软件过程与管理

SoftwareRequirementsObjectivesTointroducetheconceptsofuserandsystemrequirementsTodescribefunctionalandnon-functionalrequirementsToexplainhowsoftwarerequirementsmaybeorganisedinarequirementsdocumentTopicscoveredFunctionalandnon-functionalrequirementsUserrequirementsSystemrequirementsInterfacespecificationThesoftwarerequirementsdocumentRequirementsengineeringTheprocessofestablishingtheservicesthatthecustomerrequiresfromasystemandtheconstraintsunderwhichitoperatesandisdeveloped.Therequirementsthemselvesarethedescriptionsofthesystemservicesandconstraintsthataregeneratedduringtherequirementsengineeringprocess.Whatisarequirement?Itmayrangefromahigh-levelabstractstatementofaserviceorofasystemconstrainttoadetailedmathematicalfunctionalspecification.ThisisinevitableasrequirementsmayserveadualfunctionMaybethebasisforabidforacontract-thereforemustbeopentointerpretation;Maybethebasisforthecontractitself-thereforemustbedefinedindetail;Boththesestatementsmaybecalledrequirements.Requirementsabstraction(Davis)TypesofrequirementUserrequirementsStatementsinnaturallanguageplusdiagramsoftheservicesthesystemprovidesanditsoperationalconstraints.Writtenforcustomers.SystemrequirementsAstructureddocumentsettingoutdetaileddescriptionsofthesystem’sfunctions,servicesandoperationalconstraints.Defineswhatshouldbeimplementedsomaybepartofacontractbetweenclientandcontractor.DefinitionsandspecificationsRequirementsreadersFunctionalandnon-functionalrequirementsFunctionalrequirementsStatementsofservicesthesystemshouldprovide,howthesystemshouldreacttoparticularinputsandhowthesystemshouldbehaveinparticularsituations.Non-functionalrequirementsconstraintsontheservicesorfunctionsofferedbythesystemsuchastimingconstraints,constraintsonthedevelopmentprocess,standards,etc.DomainrequirementsRequirementsthatcomefromtheapplicationdomainofthesystemandthatreflectcharacteristicsofthatdomain.FunctionalrequirementsDescribefunctionalityorsystemservices.Dependonthetypeofsoftware,expectedusersandthetypeofsystemwherethesoftwareisused.Functionaluserrequirementsmaybehigh-levelstatementsofwhatthesystemshoulddobutfunctionalsystemrequirementsshoulddescribethesystemservicesindetail.TheLIBSYSsystemAlibrarysystemthatprovidesasingleinterfacetoanumberofdatabasesofarticlesindifferentlibraries.Userscansearchfor,downloadandprintthesearticlesforpersonalstudy.ExamplesoffunctionalrequirementsTheusershallbeabletosearcheitheralloftheinitialsetofdatabasesorselectasubsetfromit.Thesystemshallprovideappropriateviewersfortheusertoreaddocumentsinthedocumentstore.Everyordershallbeallocatedauniqueidentifier(ORDER_ID)whichtheusershallbeabletocopytotheaccount’spermanentstoragearea.RequirementsimprecisionProblemsarisewhenrequirementsarenotpreciselystated.Ambiguousrequirementsmaybeinterpretedindifferentwaysbydevelopersandusers.Considertheterm‘appropriateviewers’Userintention-specialpurposeviewerforeachdifferentdocumenttype;Developerinterpretation-Provideatextviewerthatshowsthecontentsofthedocument.RequirementscompletenessandconsistencyInprinciple,requirementsshouldbebothcompleteandconsistent.CompleteTheyshouldincludedescriptionsofallfacilitiesrequired.ConsistentThereshouldbenoconflictsorcontradictionsinthedescriptionsofthesystemfacilities.Inpractice,itisimpossibletoproduceacompleteandconsistentrequirementsdocument.Non-functionalrequirementsThesedefinesystempropertiesandconstraintse.g.reliability,responsetimeandstoragerequirements.ConstraintsareI/Odevicecapability,systemrepresentations,etc.ProcessrequirementsmayalsobespecifiedmandatingaparticularCASEsystem,programminglanguageordevelopmentmethod.Non-functionalrequirementsmaybemorecriticalthanfunctionalrequirements.Ifthesearenotmet,thesystemisuseless.Non-functionalclassificationsProductrequirementsRequirementswhichspecifythatthedeliveredproductmustbehaveinaparticularwaye.g.executionspeed,reliability,etc.OrganisationalrequirementsRequirementswhichareaconsequenceoforganisationalpoliciesandprocedurese.g.processstandardsused,implementationrequirements,etc.ExternalrequirementsRequirementswhicharisefromfactorswhichareexternaltothesystemanditsdevelopmentprocesse.g.interoperabilityrequirements,legislativerequirements,etc.Non-functionalrequirementtypesNon-functionalrequirementsexamplesProductrequirement8.1 TheuserinterfaceforLIBSYSshallbeimplementedassimpleHTMLwithoutframesorJavaapplets.Organisationalrequirement9.3.2ThesystemdevelopmentprocessanddeliverabledocumentsshallconformtotheprocessanddeliverablesdefinedinXYZCo-SP-STAN-95.Externalrequirement7.6.5Thesystemshallnotdiscloseanypersonalinformationaboutcustomersapartfromtheirnameandreferencenumbertotheoperatorsofthesystem.GoalsandrequirementsNon-functionalrequirementsmaybeverydifficulttostatepreciselyandimpreciserequirementsmaybedifficulttoverify.GoalAgeneralintentionoftheusersuchaseaseofuse.Verifiablenon-functionalrequirementAstatementusingsomemeasurethatcanbeobjectivelytested.Goalsarehelpfultodevelopersastheyconveytheintentionsofthesystemusers.ExamplesAsystemgoalThesystemshouldbeeasytousebyexperiencedcontrollersandshouldbeorganisedinsuchawaythatusererrorsareminimised.Averifiablenon-functionalrequirementExperiencedcontrollersshallbeabletouseallthesystemfunctionsafteratotaloftwohourstraining.Afterthistraining,theaveragenumberoferrorsmadebyexperiencedusersshallnotexceedtwoperday.RequirementsmeasuresRequirementsinteractionConflictsbetweendifferentnon-functionalrequirementsarecommonincomplexsystems.SpacecraftsystemTominimiseweight,thenumberofseparatechipsinthesystemshouldbeminimised.Tominimisepowerconsumption,lowerpowerchipsshouldbeused.However,usinglowpowerchipsmaymeanthatmorechipshavetobeused.Whichisthemostcriticalrequirement?DomainrequirementsDerivedfromtheapplicationdomainanddescribesystemcharacteristicsandfeaturesthatreflectthedomain.Domainrequirementsbenewfunctionalrequirements,constraintsonexistingrequirementsordefinespecificcomputations.Ifdomainrequirementsarenotsatisfied,thesystemmaybeunworkable.LibrarysystemdomainrequirementsThereshallbeastandarduserinterfacetoalldatabaseswhichshallbebasedontheZ39.50standard.Becauseofcopyrightrestrictions,somedocumentsmustbedeletedimmediatelyonarrival.Dependingontheuser’srequirements,thesedocumentswilleitherbeprintedlocallyonthesystemserverformanuallyforwardingtotheuserorroutedtoanetworkprinter.TrainprotectionsystemThedecelerationofthetrainshallbecomputedas:Dtrain=Dcontrol+Dgradient whereDgradientis9.81ms2*compensatedgradient/alphaandwherethevaluesof9.81ms2/alphaareknownfordifferenttypesoftrain.DomainrequirementsproblemsUnderstandabilityRequirementsareexpressedinthelanguageoftheapplicationdomain;Thisisoftennotunderstoodbysoftwareengineersdevelopingthesystem.ImplicitnessDomainspecialistsunderstandtheareasowellthattheydonotthinkofmakingthedomainrequirementsexplicit.UserrequirementsShoulddescribefunctionalandnon-functionalrequirementsinsuchawaythattheyareunderstandablebysystemuserswhodon’thavedetailedtechnicalknowledge.Userrequirementsaredefinedusingnaturallanguage,tablesanddiagramsasthesecanbeunderstoodbyallusers.ProblemswithnaturallanguageLackofclarityPrecisionisdifficultwithoutmakingthedocumentdifficulttoread.RequirementsconfusionFunctionalandnon-functionalrequirementstendtobemixed-up.RequirementsamalgamationSeveraldifferentrequirementsmaybeexpressedtogether.LIBSYSrequirement4..5LIBSYSshallprovideafinancialaccountingsystemthatmaintainsrecordsofallpaymentsmadebyusersofthesystem.Systemmanagersmayconfigurethissystemsothatregularusersmayreceivediscountedrates.Editorgridrequirement2.6GridfacilitiesToassistinthepositioningofentitiesonadiagram,theusermayturnonagridineithercentimetresorinches,viaanoptiononthecontrolpanel.Initially,thegridisoff.Thegridmaybeturnedonandoffatanytimeduringaneditingsessionandcanbetoggledbetweeninchesandcentimetresatanytime.Agridoptionwillbeprovidedonthereduce-to-fitviewbutthenumberofgridlinesshownwillbereducedtoavoidfillingthesmallerdiagramwithgridlines.RequirementproblemsDatabaserequirementsincludesbothconceptualanddetailedinformationDescribestheconceptofafinancialaccountingsystemthatistobeincludedinLIBSYS;However,italsoincludesthedetailthatmanagerscanconfigurethissystem-thisisunnecessaryatthislevel.GridrequirementmixesthreedifferentkindsofrequirementConceptualfunctionalrequirement(theneedforagrid);Non-functionalrequirement(gridunits);Non-functionalUIrequirement(gridswitching).StructuredpresentationGuidelinesforwritingrequirementsInventastandardformatanduseitforallrequirements.Uselanguageinaconsistentway.Useshallformandatoryrequirements,shouldfordesirablerequirements.Usetexthighlightingtoidentifykeypartsoftherequirement.Avoidtheuseofcomputerjargon.SystemrequirementsMoredetailedspecificationsofsystemfunctions,servicesandconstraintsthanuserrequirements.Theyareintendedtobeabasisfordesigningthesystem.Theymaybeincorporatedintothesystemcontract.SystemrequirementsmaybedefinedorillustratedusingsystemmodelsdiscussedinChapter8.RequirementsanddesignInprinciple,requirementsshouldstatewhatthesystemshoulddoandthedesignshoulddescribehowitdoesthis.Inpractice,requirementsanddesignareinseparableAsystemarchitecturemaybedesignedtostructuretherequirements;Thesystemmayinter-operatewithothersystemsthatgeneratedesignrequirements;Theuseofaspecificdesignmaybeadomainrequirement.ProblemswithNLspecificationAmbiguityThereadersandwritersoftherequirementmustinterpretthesamewordsinthesameway.NLisnaturallyambiguoussothisisverydifficult.Over-flexibilityThesamethingmaybesaidinanumberofdifferentwaysinthespecification.LackofmodularisationNLstructuresareinadequatetostructuresystemrequirements.AlternativestoNLspecificationStructuredlanguagespecificationsThefreedomoftherequirementswriterislimitedbyapredefinedtemplateforrequirements.Allrequirementsarewritteninastandardway.Theterminologyusedinthedescriptionmaybelimited.Theadvantageisthatthemostoftheexpressivenessofnaturallanguageismaintainedbutadegreeofuniformityisimposedonthespecification.Form-basedspecificationsDefinitionofthefunctionorentity.Descriptionofinputsandwheretheycomefrom.Descriptionofoutputsandwheretheygoto.Indicationofotherentitiesrequired.Preandpostconditions(ifappropriate).Thesideeffects(ifany)ofthefunction.Form-basednodespecificationTabularspecificationUsedtosupplementnaturallanguage.Particularlyusefulwhenyouhavetodefineanumberofpossiblealternativecoursesofaction.TabularspecificationGraphicalmodelsGraphicalmodelsaremostusefulwhenyouneedtoshowhowstatechangesorwhereyouneedtodescribeasequenceofactions.DifferentgraphicalmodelsareexplainedinChapter8.SequencediagramsTheseshowthesequenceofeventsthattakeplaceduringsomeuserinteractionwithasystem.Youreadthemfromtoptobottomtoseetheorderoftheactionsthattakeplace.CashwithdrawalfromanATMValidatecard;Handlerequest;Completetransaction.SequencediagramofATMwithdrawalInterfacespecificationMostsystemsmustoperatewithothersystemsandtheoperatinginterfacesmustbespecifiedaspartoftherequirements.ThreetypesofinterfacemayhavetobedefinedProceduralinterfaces;Datastructuresthatareexchanged;Datarepresentations.Formalnotationsareaneffe