教程分布式计算

11Chapter2:SystemArchitectureFundamentalWhatWhatisaodelisintendedtoprovide,simplifiedbutdescriptionofarelevantaspectdistributedsystem2ArchitectureArchitectureArchitecturedefinethewayinwhichtheyaremappedontotheunderlyingnetworkofcomputersdeterminethedistributionofdataandcomputationaltasksamongstthephysicalnodesofthesystemevaluatingtheperformance,reliability,scalabilityandotherpropertiesofdistributedsystems.Client-serverPeerprocessVariationsoftheclient-server FundamentalFundamentalAreconcernedwithamoreformaldescriptionofthepropertiesthatarecommoninallofthearchitecturalTheinteractionmodel:dealwithperformanceandwiththedifficultyofsettingtimelimitsinadistributedsystemThefailuremodel:giveaprecisespecificationofthefaultsthatcanbeexhibitedbyprocessesandcommunicationchannelsThesecuritymodel:discussthepossiblethreatstoprocessesandcommunicationchannels455DifficutiesforandthreatstoWidelyvaryingmodesofWiderangeofsystemInternalExternalChapterChapter3:SystemArchitectureFundamental67the7theindividual–Achievedbyclassifyprocessesasserver,clientandpeerprocessesThenTheplacementoftheTheinterrelationshipsbetweenthesthefunctionsSimplifiesBuildarchitecturaldistributed8OperatingComputerComputerandnetwork99-----Arethelowest-levelhardwaresoftwareItspurposeistomaskheterogeneityandprovideaconvenientprogrammingmodele.g.OMG’sCORBA,JavaRMI,Support Remotemethodinvocation:SunGroupcommunication:NotificationofThereplicationofsharedTransmissionofmultimediaLimitationofsomesystemsrequiresupportattheapplicationlevel.E.g.transferoflargeelectronic -to-endargument’ somecommunication-relatedfunctionscanbe yandreliablyimplementedonlywiththeknowledgeandhelpoftheapplicationstandingatpointsofthecommunicationE.g.TCP,DNSandtheSystemSystemThedivisionofresponsibilitiesbetweensystemcomponents(applications,serverandotherprocesses)andtheplacementofthecomponentsoncomputersintheArch.Arch.1:BehistoricallythemostimportantandremainthemostwidelyemployedServersmayinturnbeclientsofotherArch.Arch.2:ServicesprovidedbymultiplePartitionserviceobjectsondifferent–e.g.Web,–e.g.SunNIS,Arch.Arch.3:ProxyserversandastoreofrecentlyuseddataobjectsthatiscloserthantheobjectsthemselvesE.g.,w gecacheatwebbrowserorwebproxyproxyserver5Arch.Arch.4:PeertoAllprocessesplaysimilarInteractingcooperativelytoperformadistributedMaintainconsistencyorsynchronizeatapplicationExample:apeertopeer...Ihave Whereisfile IhavefileIhavefileWhereisfileA?IhaveX!WhereisfileADBCADB CConsistentConsistentNodeidentifierhash=hash(IPKeyidentifierhash=InChordhashfunctionisSecureHashSHA-1Akeyisstoredatitssuccessor:nodewithnexthigherIDChord：保存键值的索Key NodeChordChord“Whereiskey“N90has Key(n+2k-1)mod2m,node>=n.finger[k].startthenextnodeontheidentifiercircle;finger[1]thepreviousnodeontheidentifierkisthefingertableFingerFingertablesandkeylocationswithnodes0,1,3andkeys1,2and6FingertableforNodeLookupLookupFindingsuccessorofidentifierTofindthesuccessorofanidChordreturnsthesuccessoroftheclosestprecedingfingertothisLookupLookupThefingerpointersatrepeatedlydoublingdistancesaroundthecirclecauseeachiterationoftheloopinfind_predecessortohalvethedistancetothe InanNnodeNetworkthenumberofmessagesisofO(Log路路由id 文件id的另一个file－保存在本地的id搜索如果文件 在本地，停止搜索，上传文145网络趋向于一个小世界－smallworld,类似六度隔（SixDegreesofSeparation）理因此，大部分查询只需经过少量…4459234835DistributedDistributedHash超立方体，跳表，蝶形网络...CFS,OceanStore,PAST,(文件共享put(key,getdeIPlookup(key)VariationsVariationsontheclient-serverReasonsofTheuseofmobilecodeandmobileUsersneedforlow-costcomputerswithlimitedhardwareresourcesTherequirementtoaddandremovemobiledevicesinaconvenientmannerArch.Arch.1.1:MobileForgoodinteractiveresponse,e.g.a)clientrequestresultsinthedownloadingofappletAppletb)clientinteractswiththe Arch.Arch.1.2:MobileArunningprogramthattravelsfromonecomputertoanotherinanetworkCarryingoutataskonsomeone’sbehalf,e.g.worm[XeroxPARC]Arch.1.3:NetworkDownloadoperatingsystemandanyapplicationsoftwarefromaremotefileserverAlltheapplicationdataandcodeisstoredbyafileserverUsersmayArch.Arch.1.4:ThinAGUIonacomputerthatislocaltotheExecuteapplicationprogramsonaremoteDrawback:highImplementation:X-11,VNC[AT&TComputeNetworkcomputerorArch.Arch.1.5:SpontaneousIntegratemobiledevicesandotherdevicesintoagivennetworkKeyEasyconnectiontoalocalEasyintegrationwithlocalKeydesignConvenientconnectionandLimitedmobiledevicemovearoundcontinuously,SecurityandDiscoverySpontaneousSpontaneousnetworkingina Interface--AsetoffunctionsavailableorInobject-oriented--ManyobjectscanbeencapsulatedinDistributionof--astaticclient-serverarchitectureorthemoredynamicobject-orientedmodelInterfacesandDesignDesignrequirementsfordistributedResourcesharingistakenforgranted,buteffectivedatasharingonalargescaleremainsasubstantialchallenge.PerformanceQualityofUseofcachingandDependabilityPerformancePerformanceResponsiveness.E.g.theperformanceofweb-browsingclientstheloadandperformanceoftheserveranddelayintheclientandserveroperatingsystem’scommunicationandmiddlewareservicesaswellascodeoftheservicetherateatwhichcomputationalworkisItisaffectedbyprocessingspeedsandbydatatransferratesBalancingcomputational E.g.applets,severalcomputersforaQualityQualityofReliability,security,performanceandThefailuremodel,thesecuritymodelandtheinteractionmodelQoSiscommandeeredtorefertotheabilitytomeettime-criticaldata(RSVP)QosappliestoOSsaswellasPerformanceissuesarebarstodeploydistributedReplicationandcaching,withavarietyofdifferentcacheconsistencyprotocolsE.g.Web-UseofcachingandDesignDesignrequirementsfordistributedDependabilityFaultredundancy,e.g.dataandprocessesbereplicated,messagesberetransmittede.g.locatesensitivedataincomputersthatcanbesecuredeffectivelyagainstArchitectureFundamentalChapter3:SystemAAsystemmodelshouldWhatarethemainentitiesintheHowdotheyWhatarethecharacteristicsthataffecttheirindividualandcollectivebehavior?MakeexplicitalltheassumptionsaboutthesystemweareMakegeneralizationsconcerningwhatispossibleorimpossiblebylogicalysisandmathematicalPurposeofafundamentalFundamentalFundamentalmodelsintendtoInteractionTheprocessesinteractbypassingmessages,resultingincommunicationanddelayareoftenofconsiderableIt’sdifficulttomaintainthesametimenotionindistributedsystemAccuracywithwhichoftheindependentprocessescanbecoordinatedislimitedbythetwofactsmentionedaboveFundamentalmodelsintendtoFailureafaultoccursincomputersorFailuremodelcanprovidetheabasisfor ysisofthepotentialeffectandforthedesignofsystemtobeabletotoleratefaultsofeachtypewhilecontinuingtoruncorrectlyFundamentalFundamentalmodelsintendtoSecurityModularnatureofdistributedandtheiropennessexposesthemtoattackbybothexternalandinternalagent.SecuritymodelprovidethebasisforysisofthreatstosystemandforthedesignofsystemthatareabletoresistInteractionExamplesofinteractionindistributedDNS,multipleserverprocessescooperatewithoneanotherP2Pvoiceconferencewithstrictreal-timeImplementationImplementationofainteractionadefinitionofthestepstobetakenbyeachoftheprocessesofwhichthesystemiscomposed,includingthetransmissionofmessagesbetweenthemTheproceedingrateandtransmissiontimingcannotbepredicted.difficulttodescribeallthestates,becauseoffailuresofprocessesandmessageItisimpossibletomaintainaglobalnotionofTwofactorsaffectinginteractingCommunicationperformanceisalwaysalimitedcharacteristicPerformancePerformanceofcommunicationthetimetakenforthe stringofbitstransmittedthroughanetworktoreachitsdestinationaccessingOScommunicationtotalamountofinformationthatcanbetransmittedovercomputernetworkinagiventimevariationinthetimetakentodeliveraseriesofE.g.consecutivesamplesofaudiodataareplayeddifferingtimefromaperfectreferenceTiming e.g.,GPS,LogicalomputerclockClockdrift therelativeamountComputerclocksandtimingTwoTwovariantsoftheinteractionSynchronousdistributedThetimetoexecuteeachstepofaprocesshasknowlowerandupperEaessagetransmittedoverachannelisreceivedwithinaknownboundedtimeEachprocesshasalocalclockwhosedriftratefromrealtimehasaknownTwoTwovariantsoftheinteractionAsynchronousdistributedsystem–noboundsonProcessexecutione.g.eachstepmaytakeanarbitrarilylongMessagetransmissione.g.amessagemaybereceivedafteranarbitrarilylongtimeClockdriftthedriftrateofaclockisExamplesExamplesofSyn.DSandAsyn.Asynchronous–VoiceConferenceEventEventX1m1 4mY22ZreceiveA receivereceivereceiveExampleofdisorderofAgroupincludingX,Y,ZandXsend“Meeting”toall;YandZreply“Re:Meeting”toAtA,themessagesreceivedareZ.“Re:X.“Meeting”,Y.“Re:FailureFailureDefinethewaysinwhichfailuremayoccurinordertoprovideanunderstandingoftheeffectsoffailuresTaxonomy[HadzilacosandToueg,OmissionArbitraryTime1.1.OmissionAprocessorcommunicationchannelfailstoperformactionsthatitissupposedtodoProcessomissionfailure:Fail-stop:Crashthatcanbedetectedbyotherprocessescertainly,e.g.,bytimeoutsinsynchronousDSCommunicationomissionfailures:drop–Sendomission,receiveomission,channelBenign 1.1.OmissionSendpqOutgoingmessageingmessageTheworstpossiblefailureArbitrarilyomitintendedprocessingstepsstepsortakeunintendedprocessingsteps.–E.g.returnawrongvalueinresponsetoananArbitraryfailuresinprocessishardtobeArbitraryfailuresincommunicationchannelexistbutrare.–E.g.checksum,sequence)2.ArbitraryClassoffailureAffectsFail-stopProcessPcessesmaydetectthisCrashProcessProcesshaltsandremainshalted.OthermaynotbeabletodetectthisOmissionChannelAmessageinsertedinanoutgoingmessagebufferarrivesattheotherend’s ingmessagebuffer. ProcessAprocesscompletesasend,butthemessageisnot putinitsoutgoingmessagebuffer. Amessageisputinaprocess’s ingmessage buffer,butthatprocessdoesnotreceiveit. ProcessorProcess/channelexhibitsarbitrarybehaviour:itmay )channel send/transmitarbitrarymessagesatarbitrarytimes,commitomissions;aprocessmaystoportakeanincorrectstep.3.TimingApplicableinsyn.distributedsystem,butnotinasyn.distributedsystemClassof Process’slocalclockexceedstheonitsrateofdriftfromrealPerformance ProcessexceedstheboundsonthebetweentwoPerformance thanthestatedMaskingMaskinge.g.replicatede.g.Checksum:arbitraryfailure->omissionReliabilityofone-to-oneanymessageintheoutgoingmessagebufferiseventuallydeliveredtothe ingmessagethemessagereceivedisidenticaltoonesent,nomessagesaredeliveredtwice,againstretransmitprotocolsandspuriousmessagesSecuritySecurityThesecurityofadistributedTheThecommunicationTheProtectingtheAccessrights:whoisallowedtoperformtheoperationsofanobjectPrincipal:theauthoritywhohassomerightso