formal method in software engineering：軟件工程中的形式化方法

SE561SoftwareSystemRequirementsFormalMethodsSoftwareEngineeringandFormalMethodsEverysoftwareengineeringmethodologyisbasedonarecommendeddevelopmentprocessproceedingthroughseveralphases:Requirements,Specification,DesignCoding,UnitTestingIntegrationandSystemTesting,MaintenanceFormalmethodscanBeafoundationfordesigningsafetycriticalsystemsBeafoundationfordescribingcomplexsystemsProvidesupportforprogramdevelopment

WhatareFormalMethods?

TechniquesandtoolsbasedonmathematicsandformallogicCanassumevariousformsandlevelsofrigorInformalLowMediumHigh

WhyConsiderFormalMethods?

ThedevelopmentofaformalspecificationprovidesinsightsandanunderstandingofthesoftwarerequirementsandsoftwaredesignClarifycustomers’requirementsRevealandremoveambiguity,inconsistencyandincompletenessFacilitatecommunicationofrequirementordesignProvidesabasisforanelegantsoftwaredesignTraceabilitySystem-levelrequirementsshouldbetraceabletosubsystemsorcomponentsFormalMethodsConceptsFormalSpecificationMethodsFormalspecificationFormalProofsModelcheckingAbstractionFormalSpecificationThetranslationofnon-mathematicaldescription(diagrams,table,naturallanguage)intoaformalspecificationlanguageItrepresentsaconcisedescriptionofhigh-levelbehaviorandpropertiesofasystemWell-definedlanguagesemanticssupportformaldeductionaboutthespecificationTypeofFormalSpecificationsModelOriented:Constructamodelofthesystembehaviorusingmathematicalobjectslikesets,sequencesetc.Statecharts,SCR,VDM,ZPetriNets,CCS,CSP,AutomatatheoreticmodelsPropertyOriented:Useasetofnecessarypropertiestodescribesystembehavior,suchasaxioms,rulesetc.AlgebraicsemanticsTemporallogicmodels.FormalProofsProofisanessentialpartofspecificationProofsareconstructedasaseriesofsmallsteps,eachofwhichisjustifiedusingasmallsetofrulesProofscanbedonemanually,butusuallyconstructedwithsomeautomatedassistanceModelCheckingAtechniquereliesonbuildingafinitemodelofasystemandcheckingthatadesiredpropertyholdsinthatmodelTwogeneralapproachestemporalmodelcheckingautomatonmodelcheckingUsemodelcheckersSMVAbstractionRepresentationoftheprogramusingasmallermodelAllowsyoutofocusonthemostimportantcentralpropertiesandcharacteristicsGettingtherightlevelofabstractionisveryimportantinaspecification.

MathematicalModels

AbstractrepresentationsofasystemusingmathematicalentitiesandconceptsModelshouldcapturestheessentialcharacteristicsofthesystemwhileignoringirrelevantdetailsModelcanbeanalyzedusingmathematicalreasoningtoprovesystempropertiesorderivenewbehaviors.TwotypesContinuousmodelsDiscretemodelsFormalSpecificationProcessModelClarifyrequirementsandhighleveldesignArticulateimplicitassumptionsIdentifyundocumentedorunexpectedassumptionsExposedefectsIdentifyexceptionsEvaluatetestcoverageCleanroomsoftwaredevelopmentSpendalotofeffort"up-front"topreventdefectsFormalspecificationIncrementaldevelopmentStatisticalmethodstoensurereliabilityCleanroomProcessFormalspecificationusingastatetransitionmodelStructuredprogramming-limitedcontrolandabstractionconstructsareusedProgramresemblesstatemachineStaticverificationusingrigorousinspectionsMathematicalargumentsStatisticaltestingofthesystemreliability

CleanroomProcessCleanroomProcessIncrementaldevelopmentAllowsfreezingofrequirements,soformalworkcanproceedWorkoncriticalfunctionalityinearlyrevisions,soitreceivesthemosttestingCleanroomProcessSpecificationteam.DevelopandmaintainsystemspecificationDevelopmentteam.Developandverify(mathematically)thesoftware.ThesoftwareisnotexecutedorevencompiledduringthisprocessCertificationteam.Developsetofstatisticalteststoexercisethesoftwareafterdevelopment.ReliabilitygrowthmodelsusedtodeterminewhenreliabilityisacceptableTestResultsSuccessfulinthefieldFewerrorsNotmoreexpensivethanotherprocessesGenerallyworkableHigherqualitycoderesulted

BenefitsofFormalSpecifications

HigherlevelofrigorleadstobetterproblemunderstandingDefectsareuncoveredthatwouldbemissedusingtraditionalspecificationmethodsAllowsearlierdefectidentificationFormalspecificationlanguagesemanticsallowchecksforself-consistencyEnablestheuseofformalproofstoestablishfundamentalsystempropertiesandinvariantsLimitationstoFormalMethodsRequiresasoundmathematicalknowledgeofthedeveloperDifferentaspectsofadesignmayberepresentedbydifferentformalspecificationmethodsUsefulforconsistencychecks,butformalmethodscannotguaranteethecompletenessofaspecificationsForthemajorityofsystemsDoesnotoffersignificantcostorqualityadvantagesoverothersReviewWhatWelearned…FundamentalrequirementsengineeringconceptsRequirementsengineeringprocessesRequirementsengineeringtechniquesRequirementsEngineeringConceptsRequirements–definewhatasystemisrequiredtodoandtheconstraintsunderwhichitisrequiredtooperateRequirementsengineering–allactivitiesinvolvedindiscovering,documenting,andmaintainingasetofrequirementsforacomputer-basedsystemThetermengineeringimpliesthatsystematicandrepeatabletechniques(basedonBestPractices)shouldbeusedThefirststepinsystemdevelopmentIncludeFunctionalrequirementsNon-functionalrequirementsStakeholdersSoftwareengineers,systemend-users,managersofsystemend-users,externalregulators,domainexpertsRequirementsEngineeringProcessesRequirementsEngineeringProcessesIBMRationalRequisitProforrequirementsdocumentationandmanagementSRStemplateforfinalspecificationRequirementsEngineeringTechniquesProcessofrequirementsengineering(RE)isusuallyguidedbyarequirementsmethodRequirementmethodsaresystematicwaysofproducingsystemmodelsSystemmodelsareimportantbridgesbetweentheanalysisandthedesignprocessTypesStructuredanalysisObject-orientedanalysisRequirementsEngineeringTechniquesDataflowmodelingOneofthemostpopularstructuredmethodsDFDprovidesadescriptionofasystembasedonmodelingthetransformationalprocessesofasystem,thecollections(stores)ofdatathatthesystemmanipulates,andtheflowsofdatabetweentheprocesses,storesandtheoutsideworld.TheDFDdescribesthefunctionalviewpointofthesysteme.g.itdescribesthesystemintermsofitsoperation(tasks).Conductedhierarchically.RequirementsEngineeringTechniquesObject-orientedapproachintegratedataandf