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AlternativeConcurrencyControlMethodsR&G-Chapter17RoadmapSofar:Correctnesscriterion:serializabilityLock-basedCCtoenforceserializabilityStrict2PLDeadlocksLockinggranularitiesTreelockingprotocolsPhantomsToday:AlternativeCCmechanismsOptimisticCC(Kung-Robinson)Lockingisaconservativeapproachinwhichconflictsareprevented.
Disadvantages:Lockmanagementoverhead.
Deadlockdetection/resolution.
Lockcontentionforheavilyusedobjects.
Lockingis"pessimistic"becauseitassumesthatconflictswillhappen.
WhatifconflictsarerareWemightgetbetterperformancebynotlocking,andinsteadcheckingforconflictsatcommittime.
Kung-RobinsonModelXactshavethreephases:READ:Xactsreadfromthedatabase,butmakechangestoprivatecopiesofobjects.
VALIDATE:Checkforconflicts.
WRITE:Makelocalcopiesofchangespublic.
RVWValidationIdea:testconditionsthataresufficienttoensurethatnoconflictoccurred.
EachXactassignedanumericid.
Justuseatimestamp.
AssignedatendofREADphase.
ReadSet(Ti):SetofobjectsreadbyXactTi.
WriteSet(Ti):SetofobjectsmodifiedbyTi.
Test1ForalliandjsuchthatTiWTS(O):AllowTtoreadO.
ResetRTS(O)tomax(RTS(O),TS(T))ChangetoRTS(O)onreadsmustbewrittentodisk!
Thisandrestartsrepresentoverheads.
TimestampCC:WhenXactTwantstoWriteObjectOIfTS(T)WTS(O))untilwriterofOcommits.
SimilartowritersholdingXlocksuntilcommit,butstillnotquite2PL.
T1T2W(A)R(A)W(B)CommitUnrecoverableschedulesareallowedbyTimestampCC!
Recoverableschedule:xactscommitonlyafter(andif)allxactswhosechangestheyreadcommitAweakerconditionthanAvoidCascadingRollbackMultiversionTimestampCCIdea:Letwritersmakea"new"copywhilereadersuseanappropriate"old"copy:OO'O''MAINSEGMENT(CurrentversionsofDBobjects)VERSIONPOOL(Olderversionsthatmaybeusefulforsomeactivereaders.
)Readersarealwaysallowedtoproceed.
Butmaybeblockeduntilwritercommits.
MultiversionCC(Contd.
)Eachversionofanobjecthasitswriter'sTSasitsWTS,andtheTSoftheXactthatmostrecentlyreadthisversionasitsRTS.
Versionsarechainedbackward;wecandiscardversionsthatare"toooldtobeofinterest".
EachXactisclassifiedasReaderorWriter.
Writermaywritesomeobject;Readerneverwill.
XactdeclareswhetheritisaReaderwhenitbegins.
ReaderXactForeachobjecttoberead:FindsnewestversionwithWTSReaderXactsareneverrestarted.
However,mightblockuntilwriteroftheappropriateversioncommits.
ToldnewWTStimelineWriterXactToreadanobject,followsreaderprotocol.
Towriteanobject:FindsnewestversionVs.
t.
WTSIfRTS(V))Else,rejectwrite.
ToldnewWTSCVVRTS(V)SummaryOptimisticCCusingend-of-xact"validation"Goodif:Read-dominatedworkloadSystemhaslotsofextraresourcesMostrealsystemsuselockingSummary(Contd.
)Anotheralternative:TimestampCCDecidelogicalxactexecutionorderwhenxactsentersystemEnforcebycomparingxacttimestampswithobjecttimestampsVariant:MultiversionCCKeepout-of-dateversionsofobjects,so"old"readersdon'thavetorestart(theycanrun"inthepast")OracleusesaflavorofmultiversionCC