浙江大学数据库系统概念PPT第十六章

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1、Chapter16:ConcurrencyControlLock-BasedProtocolsMultipleGranularityDeadlockHandlingInsertandDeleteOperationsConcurrencyinIndexStructuresLock-BasedProtocolsAlockisamechanismtocontrolconcurrentaccesstoadataitemDataitemscanbelockedintwomodes:1.exclusive(X)mode.Dataitemcanbeb

2、othreadaswellaswritten.X-lockisrequestedusinglock-Xinstruction.2.shared(S)mode.Dataitemcanonlyberead.S-lockisrequestedusinglock-Sinstruction.Lockrequestsaremadetoconcurrency-controlmanager.Transactioncanproceedonlyafterrequestisgranted.Lock-BasedProtocols(Cont.)Lock-comp

3、atibilitymatrixAtransactionmaybegrantedalockonanitemiftherequestedlockiscompatiblewithlocksalreadyheldontheitembyothertransactionsAnynumberoftransactionscanholdsharedlocksonanitem,butifanytransactionholdsanexclusiveontheitemnoothertransactionmayholdanylockontheitem.Ifalo

4、ckcannotbegranted,therequestingtransactionismadetowaittillallincompatiblelocksheldbyothertransactionshavebeenreleased.Thelockisthengranted.Lock-BasedProtocols(Cont.)Exampleofatransactionperforminglocking:T2:lock-S(A);read(A);unlock(A);lock-S(B);read(B);unlock(B);display(

5、A+B)Lockingasaboveisnotsufficienttoguaranteeserializability—ifAandBgetupdatedin-betweenthereadofAandB,thedisplayedsumwouldbewrong.Alockingprotocolisasetofrulesfollowedbyalltransactionswhilerequestingandreleasinglocks.Lockingprotocolsrestrictthesetofpossibleschedules.Basi

6、cally,howtogenerateacorrect(serializable)schedule?PitfallsofLock-BasedProtocolsConsiderthepartialscheduleNeitherT3norT4canmakeprogress—executinglock-S(B)causesT4towaitforT3toreleaseitslockonB,whileexecutinglock-X(A)causesT3towaitforT4toreleaseitslockonA.Suchasituationisc

7、alledadeadlock.TohandleadeadlockoneofT3orT4mustberolledbackanditslocksreleased.PitfallsofLock-BasedProtocols(Cont.)Thepotentialfordeadlockexistsinmostlockingprotocols.Deadlocksareanecessaryevil.Starvationisalsopossibleifconcurrencycontrolmanagerisbadlydesigned.Forexampl

8、e:AtransactionmaybewaitingforanX-lockonanitem,whileasequenceofothertransactionsrequestandaregrantedanS-