分布式系统设计.07

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1、DistributedSystemsPrinciplesandParadigmsMaartenvanSteenVUAmsterdam,Dept.ComputerScienceRoomR4.20,steen@cs.vu.nlChapter07:Consistency&ReplicationVersion:November19,2009ContentsChapter01:Introduction02:Architectures03:Processes04:Communication05:Naming06:Synchronization07:Consistency

2、&Replication08:FaultTolerance09:Security10:DistributedObject-BasedSystems11:DistributedFileSystems12:DistributedWeb-BasedSystems13:DistributedCoordination-BasedSystems2/42Consistency&ReplicationConsistency&replicationIntroduction(what’sitallabout)Data-centricconsistencyClient-centr

3、icconsistencyReplicamanagementConsistencyprotocols3/42Consistency&Replication7.1IntroductionPerformanceandscalabilityMainissueTokeepreplicasconsistent,wegenerallyneedtoensurethatallconflictingoperationsaredoneinthethesameordereverywhereConflictingoperationsFromtheworldoftransactions:

4、Read–writeconflict:areadoperationandawriteoperationactconcurrentlyWrite–writeconflict:twoconcurrentwriteoperationsIssueGuaranteeingglobalorderingonconflictingoperationsmaybeacostlyoperation,downgradingscalabilitySolution:weakenconsistencyrequirementssothathopefullyglobalsynchronizatio

5、ncanbeavoided4/42Consistency&Replication7.2Data-CentricConsistencyModelsData-centricconsistencymodelsConsistencymodelAcontractbetweena(distributed)datastoreandprocesses,inwhichthedatastorespecifiespreciselywhattheresultsofreadandwriteoperationsareinthepresenceofconcurrency.Essential

6、Adatastoreisadistributedcollectionofstorages:ProcessProcessProcessLocalcopyDistributeddatastore5/42Consistency&Replication7.2Data-CentricConsistencyModelsContinuousConsistencyObservationWecanactuallytalkaaboutadegreeofconsistency:replicasmaydifferintheirnumericalvaluereplicasmaydif

7、ferintheirrelativestalenesstheremaybedifferenceswithrespectto(numberandorder)ofperformedupdateoperationsConitConsistencyunit)specifiesthedataunitoverwhichconsistencyistobemeasured.6/42Consistency&Replication7.2Data-CentricConsistencyModelsExample:ConitReplicaAReplicaBConitConitx=6;y

8、=3x=2;y=5OperationResultOp