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Chapter16:ConcurrencyControlLock-BasedProtocolsMultipleGranularityDeadlockHandlingInsertandDeleteOperationsConcurrencyinIndexStructuresLock-BasedProtocolsAlockisamechanismtocontrolconcurrentaccesstoadataitemDataitemscanbelockedintwomodes:
1.exclusive(X)mode.Dataitemcanbebothreadaswellaswritten.X-lockisrequestedusinglock-Xinstruction.
2.shared(S)mode.Dataitemcanonlyberead.S-lockisrequestedusinglock-Sinstruction.Lockrequestsaremadetoconcurrency-controlmanager.Transactioncanproceedonlyafterrequestisgranted.Lock-BasedProtocols(Cont.)Lock-compatibilitymatrixAtransactionmaybegrantedalockonanitemiftherequestedlockiscompatiblewithlocksalreadyheldontheitembyothertransactionsAnynumberoftransactionscanholdsharedlocksonanitem,butifanytransactionholdsanexclusiveontheitemnoothertransactionmayholdanylockontheitem.Ifalockcannotbegranted,therequestingtransactionismadetowaittillallincompatiblelocksheldbyothertransactionshavebeenreleased.Thelockisthengranted.PitfallsofLock-BasedProtocolsConsiderthepartialschedule
NeitherT3norT4canmakeprogress—executinglock-S(B)causesT4towaitforT3toreleaseitslockonB,whileexecutinglock-X(A)causesT3
towaitforT4toreleaseitslockonA.Suchasituationiscalledadeadlock.TohandleadeadlockoneofT3orT4mustberolledback
anditslocksreleased.PitfallsofLock-BasedProtocols(Cont.)Thepotentialfordeadlockexistsinmostlockingprotocols.Deadlocksareanecessaryevil.Starvationisalsopossibleifconcurrencycontrolmanagerisbadlydesigned.Forexample:AtransactionmaybewaitingforanX-lockonanitem,whileasequenceofothertransactionsrequestandaregrantedanS-lockonthesameitem.Thesametransactionisrepeatedlyrolledbackduetodeadlocks.Concurrencycontrolmanagercanbedesignedtopreventstarvation.TheTwo-PhaseLockingProtocolTwo-PhaseLockingisaprotocolwhichensuresconflict-serializableschedules.Phase1:GrowingPhasetransactionmayobtainlockstransactionmaynotreleaselocksPhase2:ShrinkingPhasetransactionmayreleaselockstransactionmaynotobtainlocksTheprotocolassuresserializability.Itcanbeprovedthatthetransactionscanbeserializedintheorderoftheirlockpoints
(i.e.thepointwhereatransactionacquireditsfinallock).TheTwo-PhaseLockingProtocol(Cont.)Theremaybeconflictserializableschedulesthatcannotbeobtainediftwo-phaselockingisused.Toobtainconflictserializableschedulesthoughnon-two-phaselockingprotocol,weneed:EithertohaveadditionalinformationaboutthetransactionsOrtoimposesomestructureororderingonthesetofdateitemsinthedatabase.LockConversionsLockConversions:OnlyacquireXlockwhennecessary.ProvideamechanismforupgradingaSlocktoaXlock.Two-phaselockingwithlockconversions:–FirstPhase:canacquirealock-Sonitemcanacquirealock-Xonitemcanconvertalock-Stoalock-X(upgrade)–SecondPhase:canreleasealock-Scanreleasealock-Xcanconvertalock-Xtoalock-S
(downgrade)Thisprotocolassuresserializability.Butstillreliesontheprogrammertoinsertthevariouslockinginstructions.AutomaticAcquisitionofLocks
(CommercialDBMSUses)AtransactionTiissuesthestandardread/writeinstruction,withoutexplicitlockingcalls.Theoperationread(D)isprocessedas:ifTihasalockonD
thenread(D)elsebegin
ifnecessarywaituntilnoothertransactionhasalock-XonDgrantTialock-SonD;read(D)endOtherConcurrencyControlProtocolsGraph-BasedLockingProtocol–TreeprotocolMaintainpartialorderingofalldataitems.OnlyXlockGuaranteeconflictserializableTimestamp-BasedProtocolValidation-BasedProtocolMultipleGranularityAllowdataitemstobeofvarioussizesanddefineahierarchyofdatagranularities,wherethesmallgranularitiesarenestedwithinlargeronesCanberepresentedgraphicallyasatreeWhenatransactionlocksanodeinthetreeexplicitly,itimplicitlylocksallthenode'sdescendentsinthesamemode.Granularityoflocking(levelintreewherelockingisdone):finegranularity(lowerintree):highconcurrency,highlockingoverheadcoarsegranularity
(higherintree):lowlockingoverhead,lowconcurrencyExampleofGranularityHierarchy
Thehighestlevelintheexamplehierarchyistheentiredatabase.Thelevelsbelowareoftypearea,fileandrecordinthatorder.CompatibilityMatrixwith
IntentionLockModesThecompatibilitymatrixforalllockmodesis:ISIXSSIXXISIXSSIXXMultipleGranularityLockingSchemeTransactionTicanlockanodeQ,usingthefollowingrules:1.Thelockcompatibilitymatrixmustbeobserved.2.Therootofthetreemustbelockedfirst,andmaybelockedinanymode.3.AnodeQcanbelockedbyTiinSorISmodeonlyiftheparentofQiscurrentlylockedbyTiineitherIXorISmode.4.AnodeQcanbelockedbyTiinX,SIX,orIXmodeonlyiftheparentofQiscurrentlylockedbyTiineitherIXorSIXmode.5.Ticanlockanodeonlyifithasnotpreviouslyunlockedanynode(thatis,Ti
istwo-phase).6.Ti
canunlockanodeQonlyifnoneofthechildrenofQarecurrentlylockedbyTi.Observethatlocksareacquiredinroot-to-leaforder,
whereastheyarereleasedinleaf-to-rootorder.DeadlockHandlingConsiderthefollowingtwotransactions:
T1:write(X)T2:write(Y)write(Y)write(X)SchedulewithdeadlockT1T2lock-XonXwrite(X)lock-XonYwrite(X)waitforlock-XonXwaitforlock-XonYMoreDeadlockPreventionStrategiesFollowingschemesusetransactiontimestampsforthesakeofdeadlockpreventionalone.wait-diescheme—non-preemptiveoldertransactionmaywaitforyoungeronetoreleasedataitem.Youngertransactionsneverwaitforolderones;theyarerolledbackinstead.atransactionmaydieseveraltimesbeforeacquiringneededdataitemwound-waitscheme—preemptiveoldertransactionwounds(forcesrollback)ofyoungertransactioninsteadofwaitingforit.Youngertransactionsmaywaitforolderones.maybefewerrollbacksthanwait-diescheme.Wait-dievs.Wound-waitThreetransactions:A,B,Corderedbystartingtime.BheldalockondataitemY.ThenWait-dieWound-waitAacquireYWaitBrollback;AgrantedthelockonYCacquireYCrollbackWaitDeadlockprevention(Cont.)Bothinwait-dieandinwound-waitschemes,arolledbacktransactionsisrestartedwithitsoriginaltimestamp.Oldertransactionsthushaveprecedenceovernewerones,andstarvationishenceavoided.Timeout-BasedSchemes:atransactionwaitsforalockonlyforasp
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