軟件體系結(jié)構(gòu)試題

Explainindetailthearchitecturebusinesscycleandalltheactivitiesinvolvedandtherelationshipbetweenthem.

Softwareprocessisthetermgiventotheorganization,ritualization,andmanagementofsoftwaredevelopmentactivities.Whatactivitiesareinvolvedincreatingasoftwarearchitecture,usingthatarchitecturetorealizeadesign,andthenimplementingormanagingtheevolutionofatargetsystemorapplication?Theseactivitiesincludethefollowing:CreatingthebusinesscaseforthesystemUnderstandingtherequirementsCreatingorselectingthearchitectureDocumentingandcommunicatingthearchitectureAnalyzingorevaluatingthearchitectureImplementingthesystembasedonthearchitectureEnsuringthattheimplementationconformstothearchitectureARCHITECTUREACTIVITIESAsindicatedinthestructureoftheABC,architectureactivitieshavecomprehensivefeedbackrelationshipswitheachother.Wewillbrieflyintroduceeachactivityinthefollowingsubsections.CreatingtheBusinessCasefortheSystemCreatingabusinesscaseisbroaderthansimplyassessingthemarketneedforasystem.Itisanimportantstepincreatingandconstraininganyfuturerequirements.Howmuchshouldtheproductcost?Whatisitstargetedmarket?Whatisitstargetedtimetomarket?Willitneedtointerfacewithothersystems?Aretheresystemlimitationsthatitmustworkwithin?Theseareallquestionsthatmustinvolvethesystem'sarchitects.Theycannotbedecidedsolelybyanarchitect,butifanarchitectisnotconsultedinthecreationofthebusinesscase,itmaybeimpossibletoachievethebusinessgoals.UnderstandingtheRequirementsThereareavarietyoftechniquesforelicitingrequirementsfromthestakeholders.Forexample,object-orientedanalysisusesscenarios,or"usecases"toembodyrequirements.Safety-criticalsystemsusemorerigorousapproaches,suchasfinite-state-machinemodelsorformalspecificationlanguages.InChapter4(UnderstandingQualityAttributes),weintroduceacollectionofqualityattributescenariosthatsupportthecaptureofqualityrequirementsforasystem.Onefundamentaldecisionwithrespecttothesystembeingbuiltistheextenttowhichitisavariationonothersystemsthathavebeenconstructed.Sinceitisararesystemthesedaysthatisnotsimilartoothersystems,requirementselicitationtechniquesextensivelyinvolveunderstandingthesepriorsystems'characteristics.WediscussthearchitecturalimplicationsofproductlinesinChapter14(SoftwareProductLines:Re-usingArchitecturalAssets).Anothertechniquethathelpsusunderstandrequirementsisthecreationofprototypes.Prototypesmayhelptomodeldesiredbehavior,designtheuserinterface,oranalyzeresourceutilization.Thishelpstomakethesystem"real"intheeyesofitsstakeholdersandcanquicklycatalyzedecisionsonthesystem'sdesignandthedesignofitsuserinterface.Regardlessofthetechniqueusedtoelicittherequirements,thedesiredqualitiesofthesystemtobeconstructeddeterminetheshapeofitsarchitecture.Specifictacticshavelongbeenusedbyarchitectstoachieveparticularqualityattributes.WediscussmanyofthesetacticsinChapter5(AchievingQualities).Anarchitecturaldesignembodiesmanytradeoffs,andnotallofthesetradeoffsareapparentwhenspecifyingrequirements.Itisnotuntilthearchitectureiscreatedthatsometradeoffsamongrequirementsbecomeapparentandforceadecisiononrequirementpriorities.CreatingorSelectingtheArchitectureInthelandmarkbookTheMythicalMan-Month,FredBrooksarguesforcefullyandeloquentlythatconceptualintegrityisthekeytosoundsystemdesignandthatconceptualintegritycanonlybehadbyasmallnumberofmindscomingtogethertodesignthesystem'sarchitecture.Chapters5(AchievingQualities)and7(DesigningtheArchitecture)showhowtocreateanarchitecturetoachieveitsbehavioralandqualityrequirements.CommunicatingtheArchitectureForthearchitecturetobeeffectiveasthebackboneoftheproject'sdesign,itmustbecommunicatedclearlyandunambiguouslytoallofthestakeholders.Developersmustunderstandtheworkassignmentsitrequiresofthem,testersmustunderstandthetaskstructureitimposesonthem,managementmustunderstandtheschedulingimplicationsitsuggests,andsoforth.Towardthisend,thearchitecture'sdocumentationshouldbeinformative,unambiguous,andreadablebymanypeoplewithvariedbackgrounds.WediscussthedocumentationofarchitecturesinChapter9(DocumentingSoftwareArchitectures).AnalyzingorEvaluatingtheArchitectureInanydesignprocesstherewillbemultiplecandidatedesignsconsidered.Somewillberejectedimmediately.Otherswillcontendforprimacy.Choosingamongthesecompetingdesignsinarationalwayisoneofthearchitect'sgreatestchallenges.ThechaptersinPartThree(AnalyzinganArchitecture)describemethodsformakingsuchchoices.Evaluatinganarchitectureforthequalitiesthatitsupportsisessentialtoensuringthatthesystemconstructedfromthatarchitecturesatisfiesitsstakeholders'needs.Becomingmorewidespreadareanalysistechniquestoevaluatethequalityattributesthatanarchitectureimpartstoasystem.Scenario-basedtechniquesprovideoneofthemostgeneralandeffectiveapproachesforevaluatinganarchitecture.ThemostmaturemethodologicalapproachisfoundintheArchitectureTradeoffAnalysisMethod(ATAM)ofChapter11,whiletheCostBenefitAnalysisMethod(CBAM)ofChapter12providesthecriticallinktotheeconomicimplicationsofarchitecturaldecisions.ImplementingBasedontheArchitectureThisactivityisconcernedwithkeepingthedevelopersfaithfultothestructuresandinteractionprotocolsconstrainedbythearchitecture.Havinganexplicitandwell-communicatedarchitectureisthefirststeptowardensuringarchitecturalconformance.Havinganenvironmentorinfrastructurethatactivelyassistsdevelopersincreatingandmaintainingthearchitecture(asopposedtojustthecode)isbetter.EnsuringConformancetoanArchitectureFinally,whenanarchitectureiscreatedandused,itgoesintoamaintenancephase.Constantvigilanceisrequiredtoensurethattheactualarchitectureanditsrepresentationremainfaithfultoeachotherduringthisphase.Althoughworkinthisareaiscomparativelyimmature,therehasbeenintenseactivityinrecentyears.Chapter10(ReconstructingSoftwareArchitectures)willpresentthecurrentstateofrecoveringanarchitecturefromanexistingsystemandensuringthatitconformstothespecifiedarchitecture.2.Whatmakeasoftwarearchitecture“good”whataresomeofthingsyoucanlookfortodeterminewhetherornotanarchitectureisgood.

Wedivideourobservationsintotwoclusters:processrecommendationsandproduct(orstructural)recommendations.Ourprocessrecommendationsareasfollows:Thearchitectureshouldbetheproductofasinglearchitectorasmallgroupofarchitectswithanidentifiedleader.Thearchitect(orarchitectureteam)shouldhavethefunctionalrequirementsforthesystemandanarticulated,prioritizedlistofqualityattributes(suchassecurityormodifiability)thatthearchitectureisexpectedtosatisfy.Thearchitectureshouldbewelldocumented,withatleastonestaticviewandonedynamicview(explainedinChapter2),usinganagreed-onnotationthatallstakeholderscanunderstandwithaminimumofeffort.Thearchitectureshouldbecirculatedtothesystem'sstakeholders,whoshouldbeactivelyinvolvedinitsreview.Thearchitectureshouldbeanalyzedforapplicablequantitativemeasures(suchasmaximumthroughput)andformallyevaluatedforqualityattributesbeforeitistoolatetomakechangestoit.Thearchitectureshouldlenditselftoincrementalimplementationviathecreationofa"skeletal"systeminwhichthecommunicationpathsareexercisedbutwhichatfirsthasminimalfunctionality.Thisskeletalsystemcanthenbeusedto"grow"thesystemincrementally,easingtheintegrationandtestingefforts(seeChapter7,Section7.4).Thearchitectureshouldresultinaspecific(andsmall)setofresourcecontentionareas,theresolutionofwhichisclearlyspecified,circulated,andmaintained.Forexample,ifnetworkutilizationisanareaofconcern,thearchitectshouldproduce(andenforce)foreachdevelopmentteamguidelinesthatwillresultinaminimumofnetworktraffic.Ifperformanceisaconcern,thearchitectsshouldproduce(andenforce)timebudgetsforthemajorthreads.Ourstructuralrulesofthumbareasfollows:Thearchitectureshouldfeaturewell-definedmoduleswhosefunctionalresponsibilitiesareallocatedontheprinciplesofinformationhidingandseparationofconcerns.Theinformation-hidingmodulesshouldincludethosethatencapsulateidiosyncrasiesofthecomputinginfrastructure,thusinsulatingthebulkofthesoftwarefromchangeshouldtheinfrastructurechange.Eachmoduleshouldhaveawell-definedinterfacethatencapsulatesor"hides"changeableaspects(suchasimplementationstrategiesanddatastructurechoices)fromothersoftwarethatusesitsfacilities.Theseinterfacesshouldallowtheirrespectivedevelopmentteamstoworklargelyindependentlyofeachother.Qualityattributesshouldbeachievedusingwell-knownarchitecturaltacticsspecifictoeachattribute,asdescribedinChapter5(AchievingQualities).Thearchitectureshouldneverdependonaparticularversionofacommercialproductortool.Ifitdependsuponaparticularcommercialproduct,itshouldbestructuredsuchthatchangingtoadifferentproductisstraightforwardandinexpensive.Modulesthatproducedatashouldbeseparatefrommodulesthatconsumedata.Thistendstoincreasemodifiabilitybecausechangesareoftenconfinedtoeithertheproductionortheconsumptionsideofdata.Ifnewdataisadded,bothsideswillhavetochange,buttheseparationallowsforastaged(incremental)upgrade.Forparallel-processingsystems,thearchitectureshouldfeaturewell-definedprocessesortasksthatdonotnecessarilymirrorthemoduledecompositionstructure.Thatis,processesmaythreadthroughmorethanonemodule;amodulemayincludeproceduresthatareinvokedaspartofmorethanoneprocess(theA-7EcasestudyofChapter3isanexampleofemployingthisprinciple).Everytaskorprocessshouldbewrittensothatitsassignmenttoaspecificprocessorcanbeeasilychanged,perhapsevenatruntime.Thearchitectureshouldfeatureasmallnumberofsimpleinteractionpatterns(seeChapter5).Thatis,thesystemshoulddothesamethingsinthesamewaythroughout.Thiswillaidinunderstandability,reducedevelopmenttime,increasereliability,andenhancemodifiability.Itwillalsoshowconceptualintegrityinthearchitecture,which,whilenotmeasurable,leadstosmoothdevelopment.3.Youareanarchitectforabankwhatthetacticsandpatternsyoucanusetoimplementsecurityqualityattribute?

Tacticsforachievingsecuritycanbedividedintothoseconcernedwithresistingattacks,thoseconcernedwithdetectingattacks,andthoseconcernedwithrecoveringfromattacks.4.Explaintheconceptofwhole-partandinwhichsituationsandhowisitusefulforsoftwarearchitects?

Aqualityattributescenarioisaquality-attribute-specificrequirement.Itconsistsofsixparts.Sourceofstimulus.Thisissomeentity(ahuman,acomputersystem,oranyotheractuator)thatgeneratedthestimulus.Stimulus.Thestimulusisaconditionthatneedstobeconsideredwhenitarrivesatasystem.Environment.Thestimulusoccurswithincertainconditions.Thesystemmaybeinanoverloadconditionormayberunningwhenthestimulusoccurs,orsomeotherconditionmaybetrue.Artifact.Someartifactisstimulated.Thismaybethewholesystemorsomepiecesofit.Response.Theresponseistheactivityundertakenafterthearrivalofthestimulus.Responsemeasure.Whentheresponseoccurs,itshouldbemeasurableinsomefashionsothattherequirementcanbetested.5.Whatarethestepsrequiredforeffectivesoftwarereconstruction?Softwarearchitecturereconstructioncomprisesthefollowingactivities,carriedoutiteratively:Informationextraction.Thepurposeofthisactivityistoextractinformationfromvarioussources.Databaseconstruction.DatabaseconstructioninvolvesconvertingthisinformationintoastandardformsuchastheRigiStandardForm(atuple-baseddataformatintheformofrelationship<entity1><entity2>)andanSQL-baseddatabaseformatfromwhichthedatabaseiscreated.Viewfusion.Viewfusioncombinesinformationinthedatabasetoproduceacoherentviewofthearchitecture.Reconstruction.Thereconstructionactivityiswherethemainworkofbuildingabstractionsandvariousrepresentationsofthedatatogenerateanarchitecturerepresentationtakesplace.GUIDELINESThefollowingaresomepracticalconsiderationsinapplyingthisstepofthemethod.Usethe"leasteffort"extraction.Considerwhatinformationyouneedtoextractfromasourcecorpus.Isthisinformationlexicalinnature?Doesitrequirethecomprehensionofcomplexsyntacticstructures?Doesitrequiresomesemanticanalysis?Ineachcase,adifferenttoolcouldbeappliedsuccessfully.Ingeneral,lexicalapproachesarethecheapesttouse,andtheyshouldbeconsideredifyourreconstructiongoalsaresimple.Validatetheinformationyouhaveextracted.Beforestartingtofuseormanipulatethevariousviewsobtained,makesurethatthecorrectviewinformationhasbeencaptured.Itisimportantthatthetoolsbeingusedtoanalyzethesourceartifactsdotheirjobcorrectly.Firstperformdetailedmanualexaminationandverificationofasubsetoftheelementsandrelationsagainsttheunderlyingsourcecode,toestablishthatthecorrectinformationisbeingcaptured.Thepreciseamountofinformationthatneedstobeverifiedmanuallyisuptoyou.Assumingthatthisisastatisticalsampling,youcandecideonadesiredconfidencelevelandchoosethesamplingstrategytoachieveit.Extractdynamicinformationwhererequired,suchaswherethereisalotofruntimeorlatebindingandthearchitectureisdynamicallyconfigurable.GUIDELINESWhenconstructiongthedatabase,considerthefollowing.Builddatabasetablesfromtheextractedrelationstomakeprocessingofthedataviewseasierduringviewfusion.Forexample,buildatablethatstorestheresultsofaparticularquerysothatthequeryneednotberunagain.Iftheresultsarerequired,youcanaccessthemeasilythroughthetable.Aswithanydatabaseconstruction,carefullyconsiderthedatabasedesignbeforeyougetstarted.Whatwilltheprimary(andpossiblysecondary)keybe?Willanydatabasejoinsbeparticularlyexpensive,spanningmultipletables?Inreconstructionthetablesareusuallyquitesimple—ontheorderofdir_contains_dirorfunction_calls_function—andtheprimarykeyisafunctionoftheentirerow.Usesimplelexicaltoolslikeperlandawktochangetheformatofdatathatwasextractedusinganytoolsintoaformatthatcanbeusedbytheworkbench.GUIDELINESThefollowingaresomepracticalconsiderationsinapplyingthisstepofthemethod.Fusetableswhennosingleextractedtableprovidestheneededinformation.Fusetableswhenthereisambiguitywithinoneofthem,anditisnotpossibletodisambiguateusingasingletable.Considerdifferentextractiontechniquestoextractdifferentinformation;forexample,youcanusedynamicandstaticextraction.Oryoumightwanttousedifferentinstancesofthesametechnique,suchasdifferentparsersforthesamelanguage,ifyoufeelthatasingleinstancemightprovideerroneousorincompleteinformation.GUIDELINESThefollowingaresomepracticalconsiderationsinapplyingthisstepofthemethod.Bepreparedtoworkwiththearchitectcloselyandtoiterateseveraltimesonthearchitecturalabstractionsthatyoucreate.Thisisparticularlysoincaseswherethesystemhasnoexplicit,documentedarchitecture.(SeethesidebarPlaying"SpottheArchitecture.")Insuchcases,youcancreatearchitecturalabstractionsashypothesesandtestthesehypothesesbycreatingtheviewsandshowingthemtothearchitectandotherstakeholders.Basedonthefalsenegativesandfalsepositivesfound,thereconstructormaydecidetocreatenewabstractions,resultinginnewDalicodesegmentstoapply(orperhapsevennewextractionsthatneedtobedone).Whendevelopingcodesegments,trytobuildonesthataresuccinctandthatdonotlisteverysourceelement.ThecodesegmentshowninFigure10.11isanexampleofagoodsegment;anexampleofabadoneinthisregard,isshowninFigure10.12.Inthelatter,thesourceelementscomprisingthearchitecturalelementofinterestaresimplylisted;thismakesthesegmentdifficulttouse,understand,andre-use.Codesegmentscanbebasedonnamingconventions,ifthenamingconventionsareusedconsistentlythroughoutthesystem.Anexampleisonewhereallfunctions,data,andfilesthatbelongtotheInterfaceelementbeginwithi_.Codesegmentscanbebasedonthedirectorystructurewherefilesandfunctionsarelocated.Elementaggregationscanbebasedonthesedirectories.Architecturereconstructionistheeffortofredeterminingarchitecturaldecisions,givenonlytheresultofthesedecisionsintheactualartifacts(i.e.,thecodethatimplementsthem).Asreconstructionproceeds,informationmustbeaddedtore-introducethearchitecturaldecisionswhichintroducesbiasfromthereconstructorandthusreinforcestheneedforapersonknowledgeableinthearchitecturetobeinvolved.[TeamLiB]7.2DesigningtheArchitectureInthissectionwedescribeamethodfordesigninganarchitecturetosatisfybothqualityrequirementsandfunctionalrequirements.WecallthismethodAttribute-DrivenDesign(ADD).ADDtakesasinputasetofqualityattributescenariosandemploysknowledgeabouttherelationbetweenqualityattributeachievementandarchitectureinordertodesignthearchitecture.TheADDmethodcanbeviewedasanextensiontomostotherdevelopmentmethods,suchastheRationalUnifiedProcess.TheRationalUnifiedProcesshasseveralstepsthatresultinthehigh-leveldesignofanarchitecturebutthenproceedstodetaileddesignandimplementation.IncorporatingADDintoitinvolvesmodifyingthestepsdealingwiththehigh-leveldesignofthearchitectureandthenfollowingtheprocessasdescribedbyRational.ATTRIBUTE-DRIVENDESIGNADDisanapproachtodefiningasoftwarearchitecturethatbasesthedecompositionprocessonthequalityattributesthesoftwarehastofulfill.Itisarecursivedecompositionprocesswhere,ateachstage,tacticsandarchitecturalpatternsarechosentosatisfyasetofqualityscenariosandthenfunctionalityisallocatedtoinstantiatethemoduletypesprovidedbythepattern.ADDispositionedinthelifecycleafterrequirementsanalysisand,aswehavesaid,canbeginwhenthearchitecturaldriversareknownwithsomeconfidence.TheoutputofADDisthefirstseverallevelsofamoduledecompositionviewofanarchitectureandotherviewsasappropriate.NotalldetailsoftheviewsresultfromanapplicationofADD;thesystemisdescribedasasetofcontainersforfunctionalityandtheinteractionsamongthem.Thisisthefirstarticulationofarchitectureduringthedesignprocessandisthereforenecessarilycoarsegrained.Nevertheless,itiscriticalforachievingthedesiredqualities,anditprovidesaframeworkforachievingthefunctionality.ThedifferencebetweenanarchitectureresultingfromADDandonereadyforimplementationrestsinthemoredetaileddesigndecisionsthatneedtobemade.Thesecouldbe,forexample,thedecisiontousespecificobject-orienteddesignpatternsoraspecificpieceofmiddlewarethatbringswithitmanyarchitecturalconstraints.ThearchitecturedesignedbyADDmayhaveintentionallydeferredthisdecisiontobemoreflexible.ThereareanumberofdifferentdesignprocessesthatcouldbecreatedusingthegeneralscenariosofChapter4andthetacticsandpatternsofChapter5.Eachprocessassumesdifferentthingsabouthowto"chunk"thedesignworkandabouttheessenceofthedesignprocess.WediscussADDinsomedetailtoillustratehowweareapplyingthegeneralscenariosandtactics,andhencehowweare"chunking"thework,andwhatwebelieveistheessenceofthedesignprocess.WedemonstratetheADDmethodbyusingittodesignaproductlinearchitectureforagaragedooropenerwithinahomeinformationsystem.Theopenerisresponsibleforraisingandloweringthedoorviaaswitch,remotecontrol,orthehomeinformationsystem.Itisalsopossibletodiagnoseproblemswiththeopenerfromwithinthehomeinformationsystem.SampleInputTheinputtoADDisasetofrequirements.ADDassumesfunctionalrequirements(typicallyexpressedasusecases)andconstraintsasinput,asdootherdesignmethods.However,inADD,wedifferfromthosemethodsinourtreatmentofqualityrequirements.ADDmandatesthatqualityrequirementsbeexpressedasasetofsystem-specificqualityscenarios.ThegeneralscenariosdiscussedinChapter4actasinputtotherequirementsprocessandprovideachecklisttobeusedindevelopingthesystem-specificscenarios.System-specificscenariosshouldbedefinedtothedetailnecessaryfortheapplication.Inourexamples,weomitseveralportionsofafullyfleshedscenariosincetheseportionsdonotcontributetothedesignprocess.Forourgaragedoorexample,thequalityscenariosincludethefollowing:Thedeviceandcontrolsforopeningandclosingthedooraredifferentforthevariousproductsintheproductline,asalreadymentioned.Theymayincludecontrolsfromwithinahomeinformationsystem.Theproductarchitectureforaspecificsetofcontrolsshouldbedirectlyderivablefromtheproductlinearchitecture.Theprocessorusedindifferentproductswilldiffer.Theproductarchitectureforeachspecificprocessorshouldbedirectlyderivablefromtheproductlinearchitecture.Ifanobstacle(personorobject)isdetectedbythegaragedoorduringdescent,itmusthalt(alternatelyre-open)within0.1second.Thegaragedooropenershouldbeaccessiblefordiagnosisandadministrationfromwithinthehomeinformationsystemusingaproduct-specificdiagnosisprotocol.Itshouldbepossibletodirectlyproduceanarchitecturethatreflectsthisprotocol.BeginningADDWehavealreadyintroducedarchitecturaldrivers.ADDdependsontheidentificationofthedriversandcanstartassoonasallofthemareknown.Ofcourse,duringthedesignthedeterminationofwhicharchitecturaldriversarekeymaychangeeitherasaresultofbetterunderstandingoftherequirementsorasaresultofchangingrequirements.Still,theprocesscanbeginwhenthedriverrequirementsareknownwithsomeassurance.InthefollowingsectionwediscussADDitself.ADDStepsWebeginbybrieflypresentingthestepsperformedwhendesigninganarchitectureusingtheADDmethod.Wewillthendiscussthestepsinmoredetail.Choosethemoduletodecompose.Themoduletostartwithisusuallythewholesystem.Allrequiredinputsforthismoduleshouldbeavailable(constraints,functionalrequirements,qualityrequirements).Refinethemoduleaccordingtothesesteps:Choosethearchitecturaldriversfromthesetofconcretequalityscenariosandfunctionalrequirements.Thisstepdetermineswhatisimportantforthisdecomposition.Chooseanarchitecturalpatternthatsatisfiesthearchitecturaldrivers.Create(orselect)thepatternbasedonthetacticsthatcanbeusedtoachievethedrivers.Identifychildmodulesrequiredtoimplementthetactics.Instantiatemodulesandallocatefunctionalityfromtheusecasesandrepresentusingmultipleviews.Defineinterfacesofthechildmodules.Thedecompositionprovidesmodulesandconstraintsonthetypesofmoduleinteractions.Documentthisinformationintheinterfacedocumentforeachmodule.Verifyandrefineusecasesandqualityscenariosandmakethemconstraintsforthechildmodules.Thisstepverifiesthatnothingimportantwasforgottenandpreparesthechildmodulesforfurtherdecompositionorimplementation.Repeatthestepsaboveforeverymodulethatneedsfurtherdecomposition.1ChoosetheModuletoDecomposeThefollowingareallmodules:system,subsystem,andsubmodule.Thedecompositiontypicallystartswiththesystem,whichisthendecomposedintosubsystems,whicharefurtherdecomposedintosubmodules.Inourexample,thegaragedooropeneristhesystem.Oneconstraintatthislevelisthattheopenermustinteroperatewiththehomeinformationsystem.2.aChoosetheArchitecturalDriversAswesaid,architecturaldriversarethecombinationoffunctionalandqualityrequirementsthat"shape"thearchitectureortheparticularmoduleunderconsideration.Thedriverswillbefoundamongthetop-priorityrequirementsforthemodule.Inourexample,thefourscenarioswehaveshownarearchitecturaldrivers.Inthesystemsonwhichthisexampleisbased,thereweredozensofqualityscenarios.Inexaminingthem,weseearequirementforreal-timeperformance,[1]andmodifiabilitytosupportproductlines.Wealsoseearequirementthatonlinediagnosisbesupported.Alloftheserequirementsmustbeaddressedintheinitialdecompositionofthesystem.[1]A0.1-secondresponsewhenanobstacleisdetectedmaynotseemlikeatightdeadline,butwearediscussingamassmarketwhereusingaprocessorwithlimitedpowertranslatesintosubstantialcostsavings.Also,agaragedoorhasagreatdealofinertiaandisdifficulttostop.Thedeterminationofarchitecturaldriversisnotalwaysatop-downprocess.Sometimesdetailedinvestigationisrequiredtounderstandtheramificationsofparticularrequirements.Forexample,todetermineifperformanceisanissueforaparticularsystemconfiguration,aprototypicalimplementationofapieceofthesystemmayberequired.Inourexample,determiningthattheperformancerequirementisanarchitecturaldriverrequiresexaminingthemechanicsofagaragedoorandthespeedofthepotentialprocessors.Wewillbaseourdecompositionofamoduleonthearchitecturaldrivers.Otherrequirementsapplytothatmodule,but,bychoosingthedrivers,wearereducingtheproblemtosatisfyingthemostimportantones.Wedonottreatalloftherequiremen