CAP Twelve Years Later-How the Rules Have Changed

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1、COVERFEATURECAPTwelveYearsLater:Howthe“Rules”HaveChangedEricBrewer,UniversityofCalifornia,BerkeleyTheCAPtheoremassertsthatanynet-ancesmatter.CAPprohibitsonlyatinypartofthedesignworkedshared-datasystemcanhaveonlyspace:perfectavailabilityandconsistencyinthepresencetwoofthreedesirableproperti

2、es.How-ofpartitions,whicharerare.Althoughdesignersstillneedtochoosebetweenconsis-ever,byexplicitlyhandlingpartitions,tencyandavailabilitywhenpartitionsarepresent,theredesignerscanoptimizeconsistencyandisanincrediblerangeofflexibilityforhandlingpartitionsavailability,therebyachievingsometra

3、de-andrecoveringfromthem.ThemodernCAPgoalshouldoffofallthree.betomaximizecombinationsofconsistencyandavail-abilitythatmakesenseforthespecificapplication.Suchanapproachincorporatesplansforoperationduringanthedecadesinceitsintroduction,designersandpartitionandforrecoveryafterward,thushelping

4、de-researchershaveused(andsometimesabused)thesignersthinkaboutCAPbeyonditshistoricallyperceivedCAPtheoremasareasontoexploreawidevarietylimitations.Iofnoveldistributedsystems.TheNoSQLmovementalsohasapplieditasanargumentagainsttraditionalWHY“2OF3”ISMISLEADINGdatabases.Theeasiestwaytoundersta

5、ndCAPistothinkoftwoTheCAPtheoremstatesthatanynetworkedshared-datanodesonoppositesidesofapartition.Allowingatleastsystemcanhaveatmosttwoofthreedesirableproperties:onenodetoupdatestatewillcausethenodestobecomeinconsistent,thusforfeitingC.Likewise,ifthechoiceisto•consistency(C)equivalenttohav

6、ingasingleup-to-datepreserveconsistency,onesideofthepartitionmustactcopyofthedata;asifitisunavailable,thusforfeitingA.Onlywhennodes•highavailability(A)ofthatdata(forupdates);andcommunicateisitpossibletopreservebothconsistency•tolerancetonetworkpartitions(P).andavailability,therebyforfeitin

7、gP.Thegeneralbeliefisthatforwide-areasystems,designerscannotforfeitPThisexpressionofCAPserveditspurpose,whichwasandthereforehaveadifficultchoicebetweenCandA.toopenthemindsofdesignerstoawiderrangeofsystemsInsomesense,theNoSQLmovementisaboutcreatingandtradeoffs;