Lecreplication.ppt
- 文档编号:1397624
- 上传时间:2022-10-22
- 格式:PPT
- 页数:18
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Lecreplication.ppt
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Spring2002,CS461,1,Replication,OutlineFailureModelsMirroringQuorums,Spring2002,CS461,2,WhyReplicate?
PerformancekeepcopyclosetoremoteuserscachingisaspecialcaseSurviveFailuresavailability:
provideserviceduringtemporaryfailurefaulttolerance:
provideservicedespitecatastrophicfailure,Spring2002,CS461,3,FaultModels,Crashedfaileddevicedoesntdoanything(i.e.,failssilently)Fail-StopfaileddevicetellsyouthatithasfailedByzantinefaileddevicecandoanythingadversaryplayingagameagainstanevilopponentopponentknowswhatyouredoingandtriestofoolyouusuallysomelimitonopponentsactions(e.g.atmostkfailures),Spring2002,CS461,4,ByzantineArmyProblem,3000BlueSoldiers,3000BlueSoldiers,4000RedSoldiers,Spring2002,CS461,5,Synchrony,AssumptionsconcerningboundednessofcomponentexecutionornetworktransmissionsSynchronousalwaysperformsfunctioninafinite&knowntimeboundAsynchronousnosuchboundFamousResult:
Agroupofprocessescannotagreeonavalueinanasynchronoussystemgivenasinglecrashfailure,Spring2002,CS461,6,NetworkPartitions,Canttellthedifferencebetweenacrashedprocessandaprocessthatsinaccessibleduetoanetworkfailure.NetworkPartition:
networkfailurethatcutsprocessesintotwoormoregroupsfullcommunicationwithineachgroupnocommunicationbetweengroupsdanger:
eachgroupthinkseveryoneelseisdead,Spring2002,CS461,7,Mirroring,Goal:
serviceuptoKfailuresApproach:
keepK+1copiesofeverythingClientsdooperationson“primary”copyPrimarymakessureothercopiesdooperationstooAdvantage:
simpleDisadvantages:
doeveryoperationKtimesuseKtimesmorestoragethannecessary,Spring2002,CS461,8,MirroringDetails,Optimization:
contactonereplicatoreadWhatifareplicafails?
getup-to-datedatafromprimaryafterrecoveringWhatifprimaryfails?
electanewprimary,Spring2002,CS461,9,ElectionProblem,Whenalgorithmterminates,allnon-failedprocessesagreeonwhichreplicaistheprimaryAlgorithmworksdespitearbitraryfailuresandrecoveriesduringtheelectionIftherearenomorefailuresandrecoveries,thealgorithmmusteventuallyterminate,Spring2002,CS461,10,BullyAlgorithm,Usefixed“peckingorder”amongprocessese.g.,usenetworkaddressesIdea:
choosethe“biggest”non-failedmachineasprimaryCorrectnessproofisdifficult,Spring2002,CS461,11,BullyAlgorithmDetails,Processstartsanelectionwheneveritrecoversorwheneverprimaryhasfailedhowknowprimaryhasfailed?
Tostartanelection,sendelectionmessagestoallmachinesbiggerthanyourselfifsomebodyrespondswithanACK,giveupifnobodyACKs,declareyourselftheprimaryOnreceivingelectionmessage,replywithACKandstartanelectionyourself(unlessinprogress),Spring2002,CS461,12,Quorums,Quorum:
asetofservermachinesDefinewhatconstitutesa“readquorum”anda“writequorum”TowriteacquirelocksonallmembersofsomewritequorumdowritesonalllockedserversreleaselocksToread:
similar,butusereadquorum,Spring2002,CS461,13,Quorums,Correctnessrequirementsanytwowritequorumsmustshareamemberanyreadquorumandanywritequorummustshareamember(readquorumsneednotoverlap)Lockingensuresthatatmostonewritehappeningatatimeneverhaveawriteandareadhappeningatthesametime,Spring2002,CS461,14,DefiningQuorums,ManyalternativesExamplewritequorummustcontainallreplicasreadquorummaycontainanyonereplicaConsequencewritesareslow,readsarefastcanwriteonlyifallreplicasareavailablecanreadifanyonereplicaisavailable,Spring2002,CS461,15,DefiningQuorums(cont),Example:
MajorityQuorumwritequorum:
anysetwithmorethanhalfthereplicasreadquorum:
anysetwithmorethanhalfthereplicasConsequencesmodestperformanceforreadandwritecanproceedaslongasmorethanhalfthereplicasareavailable,Spring2002,CS461,16,Quorums&VersionNumbers,Writeoperationwritesonlyasubsetoftheserverssomeserversareout-of-dateRemedyputversionnumberstamponeachitemineachreplicawhenacquiringlocks,getcurrentversionnumberfromeachreplicaquorumoverlaprulesensurethatonememberofyourquorumhasthelatestversion,Spring2002,CS461,17,VersionNumbers(cont),Whenreading,getthedatafromthelatestversionnumberinyourquorumWhenwriting,setversionnumberofallreplicasyouwroteequalto1+(maxversionnumberinyourquorumbeforehand)Guaranteescorrectnessevenifnorecoveryactionistakenwhenreplicarecoversfromacrash,Spring2002,CS461,18,QuorumsandPartitions,Onegrouphasawritequorum(andthususuallyareadquorum);thatgroupcandoanythingothergroupsarefrozenNogrouphasawritequorum,butsomegroupshaveareadquorumsomegroupscanreadnogroupscanwriteNogroupcontainsanyquorumeveryoneisfrozen,
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- 关 键 词:
- Lecreplication
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