軟件體系結(jié)構(gòu)之質(zhì)量屬性課件

Ch.5DesigningtheArchitectureInstructor:TanLiwei軟件設(shè)計與體系結(jié)構(gòu)軟件體系結(jié)構(gòu)之質(zhì)量屬性referenceSoftwarearchitectureinpractice–ch1,ch5,ch7contentsDesigningArchitecture:AchievingQualities軟件體系結(jié)構(gòu)之質(zhì)量屬性ArchitectureBusinessCycle(ABC)CreatingthearchitectureHowdoesthearchitectcreateanarchitecture?(Multiplechoice):Byre-usingapproachesfromotherarchitectures通過重用來自于其他體系結(jié)構(gòu)的方法

Byinventingnewapproachesoutofthinair無中生有地發(fā)明新方法

Bymagic耍魔術(shù)，變一個CreatingthearchitectureArchitectsprimarilyworkbyusingpreviously-triedsolutionsLargescale:PatternsandstylesSmallscale:TacticsStyles,patterns,andtacticsrepresentconceptualtoolsinthearchitect’s“toolbag.”Professionalarchitectsalwayskeeptheirtoolbaguptodate.風(fēng)格，模式和策略是體系架構(gòu)師的工具箱中的概念工具。專業(yè)的體系架構(gòu)師會不斷更新自己的工具箱。PatternsandstylesThemoderntermis“patterns”butearlypapersonsoftwarearchitecturewroteabout“softwarearchitecturestyles.”Stylesinarchitecturewereanalogoustostylesinhouses:Victorian(multi-story,lotsoffrilly(鑲褶邊的)wooddecorations,tallwindows,basicallysquarefootprint…)Colonial(brickfront,pillarsorcolumns,usuallysymmetricalfront…)Ranch(single-story,sprawling,notverydecorated…)大農(nóng)場PatternsandstylesIndependentcomponentpatternsCommunication-processesEventsystemsImplicitinvocationExplicitinvocationDataflowpatternsBatchsequentialPipe-and-filterLayersData-centeredpatternsBlackboardrepositoryVirtualmachinepatternsInterpretersRule-basedsystemsCall-returnpatternsMainprogramandsubroutineObjectorientedStyles->PatternsThen,thedesignpatternscommunityarrived.Architecturalstyleswereclearlyjustpatterns,whosescopeofdesignwasthewholesystem–thatis,whosescopewasthearchitecture.Now,architecturalpatternsistheterminuse.Therearebooksofarchitecturalpatterns,e.g.,Buschmann,F.,R.Meunier,H.Rohnert,P.Sommerlad,andM.Stal.1996.Pattern-OrientedSoftwareArchitecture,Volume1:ASystemofPatterns.Wiley.現(xiàn)在到了設(shè)計模式時代，體系結(jié)構(gòu)風(fēng)格顯然就是模式，其設(shè)計范圍是整個系統(tǒng)。現(xiàn)在使用的術(shù)語是體系結(jié)構(gòu)模式。關(guān)于體系結(jié)構(gòu)模式的經(jīng)典圖書ArchitecturalpatternsThesearebroadly-scopedsolutionstopreviouslyencounteredproblems.Anarchitecturalpatternisfoundrepeatedlyinpracticeisapackageofdesigndecisionshasknownpropertiesthatpermitreusedescribesaclassofarchitectures模式會在實(shí)際應(yīng)用中被不斷地重復(fù)；它是一個設(shè)計決策包；具有眾所周知的可重用的屬性；模式描述了一類體系結(jié)構(gòu)。ArchitecturalpatternsApatternisdeterminedanddescribedbyasetofelementtypesforexample,datarepositories,processes,andobjectsasetofinteractionmechanismsorconnectorsforexample,subroutinecalls,events,andpipesatopologicallayoutofthecomponentsasetofsemanticconstraintscoveringtopology,elementbehavior,andinteractionmechanismsInaddition,apatternisdescribedbywhenandwhytouseit模式的描述：組成元素的類型、交互機(jī)制或連接器、組件的拓?fù)湟?guī)劃、語義約束以及何時為什么使用該模式。ArchitecturalpatternsThesearewidelyknownandincludemanyfamiliardesignapproaches:LayeredPipe-andfilterClient-serverThinclientThickclientAsynchronousSynchronousN-tierclient-serverEtc.Peer-topeerAgent-basedsystemsService-orientedarchitecturesEtc.Observe:No“universal”listPatternscanbecombined:e.g.,layeredclient-serverPatternscanbespecializedChoiceofpatternstouseisnotrandom!注意：不可能列出所有的模式；多個模式可以組合；模式可能是專門化的；模式的選擇不是隨機(jī)的。TacticsAnarchitecturaltacticisafine-graineddesignapproachusedtoachieveaqualityattributeresponse.Tacticsarethe“buildingblocks”ofdesignfromwhicharchitecturalpatternsarecreated.體系結(jié)構(gòu)策略是一種可獲得質(zhì)量屬性響應(yīng)的細(xì)膩的設(shè)計方法。策略可看作用來搭建體系結(jié)構(gòu)模式的積木塊。TacticsWhatisitthatimpartsportabilitytoonedesign,highperformancetoanother,andintegrabilitytoathird?Theachievementofthesequalitiesreliesonfundamentaldesigndecisions.Atacticisadesigndecisionthatinfluencesthecontrolofaqualityattributeresponse.Wecallacollectionoftacticsanarchitecturalstrategy.是什么向外界傳達(dá)一個設(shè)計的靈便性、高性能和可集成性？依賴于基本的設(shè)計決策。策略是指影響質(zhì)量屬性響應(yīng)控制的設(shè)計決策。策略的集合叫做體系結(jié)構(gòu)戰(zhàn)略。AvailabilityTactics可用性策略keepfaultsfrombecomingfailuresoratleastboundtheeffectsofthefaultandmakerepairpossible.使缺陷不要成為失敗，或者至少限制缺陷的影響并使修復(fù)成為可能。AvailabilityTacticsfaultdetection.故障檢測Ping/echo,Heartbeat,Exceptionsfaultrecovery故障恢復(fù)Faultrecoveryconsistsofpreparingforrecoveryandmakingthesystemrepair.Voting,Activeredundancy,Passiveredundancy,Spare,Shadowoperation,Stateresynchronization,Checkpoint/rollbackfaultprevention故障預(yù)防Removalfromservice,Transactions,ProcessmonitorFaultdetectionPing/echoOnecomponentissuesapingandexpectstoreceivebackanecho,withinapredefinedtime,fromthecomponentunderscrutiny.usedwithinagroupofcomponentsmutuallyresponsibleforonetask.usedbyclientstoensurethataserverobjectandthecommunicationpathtotheserverareoperatingwithintheexpectedperformancebounds."Ping/echo"faultdetectorscanbeorganizedinahierarchy,inwhichalowest-leveldetectorpingsthesoftwareprocesseswithwhichitsharesaprocessor,andthehigher-levelfaultdetectorspinglower-levelones.Thisuseslesscommunicationsbandwidththanaremotefaultdetectorthatpingsallprocesses.一個組件發(fā)出ping信號，期待在預(yù)計的時間內(nèi)從要監(jiān)視的組件收到響應(yīng)它用于一組共同負(fù)責(zé)一個任務(wù)的組件。可以被客戶端用來測試服務(wù)器及通信信道能在預(yù)計的性能約束內(nèi)操作FaultdetectionHeartbeat(deadmantimer).Inthiscaseonecomponentemitsaheartbeatmessageperiodicallyandanothercomponentlistensforit.Iftheheartbeatfails,theoriginatingcomponentisassumedtohavefailedandafaultcorrectioncomponentisnotified.Theheartbeatcanalsocarrydata.Forexample,anautomatedtellermachinecanperiodicallysendthelogofthelasttransactiontoaserver.Thismessagenotonlyactsasaheartbeatbutalsocarriesdatatobeprocessed.一個組件周期性地發(fā)出心跳（heartbeat）消息，其它組件監(jiān)聽之。如果沒聽到心跳，說明前者已經(jīng)失效，通知糾錯組件。心跳也可攜帶數(shù)據(jù)。例如自動出納機(jī)周期性地向服務(wù)器發(fā)送最新交易日志。FaultdetectionExceptions.例外Onemethodforrecognizingfaultsistoencounteranexception,whichisraisedwhenoneofthefaultclassesisrecognized.Theexceptionhandlertypicallyexecutesinthesameprocessthatintroducedtheexception.發(fā)現(xiàn)錯誤的另一種方法是使用例外，它在錯誤發(fā)生時拋出。例外處理器通常位于拋出例外的進(jìn)程中。FaultrecoveryVotingProcessesrunningonredundantprocessorseachtakeequivalentinputandcomputeasimpleoutputvaluethatissenttoavoter.Ifthevoterdetectsdeviantbehaviorfromasingleprocessor,itfailsit.Thismethodisusedtocorrectfaultyoperationofalgorithmsorfailureofaprocessorandisoftenusedincontrolsystems.Ifalloftheprocessorsutilizethesamealgorithms,theredundancydetectsonlyaprocessorfaultandnotanalgorithmfault.在冗余的處理器上運(yùn)行的進(jìn)程獲得相同的輸入并計算出一個輸出值，將之送到投票器，若投票器從一個處理器檢測到異常行為，放棄之。此方法被用于糾正有缺陷的算法運(yùn)算或處理器的失效，常用于控制系統(tǒng)中。若所有的處理器使用同樣的算法，則該冗余只能檢測處理器失效，不能檢測算法失效。FaultrecoveryActiveredundancy(hotrestart).主動冗余Allredundantcomponentsrespondtoeventsinparallel.Consequently,theyareallinthesamestate.Whenafaultoccurs,thedowntimeofsystemsusingthistacticisusuallymillisecondssincethebackupiscurrentandtheonlytimetorecoveristheswitchingtime.Synchronizationisperformedbyensuringthatallmessagestoanyredundantcomponentaresenttoallredundantcomponents.所有的冗余組件同時響應(yīng)事件。結(jié)果，他們處于同樣的狀態(tài)。錯誤發(fā)生的停機(jī)時間通常是微秒級。需要進(jìn)行同步，做法是把送往任一個組件的消息同時送其他冗余組件。FaultrecoveryPassiveredundancy(warmrestart/dualredundancy/tripleredundancy).被動冗余Onecomponent(theprimary)respondstoeventsandinformstheothercomponents(thestandbys)ofstateupdatestheymustmake.Whenafaultoccurs,thesystemmustfirstensurethatthebackupstateissufficientlyfreshbeforeresumingservices.Synchronizationistheresponsibilityoftheprimarycomponent,whichmayuseatomicbroadcaststothesecondarytoguaranteesynchronizationThedowntimecanusuallybelimitedtoseconds.一個組件（主組件）對事件響應(yīng)，同時通知其他組件（備份）更新狀態(tài)。當(dāng)錯誤發(fā)生，恢復(fù)服務(wù)前系統(tǒng)必須保證備份狀態(tài)足夠新。同步是主組件的職責(zé)，它使用自動廣播來保證同步。停機(jī)時間通常是秒級。FaultrecoverySpare.備用的Astandbysparecomputingplatformisconfiguredtoreplacemanydifferentfailedcomponents.Itmustberebootedtotheappropriatesoftwareconfigurationandhaveitsstateinitializedwhenafailureoccurs.Makingacheckpointofthesystemstatetoapersistentdeviceperiodicallyandloggingallstatechangestoapersistentdeviceallowsforthesparetobesettotheappropriatestate.Thedowntimeforthistacticisusuallyminutes.使用一備用的計算平臺來替換許多不同的失敗組件。當(dāng)失敗發(fā)生時，必須重啟到適當(dāng)?shù)能浖渲孟?，并初始化狀態(tài)。做法是周期性地向持久化設(shè)備保存系統(tǒng)狀態(tài)的檢查點(diǎn)，并把所有的狀態(tài)變化記錄到持久化設(shè)備下，以便備用設(shè)備恢復(fù)。停工期通常以分鐘計。FaultrecoveryShadowoperation.鏡像操作Apreviouslyfailedcomponentmayberunin"shadowmode"forashorttimetomakesurethatitmimicsthebehavioroftheworkingcomponentsbeforerestoringittoservice.失敗的組件可以運(yùn)行于鏡像模式一段時間以保證它在恢復(fù)服務(wù)前仿真正常組件的行為。FaultrecoveryStateresynchronization.狀態(tài)再同步Thepassiveandactiveredundancytacticsrequirethecomponentbeingrestoredtohaveitsstateupgradedbeforeitsreturntoservice.Theupdatingapproachwilldependonthedowntimethatcanbesustained,thesizeoftheupdate,andthenumberofmessagesrequiredfortheupdate.Asinglemessagecontainingthestateispreferable,ifpossible.Incrementalstateupgrades,withperiodsofservicebetweenincrements,leadtocomplicatedsoftware.被動與主動冗余策略要求被恢復(fù)的組件在回到服務(wù)前使其狀態(tài)升級。升級步驟依賴于可忍受的停工期、更新的大小和更新要求的消息數(shù)量。如果可能的話，包含狀態(tài)的簡單消息是首選的。FaultrecoveryCheckpoint/rollback.檢查點(diǎn)/回滾Acheckpointisarecordingofaconsistentstatecreatedeitherperiodicallyorinresponsetospecificevents.Sometimesasystemfailsinanunusualmanner,withadetectablyinconsistentstate.Inthiscase,thesystemshouldberestoredusingapreviouscheckpointofaconsistentstateandalogofthetransactionsthatoccurredsincethesnapshotwastaken.檢查點(diǎn)是周期性或在發(fā)生特定事件時創(chuàng)建的一致性狀態(tài)的記錄。有時系統(tǒng)以不尋常的方式失敗于可檢測的不一致狀態(tài)。此時，系統(tǒng)應(yīng)該用以前的一個一致狀態(tài)的檢查點(diǎn)和檢查點(diǎn)之后發(fā)生的事務(wù)記錄來恢復(fù)。FaultpreventionRemovalfromservice.移除服務(wù)Thistacticremovesacomponentofthesystemfromoperationtoundergosomeactivitiestopreventanticipatedfailures.Transactions.事務(wù)Atransactionisthebundlingofseveralsequentialstepssuchthattheentirebundlecanbeundoneatonce.Processmonitor.進(jìn)程監(jiān)視器Onceafaultinaprocesshasbeendetected,amonitoringprocesscandeletethenonperformingprocessandcreateanewinstanceofit,initializedtosomeappropriatestateasinthesparetactic.SummaryofavailabilitytacticsModifiabilityTacticsTacticstocontrolmodifiabilityhaveastheirgoalcontrollingthetimeandcosttoimplement,test,anddeploychanges.控制可修改性的策略致力于控制軟件實(shí)現(xiàn)、測試、和配置改變的時間和成本可根據(jù)目標(biāo)把修改策略分組:(1)localizemodifications:減少直接被修改影響的模塊數(shù)量(2)Preventtherippleeffect:把修改限制到局部模塊(3)Deferbindingtime:控制部署時間和成本LocalizeModifications局部修改受影響的模塊數(shù)與修改的成本沒有精確的關(guān)系，但把修改限制到少量的模塊通?？梢詼p少成本。這里給出5種限制范圍的策略：Maintainsemanticcoherence.保持語義一致性Semanticcoherencereferstotherelationshipsamongresponsibilitiesinamodule.Thegoalistoensurethatalloftheseresponsibilitiesworktogetherwithoutexcessiverelianceonothermodules.語義一致性指一個模塊中各職責(zé)間的關(guān)系。目的是保證模塊內(nèi)各職責(zé)一起工作而沒有對其它模塊的額外依賴。LocalizeModifications局部修改Anticipateexpectedchanges預(yù)見期望的修改Consideringthesetofenvisionedchangesprovidesawaytoevaluateaparticularassignmentofresponsibilities.Thetacticofanticipatingexpectedchangesdoesnotconcernitselfwiththecoherenceofamodule’sresponsibilitiesbutratherwithminimizingtheeffectsofthechanges.Inrealitythistacticisdifficulttousebyitselfsinceitisnotpossibletoanticipateallchanges.Forthatreason,itisusuallyusedinconjunctionwithsemanticcoherence.預(yù)先考慮將來的修改提供了一種評估職責(zé)分派的方法。這種方法并不關(guān)心模塊職責(zé)的一致性本身，而是要把修改帶來的影響降到最低?，F(xiàn)實(shí)中這種策略很難單獨(dú)使用，因?yàn)轭A(yù)期所有的修改是不可能的，因此，常常與語義一致性聯(lián)合使用LocalizeModifications局部修改Generalizethemodule.使模塊一般化Makingamodulemoregeneralallowsittocomputeabroaderrangeoffunctionsbasedoninput.Theinputcanbethoughtofasdefiningalanguageforthemodule.Themoregeneralamodule,themorelikelythatrequestedchangescanbemadebyadjustingtheinputlanguageratherthanbymodifyingthemodule使模塊更一般化允許計算更多的基于輸入的功能。該輸入可看著為模塊定義了一種語言。模塊越一般化，要求的修改就越能通過調(diào)整輸入語言來實(shí)現(xiàn)，而不用修改模塊。LocalizeModifications局部修改Limitpossibleoptions.限制可能的選項(xiàng)Modifications,especiallywithinaproductline,maybefarrangingandhenceaffectmanymodules.Restrictingthepossibleoptionswillreducetheeffectofthesemodifications.修改，特別是生產(chǎn)線上的修改，范圍非常廣泛，因此會影響許多模塊。限制可能的選項(xiàng)可以減少修改帶來的影響。PreventRippleEffects防止?jié)i漪效應(yīng)Arippleeffectfromamodificationisthenecessityofmakingchangestomodulesnotdirectlyaffectedbyit.修改的漣漪效應(yīng)指當(dāng)修改一個模塊時，必須修改另一個受到間接影響的模塊的必要性。IfmoduleAischangedtoaccomplishaparticularmodification,thenmoduleBischangedonlybecauseofthechangetomoduleA.Bhastobemodifiedbecauseitdepends,insomesense,onA.比如，由于模塊B因?yàn)槟撤N原因依賴于模塊A，當(dāng)模塊A被修改時，那么都有哪些依賴呢？PreventRippleEffects-typesofdependenciesSyntaxof語法data.ForBtocompile(orexecute)correctly,thetype(orformat)ofthedatathatisproducedbyAandconsumedbyBmustbeconsistentwiththetype(orformat)ofdataassumedbyB.B要使用A產(chǎn)生的數(shù)據(jù)，要求A提供的數(shù)據(jù)與B所要求的在類型（或格式）上一致Service.ForBtocompileandexecutecorrectly,thesignatureofservicesprovidedbyAandinvokedbyBmustbeconsistentwiththeassumptionsofB.B使用A提供的服務(wù)，服務(wù)的簽名必須一致。PreventRippleEffects-typesofdependenciesSemanticsof語義data.ForBtoexecutecorrectly,thesemanticsofthedataproducedbyAandconsumedbyBmustbeconsistentwiththeassumptionsofB.生產(chǎn)者提供的數(shù)據(jù)的語義必須與消費(fèi)者所需要的一致。service.ForBtoexecutecorrectly,thesemanticsoftheservicesproducedbyAandusedbyBmustbeconsistentwiththeassumptionsofB.服務(wù)的語義也要一致。PreventRippleEffects-typesofdependenciesSequenceof順序data.ForBtoexecutecorrectly,itmustreceivethedataproducedbyAinafixedsequence.Forexample,adatapacket‘sheadermustprecedeitsbodyinorderofreception(asopposedtoprotocolsthathavethesequencenumberbuiltintothedata).B的正確執(zhí)行依賴于從A收到的數(shù)據(jù)的順序control.ForBtoexecutecorrectly,Amusthaveexecutedpreviouslywithincertaintimingconstraints.Forexample,Amusthaveexecutednolongerthan5msbeforeBexecutes.B的正確執(zhí)行依賴于A在之前的某個時間約束下執(zhí)行完P(guān)reventRippleEffects-typesofdependenciesIdentityofaninterfaceofA.接口的標(biāo)識Amayhavemultipleinterfaces.ForBtocompileandexecutecorrectly,theidentity(nameorhandle)oftheinterfacemustbeconsistentwiththeassumptionsofB.A可能有多個接口，B的正確執(zhí)行要求接口的標(biāo)識（名稱或操作）與B所假定的一致LocationofA(runtime).運(yùn)行時的位置ForBtoexecutecorrectly,theruntimelocationofAmustbeconsistentwiththeassumptionsofB.Forexample,BmayassumethatAislocatedinadifferentprocessonthesameprocessor.B的正確執(zhí)行要求A運(yùn)行的位置與B的假定一致。如B可能要求B處于同一處理器的不同進(jìn)程中。PreventRippleEffects-typesofdependenciesQualityofservice/dataprovidedbyA.質(zhì)量ForBtoexecutecorrectly,somepropertyinvolvingthequalityofthedataorserviceprovidedbyAmustbeconsistentwithB‘sassumptions.B的正確執(zhí)行依賴于A提供的數(shù)據(jù)或服務(wù)的質(zhì)量Forexample,dataprovidedbyaparticularsensormusthaveacertainaccuracyinorderforthealgorithmsofBtoworkcorrectly.比如要求傳感器提供的數(shù)據(jù)達(dá)到一定的精度PreventRippleEffects-typesofdependenciesExistenceofA.B的正確執(zhí)行要求A必須存在ForBtoexecutecorrectly,Amustexist.Forexample,ifBisrequestingaservicefromanobjectA,andAdoesnotexistandcannotbedynamicallycreated,thenBwillnotexecutecorrectly.ResourcebehaviorofA.ForBtoexecutecorrectly,theresourcebehaviorofAmustbeconsistentwithB‘sassumptions.ThiscanbeeitherresourceusageofA(AusesthesamememoryasB)orresourceownership(BreservesaresourcethatAbelievesitowns).B對A的資源行為有要求，比如要求A使用某資源或擁有某資源PreventRippleEffects-tacticsNoticethatnoneofourtacticsnecessarilypreventtherippleofsemanticchanges.Hideinformation.隱藏信息Informationhidingisthedecompositionoftheresponsibilitiesforanentity(asystemorsomedecompositionofasystem)intosmallerpiecesandchoosingwhichinformationtomakeprivateandwhichtomakepublic.把實(shí)體的職責(zé)分解為若干較小的部分，并選擇哪些應(yīng)該是private，哪些public。即把易變的部分分拆出去。PreventRippleEffects-tacticsMaintainexistinginterfaces.保持現(xiàn)有接口IfBdependsonthenameandsignatureofaninterfaceofA,maintainingthisinterfaceanditssyntaxallowsBtoremainunchanged.若B依賴于A的接口名稱和基調(diào)，保持接口及其語義允許B保持不變。PatternsthatimplementthistacticincludeAddinginterfaceAddingadapterProvidingastubA.PreventRippleEffects-tacticsRestrictcommunicationpaths.限制通信路徑Restrictthemoduleswithwhichagivenmodulesharesdata.Thatis,reducethenumberofmodulesthatconsumedataproducedbythegivenmoduleandthenumberofmodulesthatproducedataconsumedbyit.限制共享數(shù)據(jù)的模塊。即限制為同一模塊提供數(shù)據(jù)的生產(chǎn)者的數(shù)量，或限制消費(fèi)者的數(shù)量。Thiswillreducetherippleeffectsincedataproduction/consumptionintroducesdependenciesthatcauseripples.PreventRippleEffects–tacticsUseanintermediary.使用中間物IfBhasanytypeofdependencyonAotherthansemantic,itispossibletoinsertanintermediarybetweenBandAthatmanagesactivitiesassociatedwiththedependency.Theintermediaryare:data(syntax).Repositories(bothblackboardandpassive)actasintermediariesbetweentheproducerandconsumerofdata.service(syntax).Thefacade,bridge,mediator,strategy,proxy,andfactorypatternsallprovideintermediariesthatconvertthesyntaxofaservicefromoneformintoanother.若B對A有任何非語義的依賴，在B和A之間插入一個中間物來管理與依賴相關(guān)的活動是可能的。中間物可以是：數(shù)據(jù)（語法）。倉儲是數(shù)據(jù)的生產(chǎn)者和消費(fèi)者之間的中間物。服務(wù)。把服務(wù)從一種形式轉(zhuǎn)換為另一種形式的語法。PreventRippleEffects-tacticsidentityofaninterfaceofA.Abrokerpatterncanbeusedtomaskchangesintheidentityofaninterface.IfBdependsontheidentityofaninterfaceofAandthatidentitychanges,byaddingthatidentitytothebrokerandhavingthebrokermaketheconnectiontothenewidentityofA,Bcanremainunchanged.locationofA(runtime).AnameserverenablesthelocationofAtobechangedwithoutaffectingB.Aisresponsibleforregisteringitscurrentlocationwiththenameserver,andBretrievesthatlocationfromthenameserver.中間物可以是：

A的接口身份。使用broker(中間人)來屏蔽接口的改變

A的位置（運(yùn)行時）。使用一個名字服務(wù)器使得A的位置的改變不會影響到B。PreventRippleEffects-tacticsresourcebehaviorofAorresourcecontrolledbyA.Aresourcemanagerisanintermediarythatisresponsibleforresourceallocation.Certainresourcemanagers(e.g.,thosebasedonRateMonotonicAnalysisinreal-timesystems)canguaranteethesatisfactionofallrequestswithincertainconstraints.A,ofcourse,mustgiveupcontroloftheresourcetotheresourcemanager.existenceofA.Thefactorypatternhastheabilitytocreateinstancesasneeded,andthusthedependenceofBontheexistenceofAissatisfiedbyactionsofthefactory.中間物可以是：

A的資源行為。使用資源管理器來負(fù)責(zé)資源分配。

A的存在。工廠模式有在需要時創(chuàng)建實(shí)例的能力，故B在A的存在上的依賴可通過工廠的動作來解決。DeferBindingTime延遲綁定時間Thetwotacticcategorieswehavediscussedaredesignedtominimizethenumberofmodulesthatrequirechangingtoimplementmodification.Ourmodificationscenariosincludetwoelementsthatarenotsatisfiedbyreducingthenumberofmodulestobechanged–timetodeployandallowingnondeveloperstomakechanges.Deferringbindingtimesupportsbothofthosescenariosatthecostofrequiringadditionalinfrastructuretosupportthelatebinding.前邊討論的兩類策略用來最小化在實(shí)現(xiàn)修改時要求改變的模塊數(shù)量。我們的修改場景中包括不通過減少改變的模塊數(shù)量來滿足的兩個元素－配置時間和允許非開發(fā)人員進(jìn)行改變。延遲綁定時間支持這兩個元素，但需要額外的基礎(chǔ)構(gòu)造來支持延遲綁定DeferBindingTime延遲綁定時間Manytacticsareintendedtohaveimpactatloadtimeorruntime,suchasthefollowingRuntimeregistrationsupportsplug-and-playoperationatthecostofadditionaloverheadtomanagetheregistration.Publish/subscriberegistration,forexample,canbeimplementedateitherruntimeorloadtime.Configurationfilesareintendedtosetparametersatstartup.Polymorphismallowslatebindingofmethodcalls.Componentreplacementallowsloadtimebinding.Adherencetodefinedprotocolsallowsruntimebindingofindependentprocesses.許多策略試圖影響裝載時間或運(yùn)行時間，例如：運(yùn)行時注冊配置文件試圖在啟動時設(shè)置參數(shù)。多態(tài)性允許推遲方法調(diào)用的綁定。組件替換允許裝載時綁定。SummaryofmodifiabilitytacticsPerformanceTacticsperformancetacticsistogeneratearesponsetoaneventarrivingatthesystemwithinsometimeconstraint.性能上的策略就是在某個時間約束內(nèi)對到達(dá)系統(tǒng)的事件產(chǎn)生響應(yīng)PerformanceTacticsTwobasiccontributorstotheresponsetimeResourceconsumption.ResourcesincludeCPU,datastores,networkcommunicationbandwidth,andmemory,butitcanalsoincludeentitiesdefinedbytheparticularsystemunderdesign.Blockedtime.AcomputationcanbeblockedfromusingaresourcebecauseofContentionforresourcesAvailabilityofresourcesDependencyonothercomputation對響應(yīng)時間有兩個基本的貢獻(xiàn)者：資源消費(fèi)。資源包括CPU、數(shù)據(jù)存儲、網(wǎng)絡(luò)通信帶寬和內(nèi)存，也可包括由特定的系統(tǒng)定義的實(shí)體。阻塞時間。一個計算可能被阻塞到要使用的資源上，因?yàn)闋帄Z資源、資源的可用性或者對其他計算的依賴。PerformanceTacticsthreetacticcategories:resourcedemand資源需求resourcemanagement資源管理resourcearbitration資源仲裁ResourceDemandOnetacticforreducinglatencyistoreducetheresourcesrequiredforprocessinganeventstream.Waystodothisincludethefollowing.Increasecomputationalefficiency.Onestepintheprocessingofaneventoramessageisapplyingsomealgorithm.Improvingthealgorithmsusedincriticalareaswilldecreaselatency.Reducecomputationaloverhead.Ifthereisnorequestforaresource,processingneedsarereduced.資源請求的來源是事件流。資源需求的兩個特征是資源流中的事件間的時間間隔（請求資源的頻率）和每次請求消耗多少資源。減少延遲的一個策略是減少處理事件流所需要的資源。具體做法包括：提高計算效率。即改善臨界區(qū)中使用的算法。減少計算的管理開銷。如果一個資源可以不要，那么處理就被簡化了。ResourceDemandAnothertacticforreducinglatencyistoreducethenumberofeventsprocessed.Thiscanbedoneinoneoftwofashions.Manageeventrate.Ifitispossibletoreducethesamplingfrequencyatwhichenvironmentalvariablesaremonitored,demandcanbereduced.Controlfrequencyofsampling.Ifthereisnocontroloverthearrivalofexternallygeneratedevents,queuedrequestscanbesampledatalowerfrequency,possiblyresultinginthelossofrequests.另一個策略是減少處理的事件數(shù)量。這也可以分兩個方面：管理事件速率。如果可能減少監(jiān)控的環(huán)境變量的抽樣頻率，則資源需求可減少?？刂瞥闃宇l率。如果對外部產(chǎn)生事件的到達(dá)沒有控制，隊列中的請求可能以較低的頻率采樣，可能導(dǎo)致請求丟失。ResourceDemandOthertacticsforreducingormanagingdemandinvolvecontrollingtheuseofresources.Boundexecutiontimes.Placealimitonhowmuchexecutiontimeisusedtorespondtoanevent.Sometimesthismakessenseandsometimesitdoesnot.Foriterative,data-dependentalgorithms,limitingthenumberofiterationsisamethodforboundingexecutiontimes.Boundqueuesizes.Thiscontrolsthemaximumnumberofqueuedarrivalsandconsequentlytheresourcesusedtoprocessthearrivals.減少或管理資源要求的另一策略是控制資源的使用：控制執(zhí)行時間。對用于響應(yīng)事件的執(zhí)行時間進(jìn)行限制。有時這樣做有意義有時沒有。對迭代的數(shù)據(jù)相關(guān)算法，限制迭代的次數(shù)是一種控制執(zhí)行時間的方法?？刂脐犃谐叽纭＜纯刂脐犃兄械竭_(dá)的請求的最大數(shù)量。因而控制了使用的資源。ResourceManagementIntroduceconcurrency.Ifrequestscanbeprocessedinparallel,theblockedtimecanbereduced.Maintainmultiplecopiesofeitherdataorcomputations.Clientsinaclient-serverpatternarereplicasofthecomputation.Increaseavailableresources.Fasterprocessors,additionalprocessors,additionalmemory,andfasternetworksallhavethepotentialforreducinglatency.即使資源請求沒法控制，對資源的管理也會影響響應(yīng)時間，一些控制策略包括：引入并發(fā)。如果請求可以并行處理，阻塞時間可以減少。維持?jǐn)?shù)據(jù)或計算的多個拷貝。如CS模式中的客戶是計算的多個復(fù)制品。復(fù)制的目的是減少競爭。增加可用的資源。如更快的處理器，更多的處理器，增加內(nèi)存，更快的網(wǎng)絡(luò)，都是減少延遲的潛在方法。ResourceArbitrationFirst-in/First-out.FIFOqueuestreatallrequestsforresourcesasequalsandsatisfytheminturn.Fixed-priorityscheduling.Fixed-priorityschedulingassignseachsourceofresourcerequestsaparticularpriorityandassignstheresourcesinthatpriorityorder.semanticimportancedeadlinemonotonicratemonotonic有資源競爭,就有資源計劃,如處理器、緩存、網(wǎng)絡(luò)等。架構(gòu)師的目標(biāo)就是要理解各資源的使用特性，選擇諧調(diào)的調(diào)度策略。一種安排策略概念上有兩部分：優(yōu)先權(quán)分配和調(diào)度。優(yōu)先權(quán)安排可以象先進(jìn)先出那么簡單，也可以與請求的最終期限或語義重要性綁在一起。安排的標(biāo)準(zhǔn)包括最優(yōu)資源使用率、請求的重要程度、最小化使用的資源共享數(shù)量、最低延遲、最大化吞吐量、防止餓死以保證公平等等。架構(gòu)死需要意識到可能沖突的標(biāo)準(zhǔn)，以及所選策略對滿足標(biāo)準(zhǔn)的影響。只有當(dāng)指派的資源可用，高優(yōu)先權(quán)的事件流才被調(diào)度。有時這依賴于資源搶占策略。如：隨時搶占、僅在特定的搶占點(diǎn)搶占、不能搶占正執(zhí)行的進(jìn)程資源等。幾種常用的調(diào)度策略如下：先進(jìn)先出。使用FIFO隊列，平等對待所有的資源請求，輪流處理。固定優(yōu)先權(quán)。為每個資源請求源賦予特定的優(yōu)先權(quán)，并安該順序分派資源。分派依據(jù)包括：語義重要性、最終期限單調(diào)、速率單調(diào)（即對周期性事件流，周期短優(yōu)先）ResourceArbitrationDynamicpriorityschedulingroundrobin.Roundrobinisaschedulingstrategythatorderstherequestsandthen,ateveryassignmentpossibility,assignstheresourcetothenextrequestinthatorder.earliestdeadlinefirst.Earliestdeadlinefirstassignsprioritiesbasedonthependingrequestswiththeearliestdeadline.Fixed-priorityscheduling.動態(tài)優(yōu)先權(quán)調(diào)度循環(huán)調(diào)度（輪式調(diào)度）。把資源排成環(huán)，每次調(diào)度后，環(huán)中的下一個優(yōu)先權(quán)最高。最早最終期限優(yōu)先。ResourceArbitrationStaticscheduling.Acyclicexecutivescheduleisaschedulingstrategywherethepreemptionpointsandthesequenceofassignmenttotheresourcearedeterminedoffline.靜態(tài)調(diào)度循環(huán)執(zhí)行調(diào)度是這樣一種調(diào)度策略，搶占點(diǎn)和資源分派順序離線決定。SummaryofperformancetacticsSecurityTacticsTacticsforachievingsecuritycanbedividedintothoseconcernedwithresistingattacksthoseconcernedwithdetectingattacksthoseconcernedwithrecoveringfromattacks.獲取安全性的策略可以分幾類：那些與抵抗攻擊相關(guān)的；檢測攻擊相關(guān)的；從攻擊中恢復(fù)；ResistingattacksAuthenticateusers.Authentication（鑒定）isensuringthatauserorremotecomputerisactuallywhoitpurportstobe.Passwords,one-timepasswords,digitalcertificates,andbiometricidentificationsprovideauthentication.Authorizeusers.Authorization（授權(quán)）isensuringthatanauthenticateduserhastherightstoaccessandmodifyeitherdataorservices.抵抗攻擊的方法：鑒別用戶。確保用戶或遠(yuǎn)程計算機(jī)確實(shí)是它所聲稱的那一個。相關(guān)技術(shù)包括口令、一次性口令、數(shù)字證書、生物測定等。授權(quán)用戶。給予授權(quán)用戶訪問或修改數(shù)據(jù)的權(quán)力。ResistingattacksMaintaindataconfidentiality（機(jī)密）.Datashouldbeprotectedfromunauthorizedaccess.Confidentialityisusuallyachievedbyapplyingsomeformofencryptiontodataandtocommunicationlinks.Encryptionprovidesextraprotectiontopersistentlymaintaineddatabeyondthatavailablefromauthorization.Communicationlinkstypicallydonothaveauthorizationcontrols.Encryptionistheonlyprotectionforpassingdataoverpubliclyaccessiblecommunicationlinks.維持?jǐn)?shù)據(jù)機(jī)密性。機(jī)密性通常通過某種形式的機(jī)密技術(shù)來達(dá)到。對于通信鏈路來說，一般沒有授權(quán)控制方法，加密是唯一的保護(hù)手段。ResistingattacksMaintainintegrity.Datashouldbedeliveredasintended.Itcanhaveredundantinformationencodedinit,suchaschecksumsorhashresults,whichcanbeencryptedeitheralongwithorindependentlyfromtheoriginaldata.Limitexposure.Attackstypicallydependonexploitingasingleweaknesstoattackalldataandservicesonahost.Thearchitectcandesigntheallocationofservicestohostssothatlimitedservicesareavailableoneachhost.維護(hù)完整性 數(shù)據(jù)應(yīng)按意圖交付。它可能包含冗余信息如檢測位或哈希碼，必須全部交付給接收方。限制曝露 典型攻擊是從主機(jī)的一個弱點(diǎn)入手進(jìn)而攻擊所有數(shù)據(jù)和服務(wù)。架構(gòu)師可設(shè)計服務(wù)對主機(jī)的分配以便限制每個主機(jī)可用的服務(wù)。ResistingattacksLimitaccess.Firewallsrestrictaccessbasedonmessagesourceordestinationport.限制訪問 使用防火墻基于消息來源或目標(biāo)端口限制訪問