software reliability modeling.docx
- 文档编号:29894585
- 上传时间:2023-08-03
- 格式:DOCX
- 页数:7
- 大小:19.35KB
software reliability modeling.docx
《software reliability modeling.docx》由会员分享,可在线阅读,更多相关《software reliability modeling.docx(7页珍藏版)》请在冰豆网上搜索。
softwarereliabilitymodeling
SOFTWARERELIABILITYMODELING---
WHEREWEAREANDWHERESHOULDWEBEGOING?
THENEEDFORSOFTWARERELIABILITYMODELING
Itmaybearguedthatsoftwarereliabilitymetricsareneeded,mostimportantly,becausenofieldcanreallymatureuntilitcanbedescriedinaquantitativefashion.However,therearealsosomeveryspecificreasonsforaquantitativeapproachtosoftwarereliability.Oneneedssoftwarefiguresinordertodoagoodjobofsystemengineering:
toexaminethetradeoffsbetweenliabilityandcostandreliabilityandschedules,todeterminewhatreliabilityfigureoptimizesoveralllifecyclecosts,toplanallocationofresources,andspecifyreliabilitytoacontractorwhoisdevelopingsoftwareforyou.Anotherlargeareaofapplicationisprojectmanagement,wheresoftwarereliability,measuresareneededforprogressmonitoring,schedulingandinvestigationofmanagerialalternatives.Thelengthofatestperiodandhencetheoveralllengthofaprojectishighlycorrelatedwiththereliabilityrequirementsfortheproject.Therefore,reliabilitiesareintimatelytiedupwithschedules.Changesinresourcesavailabletotheprojectaffectbothreliabilityandschedulesandonecanbeexchangedfortheother.Reliabilitymetricsofferanexcellentmeansofevaluatingtheperformanceofoperationalsoftwareandcontrollingchangestoit.Sincechangeusuallyinvolvesadegradationofreliability,onemayusereliabilityperformanceobjectivesasameansfordeterminingwhensoftwarechangescanbeallowedandperhapsevenhowlargetheycanbe.Finally,reliabilityisoneoftheimportantparametersthatshouldbeusedininvestigatingthebenefits(orlackofbenefits)ofproposednewsoftwareengineeringtechnology.
SOFTWARERELIABILITYFUNCTIONS
Hecht[1]hascategorizedsoftwarereliabilityfunctionsintomeasurement,estimationandprediction.Thisclassificationisusedinthispaperwithsomemodificationandextension.Softwarereliabilityisdefinedastheprobabilitythataprogramwillexecutewithoutfailurecausedbysoftwareforaspecifiedtimeinaspecifiedenvironment.Theterm"failure"referstoanunacceptabledeparturefromproperoperation.Theterm"unacceptable"mustbedefinedbythecustomer.The"measurement"ofsoftwarereliabilityisbasedonfailureintervaldataobtainedbyrunningtheprograminitsactualoperatingenvironment.Softwarereliability"estimation"referstotheprocessofdeterminingsoftwarereliabilitymetricsbasedonoperationinatestenvironment.Itshouldbenotedthatestimationcanbeperformedwithrespecttopresentorfuturereliabilityquantities.Thetermsoftwarereliability"prediction"referstotheprocessofcomputingsoftwarereliabilityquantitiesfromprogramdatawhichdoesnotincludefailureintervals.Typically,softwarereliabilitypredictiontakesintoaccountfactorssuchassizeandcomplexityoftheprogram,andisnormallyperformedduringaprogramphasepriortotest.Notethatfutureestimationmightbethoughtofbysomeasprediction;wearedeliberatelymakingacarefuldistinctioninterminology.
Thevariousapplicationsofsoftwarereliabilitymetricsarecloselytiedtothethreefunctionsthathavejustbeendefined.Systemengineeringprimarilyreliesuponprediction;projectmanagement,uponestimation;andoperationalsoftwaremanagementandevaluationofsoftwareengineeringtechnology,uponmeasurement.
SOFTWARERELIABILITYMODELS
Mostoftheworkthathasbeendoneinthefieldofsoftwarereliabilityfallsinoneofsixcategories:
calendartimemodels,theexecutiontimemodel,Bayesianmodels,semi-Markovmodels,deterministicmodelsandinputspaceapproaches.Theinitialapproachtosoftwarereliabilitywasthroughcalendartimemodels:
thatis,attemptsweremadetolookofreliabilityphenomenasuchasfailures,reliability,mean-time-to-failure(MTTF),etc.asfunctionsofcalendartime.Theseearlymodelsfocusedattentionontheproblemofsoftwarereliabilityandcontributedmanyvaluableconceptstowardthefurtherdevelopmentofthetheory.[2-5]
However,thefailure-inducingstressplacedonsoftwareisrelatedcloselytoexecutiontime(CPUtime)andnotcalendartime.Theexecutiontimemodel[6-11]recognizethisfact.Ithasbeenextensivelytestedonmorethan20softwaresystemsandthevalidityoftheassumptionsmadeinderivingthemodelhasbeencarefullyexamined.[12]
LittlewoodandVerrall[13]haveproposedaBayesianmodelthatisperhapsthemostmathematicallyelegantofthesoftwarereliabilitymodels,butitis,unfortunately,difficulttounderstand,andcomputationsbasedonitarelengthyandcostly.AmodelthatfocusesspecificallyontheproblemofimperfectfaultcorrectionhasbeendevelopedbyGoelandOkumoto[14];itisbasedonaviewoffaultcorrectionasasemi-Markovprocess.Theconceptofimperfectfaultcorrectionisincorporatedintheexecutiontimemodelinasimperfashion.Deterministicmodelshavebeenproposed[15,16]buttheyhavenotbeenvalidated.
Itwouldappearthatdeterministicmodelsoversimplifythefailuredetectionandcorrectionprocessandarenotefficientinusingtheinformationavailabletothem.Bayesianmodelsperhapsrepresenttheotherextreme,inthatbothfailureintervalsandfailureprocessparametersareviewedasbeingrandom.Theexecutiontimemodeltakestheintermediateapproachofconsideringfailureintervalsrandombutfailureprocessparametersasvaryingwithexecutiontimeinadeterministicfashion.
Afinalviewpoint,theinputspaceapproach,isbasedonenumeratingallthepossiblesetsofinputorenvironmentalconditionsforaprogramanddeterminingtheproportionofthesethatresultinsuccessfuloperation.Althoughthisapproachistheoreticallyappealing,thelargenumberofpossibleinputsetsforanyusefulprogrammakesitimpractical.Thecountswouldhavetobeweighedbyruntimesandfrequenciesofoperationforthevariousinputsets,inordertoprovideresultsthatwouldbecompatiblewithhardwarereliabilitytheory.
EXECUTIONTIMEMODEL
TheexecutiontimemodelpermitsthedevelopmentofrelationshipsthatindicatenumberoffailuresexperienceandpresentMTFFasfunctionsofexecutiontime(seeFigures1and2).ItrelatestotalfailuresandinitialMTFFtothenumberoffaultsinthesystem.Aninitialestimateofthenumberoffaults,priortotesting,canbedeterminedfromthesize(andperhapscomplexity)oftheprogram.Adebuggingprocessmodelisprovidedwhichrelatesexecutiontimeandcalendartimeandthusallowsexecutiontimequantitiestobeconvertedintodates.Themodelcanbeusedtomakepredictionsoftheremainingnumberoffailuretobeexperienced,theexecutiontimeandcalendartimerequiredtoreachaMTFFobjective.Ifthisobjectiveissetasthecriterionforterminatingtheproject,completiondatescanbepredicted.Astestingproceeds,twoofthekeyparametersofthemodelcanbestatisticallyreestimatedfromfailureintervalsexperienced.ThispermitstheestimationofanumberofderivedquantitiessuchaspresentMTFFandestimatedcompletiondate.Theestimatesmadearemaximumlikelihoodestimates;confidenceintervalsarealsocalculated.
Mostoftheassumptionsthatweremadeinderivingtheexecutiontimemodelhavebeenvalidated[12]andexperiencehasbeengainedwiththemodelonawidevarietyofsoftwaresystems(morethat20asofthisdate).Aprogramisavailable[17,18]tohandlethestatisticalcalculations.SampleoutputfromtheprogramisshowninFigure3.
Usercommentsindicatethattheexecutiontimemodelprovidesagoodconceptualframeworkforviewingthesoftwarefailureprocess.Itissimple,itsparametersarecloselyrelatedtothephysicalworldanditiscompatiblewithhardwarereliabilitytheory.Mostusersfeelthatthebenefitscurrentlyexceedthecosts,whicharebasicallydatacollectionandcomputation.Therehavebeentwointerestingsidebenefits.TheprocessofdefiningjustwhatconstitutesafailureandtheprocessofsettingaMTFFobjectivehavebothbeensalutaryinopeningupcommunicationbetweencustomeranddeveloper.
STATEOFTHEARTANDRESEARCHNEEDS
Softwaremeasurementcanpresentlybeachievedwithexcellentaccuracy.Figure4illustratesasoftwaresystemintheoperationalstate.Themaximumlikelihoodestimateand75%confidenceboundsareindicatedforpresentMTFF.VariationsinMTFFandthesizeoftheconfidenceintervalaregenerallyhighlycorrelatedwithperiodsoffaultcorrectionortheadditionofnewcapabilities.
Thequalityofsoftwarereliabilityestimationisdependentupontherepresentativenessoftesting;hencegoodtestplanningisesesntial.IfonedesirestoknowtheabsolutevalueoftheMTFF,knowledgeofthetestcompressionfactorisnecessary.Thetestcompressionfactorrelatestheamountoftimespentintestwiththeequivalentamountofoperatingtimerepresented.Itisknowntheoreticallyhowtocomputethisnumberbuttheonlypracticalapproachatpresentistoestimateitfromasimilarprojectinasimilartestenvironment.Researchactivityinthisareawoulddefinitelybebeneficial.Onemightcharacterizethepresentqualityofsoftwarereliabilityestimationasgoodforpresentestimationandfairforfutureestimation.Futureestimationalsorequirestodeterminethevaluesoftheseparametersandtheextenttowhichtheyvarybetweendifferentprojectsordifferentclassesofprojectsisurgentlyneeded.Figure5illustratesthevariationinpresentMTFFasthesystemtestphaseofaprojectproceeded(maximumlikelihoodestimateand75%confidenceboundsareindicated).Althoughtheaccuracyoftheabsoluteestimatesisdependentonthetestcompressionfactor,therelativevalues(i.e..
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- software reliability modeling