScenarios and reactionsIntroduction Diagrams Writing Guidelines

WHAT MAKES GOOD USE-CASESPECIFICATION?Lack of ambiguity Each requirement must be interpreted in a single manner.Completeness The collection of all use cases is everything that can be doneto/with the system.Consistency Requirements should not conflict with each other. If there are,tradeoffs must be detected and discussed.Avoid design Requirements should raise a need, not answer it.Introduction Diagrams Writing Guidelines

USE CASES AS MEANS OFCOMMUNICATIONCustomersDesignersUsersThe use case should stimulate a discussion about what thesystem should do, mainly with people who are outside of thedevelopment team.Introduction Diagrams Writing Guidelines

OUTLINEIntroductionUse Case DiagramsWriting Use CasesGuidelines for Effective Use Cases

A SIMPLE EXAMPLEHandle MessageCellular PhoneHandle CallExternal PhoneCompanyBill ction Diagrams Writing GuidelinesUse CaseActorsExample

FINDING ACTORSExternal objects that produce/consume data: Must serve as sources and destinations for data Must be external to the systemHumansMachinesOrganizational UnitsIntroduction Diagrams Writing GuidelinesExternal systemsSensors

ACTORS CAN BE GENERALIZEDThe child actor inherits alluse-cases associationsPerform SaleRegister ClientSales PersonShould be used if (and onlyif), the specific actor hasmore responsibility than thegeneralized one (i.e.,associated with more usecases)PerformBusiness SaleInstitutionalSales PersonCancel SaleSales ManagerExampleIntroduction Diagrams Writing Guidelines

LINKING USE-CASESLinking enables flexibility in requirements specification Isolating functionality Enabling functionality sharing Breaking functionality into manageable chunksThree mechanism are used: Include Extend InheritanceIntroduction Diagrams Writing Guidelines

USE-CASE LEVELSBase Use Case:Used directly bythe userPerformSaleUser goalsSub-functionalityChooseProductsFill-inbilling infoAlistair Cockburn “Writing Effective Use Cases”Introduction Diagrams Writing Guidelines

THE “INCLUDE” CONSTRUCTInclude is used when: Decomposing complicated behavior Centralizing common behaviorThe base use case explicitly incorporates the behavior of anotheruse case at a location specified in the base.PerformSale include Fill-in billinginfoExampleIntroduction Diagrams Writing Guidelines

EXTEND – GRAPHICAL REPRESENTATIONThe base use case can incorporate another use case at certainpoints, called extension points.Note the direction of the arrow The base use-case does not know which use-case extends it extend Product is a giftPerform SaleAfter checkoutGift wrapProductsExampleIntroduction Diagrams Writing Guidelines

EXAMPLE: AMAZONShopping CartProduct PageReview WritingIntroduction Diagrams Writing Guidelines

EXAMPLE – After nkSupplier«extend»«extend»WriteReviewAdd tocartCustomer«include»Checkout«extend»user is nota memberLoginHandle OrderStatusIntroduction Diagrams Writing Guidelines«include»Register

GENERALIZATION BETWEENUSE-CASESThe child use case inherits the behavior parent use case: The interaction (described in the textual description) Use case links (associations, include, extend, generalization)Child use-case can substitute parent Use caseOverriding occurs through the textual descriptionHandle CallHandle Sale CallCustomerRepresentativeIntroduction Diagrams Writing Guidelines1. Transfer call to availablerepresentative2. Mark representative as busy3. Start record call4. Stop record call5. Log call details6. Mark representative as freeHandle TechnicalAssistance CallTech AssistantRepresentativeExample

GENERALIZATION HAZARDSCombining generalizations of actors and use-cases can bedangerousSubmit and GetGradeSubmit ExamUndergrad StudentUndergrad StudentSubmit ExamSubmit ThesisGraduate StudentSubmit ThesisGraduate StudentBad: Undergrad can submitGood: Only graduatethesisstudent can submit thesisIntroduction Diagrams Writing Guidelines

EXAMPLE: EASTERN STATE UNIVERSITY(ESU) REGISTRATION SYSTEM.1. Professors indicate which courses they will teach online.2. A course catalog can be printed3. Allow students to select on-line four courses forupcoming semester.4. No course may have more than 10 students or lessthan 3 students.5. When the registration is completed, the system sendsinformation to the billing system.6. Professors can obtain course rosters on-line.7. Students can add or drop classes on-line.Introduction Diagrams Writing Guidelines

EXAMPLE: EASTERN STATE UNIVERSITY(ESU) REGISTRATION SYSTEM.Introduction Diagrams Writing Guidelines

OUTLINEIntroductionUse Case DiagramsWriting Use CasesGuidelines for Effective Use Cases

OUTLINEIntroductionUse Case DiagramsWriting Use CasesGuidelines for Effective Use Cases

HOW TO MODEL?Bottom-up ProcessStarting with throwing allscenarios on the page, andthen combining aphformatFontformatTop-down ProcessStarting with an overview ofthe system, and then splittingUse-casesFileactionsFormatting actionsViewingActions

HOW TO MODEL – CONT’DMost of the analysis process are actuallyCombined

COMBINING PROCESSESNumber Limit: The diagram should have between 3 to 10 base use-case. No morethan 15 use cases (base included extending).Abstraction: All use-cases should be in similar abstraction levels.Size: Use cases should be described in half a page or more.Interaction: Use-cases which are carried out as part of the same interaction.UCUCIntroduction Diagrams Writing GuidelinesUC

DIVIDING PROCESSESSize: If a use-cases takes more than a page, considerinclude/extendWeak dependency: If the dependency between two parts of a use-case is weak,they should be divided.UCIntroduction Diagrams Writing Guidelines

MORE GUIDELINESFactor out common usages that are required by multiple usecases If the usage is required use include If the base use case is complete and the usage may beoptional, consider use extend A use case diagram should: contain only use cases at the same level of abstraction include only actors who are requiredIntroduction Diagrams Writing Guidelines

WHEN ARE WE DONE?When every actor is specified.When every functional requirement has a use-case whichsatisfies it.A tractability matrix can help us determine it: Use Cases RequirementsIntroduction Diagrams Writing Guidelines

SUMMARY Introductionto the Unified Modeling Language (UML)To Use Case Diagram Use Case DiagramsDual presentation of use-casesInclude, Extend, Inheritance Writing Use CasesPreconditions & Post-conditionsMain scenario vs. Alternative Flow Guidelines for Effective Use Cases

A use case is a contract of an interaction between the system and an actor. A full use-case model comprise of: A diagram, describing relations between use- cases and actors. A document describing the use case in details . Register User . Use case in diagram . Use Case in script admin . Introduction Diagrams Writing Guidelines