分布式系统原理与范例_7_ConsistencyReplication.pdf

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1、DistributedSystemsConsistency&ReplicationOutlineIntroduction(what‟sitallabout)Data-centricconsistencyClient-centricconsistencyReplicamanagementConsistencyprotocolsReplicationReplication:CreatingandusingmultiplecopiesofdataorservicesWhyreplicate?Improvereliability•Datasurviva

2、l•Availability•Increaseconfidence:e.g.dealwithbyzantinefailuresImproveperformance•Scaling•ReduceaccesstimesDatavs.ComputationReplication:Itcouldbedatareplicationifthesamedataisstoredonmultiplestoragedevices–Eg,Websitemirror,browsercache,DNSorcomputationreplicationifthesamecomput

3、ingtaskisexecutedmanytimes.Whataretheissues?UpdatingreplicasConsistency(howtodealwithupdateddata)•(luckily)applicationsdonotalwaysrequirestrictconsistencyHowareupdatesdistributed?ReplicamanagementHowmanyreplicas?Wheretoplacethem?Whentogetridofthem?Redirection/RoutingWhichre

4、plicashouldclientsuse?ScalabilityCONFILCTmanagementoverheadsExample:CostsAsascalingtechnique,maynotalwaysbeapplicable.AccessreplicaNUpdatereplicaMtimespersecondtimespersecondPAsascalingtechnique,maynotalwaysbeapplicable.WhatifN<

5、asconsistent,wegenerallyneedtoensurethatallconflictingoperationsaredoneinthethesameordereverywhereConflictingoperations:Fromtheworldoftransactions:Read–writeconflict:areadoperationandawriteoperationactconcurrentlyWrite–writeconflict:twoconcurrentwriteoperationsProblem:Guaranteei

6、ngglobalorderingonconflictingoperationsmaybeacostlyoperation,downgradingscalabilitySolution:weakenconsistencyrequirementssothathopefullyglobalsynchronizationcanbeavoidedOutlineIntroduction(what‟sitallabout)Data-centricconsistencyClient-centricconsistencyReplicamanagementConsis

7、tencyprotocolsClient‘sviewofdata-storeIdeally:blackbox--complete„transparency‟overhowdataisstoredandmanagedManagement’systemviewondatastoreManagementsystem:ControlstheallocatedresourcesandaimstoprovidetransparencyIntheFollowingConsistencymodel:contractbetweenthedatastoreandthecli

8、entsthedatastoresp