浙江大學數(shù)據(jù)庫系統(tǒng)概念PPT第十六章

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