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1、RSSSubscribe:RSSfeedHighlyScalableBlogArticlesonBigData,HPC,andHighlyScalableSoftwareEngineeringDistributedAlgorithmsinNoSQLDatabasesPostedonSeptember18,20129ScalabilityisoneofthemaindriversoftheNoSQLmovement.Assuch,itencompassesdistributedsystemcoordinati
2、on,failover,resourcemanagementandmanyothercapabilities.Itsoundslikeabigumbrella,anditis.AlthoughitcanhardlybesaidthatNoSQLmovementbroughtfundamentallynewtechniquesintodistributeddataprocessing,ittriggeredanavalancheofpracticalstudiesandreal-lifetrialsofdif
3、ferentcombinationsofprotocolsandalgorithms.Thesedevelopmentsgraduallyhighlightasystemofrelevantdatabasebuildingblockswithprovenpracticalefficiency.InthisarticleI’mtryingtoprovidemoreorlesssystematicdescriptionoftechniquesrelatedtodistributedoperationsinNoS
4、QLdatabases.Intherestofthisarticlewestudyanumberofdistributedactivitieslikereplicationoffailuredetectionthatcouldhappeninadatabase.Theseactivities,highlightedinboldbelow,aregroupedintothreemajorsections:DataConsistency.Historically,NoSQLpaidalotofattention
5、totradeoffsbetweenconsistency,fault-toleranceandperformancetoservegeographicallydistributedsystems,low-latencyorhighlyavailableapplications.Fundamentally,thesetradeoffsspinarounddataconsistency,sothissectionisdevoteddatareplicationanddatarepair.DataPlaceme
6、nt.Adatabaseshouldaccommodateitselftodifferentdatadistributions,clustertopologiesandhardwareconfigurations.Inthissectionwediscusshowtodistributeorrebalancedatainsuchawaythatfailuresarehandledrapidly,persistenceguaranteesaremaintained,queriesareefficient,an
7、dsystemresourcelikeRAMordiskspaceareusedevenlythroughoutthecluster.SystemCoordination.Coordinationtechniqueslikeleaderelectionareusedinmanydatabasestoimplementsfault-toleranceandstrongdataconsistency.However,evendecentralizeddatabasestypicallytracktheirglo
8、balstate,detectfailuresandtopologychanges.Thissectiondescribesseveralimportanttechniquesthatareusedtokeepthesysteminacoherentstate.DataConsistencyItiswellknownandfairlyobviousthatingeographicallydistributedsy