課程資源-復(fù)試數(shù)據(jù)庫chtransaction

Chapter10

UpdateTransaction2022/12/23UpdateTransaction2ch10UpdateTransactionDef.10.1TransactionAtransactionisameanstopackagetogetheranumberofdatabaseoperationsperformedbyaprocess,sothedatabasesystemcanprovideseveralguarantees,calledtheACIDproperties.BEGINTRANSACTIONop1;op2;opN;ENDTRANSACTION2022/12/23UpdateTransaction3ch10UpdateTransactionBeginatransactionfirstdatabaseoperationEndatransactionbyexecutingBEGINTRANSACTIONop1;op2;opN;ENDTRANSACTIONexecsqlcommitwork;orexecsqlrollbackwork;2022/12/23UpdateTransaction4TransactionsManyenterprisesusedatabasestostoreinformationabouttheirstatee.g.,BalancesofalldepositorsatabankWhenaneventoccursintherealworldthatchangesthestateoftheenterprise,aprogramisexecutedtochangethedatabasestateinacorrespondingwaye.g.,BankbalancemustbeupdatedwhendepositismadeSuchaprogramiscalledatransaction2022/12/23UpdateTransaction5WhatDoesaTransactionDo?ReturninformationfromthedatabaseRequestBalancetransaction:Readcustomer’sbalanceindatabaseandoutputitUpdatethedatabasetoreflecttheoccurrenceofarealworldeventDeposittransaction:Updatecustomer’sbalanceindatabaseCausetheoccurrenceofarealworldeventWithdrawtransaction:Dispensecash(andupdatecustomer’sbalanceindatabase)2022/12/23UpdateTransaction6TransactionsTheexecution

ofeachtransactionmustmaintaintherelationshipbetweenthedatabasestateandtheenterprisestate2022/12/23UpdateTransaction7Example:bankingapplicationswithfilesystemproblemsCreatinganinconsistentresultErrorsofconcurrentexecutionUncertaintyastowhenchangesepermanent2022/12/23UpdateTransaction8Problems:CreatinganinconsistentresultOurapplicationistransferringmoneyfromoneaccounttoanother(differentpages).Oneaccountbalancegetsouttodisk(runoutofbufferspace)andthenthecomputercrashes.2022/12/23UpdateTransaction9problemsErrorsofconcurrentexecutionTeller1transfersmoneyfromAcctAtoAcctBofthesamecustomer,whileTeller2isperformingacreditcheckbyaddingbalancesofAandB.Teller2canseeAaftertransfersubtracted,Bbeforetransferadded.2022/12/23UpdateTransaction10problemsUncertaintyastowhenchangesepermanentAttheveryleast,wewanttoknowwhenitissafetohandoutmoney:don'twanttoforgetwediditifsystemcrashes,thenonlydataondiskissafe.2022/12/23UpdateTransaction11TransactionsThereforeadditionalrequirementsareplacedontheexecutionoftransactionsbeyondthoseplacedonordinaryprograms:AtomicityConsistencyIsolationDurabilityACIDproperties2022/12/23UpdateTransaction12Atomicity

（原子性）Thesetofrecordupdatesthatarepartofatransactionareindivisible(eithertheyallhappenornonehappen).Thisistrueeveninpresenceofacrash.Consistency

（一致性）Ifalltheindividualprocessesfollowcertainrules(moneyisneithercreatednordestroyed)andusetransactionsright,thentheruleswon'tbebrokenbyanysetoftransactionsactingtogether.2022/12/23UpdateTransaction13Isolation

（隔離性）MeansthatoperationsofdifferenttransactionsseemnottobeinterleavedintimeasifALLoperationsofoneTxbeforeorafteralloperationsofanyotherTy.Durability

（持久性）WhenthesystemreturnstothelogicafteraCommitWorkstatement,itguaranteesthatallTxUpdatesareondisk.NowATMmachinecanhandoutmoney.ACIDguarantees2022/12/23UpdateTransaction1410.1TransactionalHistoriesActionsinthedatabaseReadsandWritesofdataitemsRi(A):transactionwithidentificationnumberi(Ti)readsdataitemA.Wj(B):transactionTjwritesB.AUpdateStatementwillresultinalotofoperations:Rj(B1)Wj(B1)Rj(B2)Wj(B2)...Rj(Bn)Wj(Bn)2022/12/23UpdateTransaction15[10.1.2]

historyorschedule（調(diào)度）Figure10.1ThejoboftheSchedulerlookatthehistoryofoperationsasitcomesinandprovidetheIsolationguarantee,bysometimesdelayingsomeoperations,andoccasionallyinsistingthatsometransactionsbeaborted.R2(A,50)W2(A,20)R1(A,20)R1(B,50)R2(B,50)W2(B,80)C1C22022/12/23UpdateTransaction1610.1TransactionalHistoriesserialschedule（串行調(diào)度）alloperationsofaTxareperformedinsequencewithnointerleavingwithothertransactions.Schedulerassuresthatthesequenceofoperationsinhistoryisequivalentineffecttosomeserialschedule.2022/12/23UpdateTransaction17Example10.1.1ThetwoelementsAandBin(10.1.2)areaccountrecordswitheachhavingbalance50tobeginwith.T1isaddingupbalancesoftwoaccountsT2istransferring30unitsfromAtoBR2(A,50)W2(A,20)R1(A,20)R1(B,50)R2(B,50)W2(B,80)C1C2resultoftransaction(schedule10.1.2)T1:A+B=70T2:A=20,B=80(failsthecreditcheck)2022/12/23UpdateTransaction18Example10.1.1(cont.)2022/12/23UpdateTransaction1910.1TransactionalHistoriesExample10.1.1(cont.)Butthiscouldneverhavehappenedinaserialschedule,wherealloperationofT1occurredbeforeorafteralloperationsofT2.(following)R1(A,50)R1(B,50)C1

R2(A,50)W2(A,20)

R2(B,50)W2(B,80)C2R2(A,50)W2(A,20)

R2(B,50)W2(B,80)C2

R1(A,20)R1(B,80)C1

or2022/12/23UpdateTransaction20ACID(1)AtomicThesetofupdatescontainedinatransactionmustsucceedorfailasaunit.ConsistentCompletetransactionaltransformationsondataelementsbringthedatabasefromoneconsistentstatetoanother.2022/12/23UpdateTransaction21ACID(2)IsolatedEventhoughtransactionsexecuteconcurrently,itappearstoeachsuccessfultransactionthatithasexecutedinaserialschedulewiththeothers.DurableOnceatransactioncommits,thechangesithasmadetothedatawillsurviveanymachineorsystemfailures.2022/12/23UpdateTransaction2210.2InterleavedRead/WriteOperationsIfaserialhistoryisalwaysconsistent,whydon'twejustenforceserialhistories?Figure10.3TxhasrelativelysmallCPUburstsandthenI/OduringwhichCPUhasnothingtodo.Whatdowewanttodo?Figure10.4:LetanotherTyrunduringslackCPUtime.2022/12/23UpdateTransaction2310.2InterleavedRead/WriteOperationsFigure10.5~6Ifwehavemanydisksinuse,wecankeeptheCPU100%occupied.WhenonethreaddoesanI/O,wanttofindanotherthreadwithcompletedI/Oreadytorunagain.2022/12/23UpdateTransaction2410.3SerializabilityandthePrecedenceGraphSerializabilitySchedule（可串行化調(diào)度）TheseriesofoperationsisEQUIVALENTtoaSerialscheduleScheduler（調(diào)度器）findasetofrulesfortheSchedulertoallowoperationsbyinterleavedtransactionsandguaranteeSerializability.Howcanweguaranteethis?2022/12/23UpdateTransaction2510.3SerializabilityandthePrecedenceGraphFirstIftwotransactionsneveraccessthesamedataitems,soWecancommuteoperationsinthehistoryofrequestspermittedbythescheduleruntilalloperationsofoneTxaretogether(serialhistory).Theoperationsdon'taffecteachother,andorderdoesn'tmatter.2022/12/23UpdateTransaction26Ifwehaveoperationsbytwodifferenttransactionsthatdoaffectthesamedataitem,whatthen?Thereareonlyfourpossibilities...R1(A)R2(A)...Thiscanbecommuted.IfthereisathirdtransactionT3,where:...R1(A)...W3(A)...R2(A)...thenthereads(R1andR2)cannotbecommuted....R1(A)W2(A)......W1(A)R2(A)......W1(A)W2(A)...Thesetwooperationscannotcommute.2022/12/23UpdateTransaction2710.3SerializabilityandthePrecedenceGraphDef.10.3.1ConflictingOperations（沖突動作）TwooperationsXi(A)andYj(B)inahistoryaresaidtoconflict

(i.e.,theordermatters)ifandonlyifthefollowingthreeconditionshold:A

BijOneofthetwooperationsXorYisawrite(W).OthercanbeRorW.2022/12/23UpdateTransaction28Def.10.3.1ConflictingOperations（沖突動作）TwooperationsXi(A)andYj(B)inahistoryA

BOperationsondistinctdataitemsneverconflict.ijOperationsconflictonlyiftheyareperformedbydifferenttransactions.TwooperationsoftheSAMEtransactionalsocannotbecommutedinahistory,why

?OneofthetwooperationsXorYisawrite(W).OthercanbeRorW.2022/12/23UpdateTransaction2910.3SerializabilityandthePrecedenceGraphFigure10.7TheThreeTypesRi(A)Wj(A)meaning,inahistory,Ri(A)followedbyWj(A)Wi(A)Rj(A)Wi(A)Wj(A)2022/12/23UpdateTransaction3010.3SerializabilityandthePrecedenceGraphDef.10.3.2AninterpretationofanarbitraryhistoryHconsistsof3parts.Adescriptionofthepurposeofthelogicbeingperformed.Specificationofprecisevaluesfordataitemsbeingreadandwritteninthehistory.Aconsistencyrule,apropertythatisobviouslypreservedbyisolatedtransactionsofthelogicdefinedin1).2022/12/23UpdateTransaction3110.3SerializabilityandthePrecedenceGraphExample10.3.1R2(A)W2(A)R1(A)R1(B)R2(B)W2(B)C1C2H1:HereisaninterpretationT1isdoingacreditcheck,addingupthebalancesofAandB.T2istransferringmoneyfromAtoB.TheConsistencyRule:Neithertransactioncreatesordestroysmoney.2022/12/23UpdateTransaction32ThescheduleH1isnotserializability(SR)becauseIfscheduleH1isequivalenttoserializabilityscheduleS(H1),thenW2(A)&R1(A)isconflictingoperations,and W2(A)<<H1R1(A),soW2(A)<<S(H1)R1(A)ItmeansthatT2<<S(H1)T1R1(B)&W2(B)isconflictingoperations,and R1(B)<<H1W2(B),soR1(B)<<S(H1)W2(B)ItmeansthatT1<<S(H1)T2so,scheduleH1isnotserializability.(non-SR)Example10.3.1R2(A)W2(A)R1(A)R1(B)R2(B)W2(B)C1C2H1:2022/12/23UpdateTransaction3310.3SerializabilityandthePrecedenceGraphExample10.3.2lostupdate(dirtywrite)H2:R1(A)R2(A)W1(A)W2(A)C1C2operations1and4implythatT1<<S(H2)T2operations2and3implythatT2<<S(H2)T1so,H2isnon-SRscheduleH2T1&T2T2&T1valueofA150190190anexampleofH2R1(A,100)R2(A,100)W1(A,140)W2(A,150)C1C22022/12/23UpdateTransaction3410.3SerializabilityandthePrecedenceGraphExample10.3.3BlindWritesH3:W1(A)W2(A)W2(B)W1(B)C1C2operations1and2implythatT1<<S(H3)T2operations3and4implythatT2<<S(H3)T1so,H3isnon-SRscheduleH3T1&T2T2&T1valueofA808050valueofB502050anexampleofH3W1(A,50)W2(A,80)W2(B,20)W1(B,50)C1C22022/12/23UpdateTransaction3510.3SerializabilityandthePrecedenceGraphDef.10.3.3.ThePrecedenceGraphAprecedencegraphforahistoryHisadirectedgraphdenotedbyPG(H).TheverticesofPG(H)correspondtothetransactionsthathaveCOMMITTEDinHmittedtransactionsdon'tcount.AnedgeTiTjexistsinPG(H)whenevertwoconflictingoperationsXiandYjoccurinthatorderinH.Thus,TiTjshouldbeinterpretedtomeanthatTimustprecedeTjinanyequivalentserialhistoryS(H).2022/12/23UpdateTransaction36Theorem10.3.4TheSerializabilityTheoremAhistoryHhasanequivalentserialexecutionS(H)ifftheprecedencegraphPG(H)containsnocircuit.Proof.Assumetherearemtransactionsinvolved,andlabelthemT1,T2,...,Tm.BecauseInanydirectedgraphwithnocircuitthereisalwaysavertexwithnoedgeenteringit,thusthereisavertex,ortransactionTk,withnoedgeenteringit.WechooseTktobeTi(1).2022/12/23UpdateTransaction37Proof.(cont.)NotethatsinceTi(1)hasnoedgeenteringit,thereisnoconflictinHthatforcessomeothertransactiontocomeearlier.Nowremovethisvertex,Ti(1),fromPG(H)andalledgesleavingit.CalltheresultinggraphPG1(H)withnocircuit.Continueinthisfashion,removingTi(2)andallit'sedgesfromPG1(H),andsoon,choosingTi(3)fromPG2(H),...,Ti(m)fromPGm-1(H).2022/12/23UpdateTransaction3810.4LockingEnsuresSerializabilityTwo-PhaseLocking(2PL)封鎖（lock）封鎖類型常用的兩種類型封鎖鎖相容矩陣2022/12/23UpdateTransaction393910.4LockingEnsuresSerializability封鎖（lock）使用封鎖技術(shù)的前提在一個事務(wù)訪問數(shù)據(jù)庫中的數(shù)據(jù)時，必須先獲得被訪問的數(shù)據(jù)對象上的封鎖，以保證數(shù)據(jù)訪問操作的正確性和一致性。封鎖的作用在一段時間內(nèi)禁止其它事務(wù)在被封鎖的數(shù)據(jù)對象上執(zhí)行某些類型的操作（由封鎖的類型決定）同時也表明：持有該封鎖的事務(wù)在被封鎖的數(shù)據(jù)對象上將要執(zhí)行什么類型的操作（由系統(tǒng)所采用的封鎖協(xié)議來決定）2022/12/23UpdateTransaction4010.4LockingEnsuresSerializability封鎖類型常用的封鎖類型有兩種排它鎖(eXclusivelock，簡稱X鎖)又被稱為‘寫封鎖’

(WL–WriteLock)共享鎖(Sharinglock，簡稱S鎖)又被稱為‘讀封鎖’

(RL–ReadLock)2022/12/23UpdateTransaction41排它鎖（X鎖）特性只有當(dāng)數(shù)據(jù)對象A沒有被其它事務(wù)封鎖時，事務(wù)T才能在數(shù)據(jù)對象A上施加‘X鎖’；如果事務(wù)T對數(shù)據(jù)對象A施加了’X鎖’，則其它任何事務(wù)都不能在數(shù)據(jù)對象A上再施加任何類型的封鎖。10.4LockingEnsuresSerializability2022/12/23UpdateTransaction42排它鎖（cont.）作用如果一個事務(wù)T申請在數(shù)據(jù)對象A上施加‘X鎖’并得到滿足，則：事務(wù)T自身可以對數(shù)據(jù)對象A作讀、寫操作，而其它事務(wù)則被禁止訪問數(shù)據(jù)對象A這樣可以讓事務(wù)T獨(dú)占該數(shù)據(jù)對象A，從而保證了事務(wù)T對數(shù)據(jù)對象A的訪問操作的正確性和一致性。缺點(diǎn)：降低了整個系統(tǒng)的并行性10.4LockingEnsuresSerializability‘X鎖’必須維持到事務(wù)T的執(zhí)行結(jié)束2022/12/23UpdateTransaction43共享鎖（S鎖）特性如果數(shù)據(jù)對象A沒有被其它事務(wù)封鎖，或者其它事務(wù)僅僅以‘S鎖’的方式來封鎖數(shù)據(jù)對象A時，事務(wù)T才能在數(shù)據(jù)對象A上施加‘S鎖’；10.4LockingEnsuresSerializability2022/12/23UpdateTransaction44共享鎖（cont.）作用如果一個事務(wù)T申請在數(shù)據(jù)對象A上施加‘S鎖’并得到滿足，則：事務(wù)T可以對‘讀’數(shù)據(jù)對象A，但不能‘寫’數(shù)據(jù)對象A不同事務(wù)所申請的‘S鎖’可以共存于同一個數(shù)據(jù)對象A上，從而保證了多個事務(wù)可以同時‘讀’數(shù)據(jù)對象A，有利于提高整個系統(tǒng)的并發(fā)性在持有封鎖的事務(wù)釋放數(shù)據(jù)對象A上的所有‘S鎖’之前，任何事務(wù)都不能‘寫’數(shù)據(jù)對象A‘S鎖’不必維持到事務(wù)T的執(zhí)行結(jié)束(依封鎖協(xié)議而定)10.4LockingEnsuresSerializability2022/12/23UpdateTransaction45‘排它鎖’與‘共享鎖’的相互關(guān)系可以用如下圖所示的‘鎖相容矩陣’來表示。當(dāng)前事務(wù)申請的鎖其它事務(wù)已持有的鎖YesYesNoS鎖YesNoNoX鎖-S鎖X鎖鎖相容矩陣10.4LockingEnsuresSerializability2022/12/23UpdateTransaction46合適(wellformed)事務(wù)如果一個事務(wù)在訪問數(shù)據(jù)庫中的數(shù)據(jù)對象A之前按照要求申請對A的封鎖，在操作結(jié)束后釋放A上的封鎖，這種事務(wù)被稱為合適事務(wù)?！线m事務(wù)’是保證并發(fā)事務(wù)的正確執(zhí)行的基本條件。10.4LockingEnsuresSerializability2022/12/23UpdateTransaction4710.4LockingEnsuresSerializabilityDef.10.4.1Two-PhaseLocking(2PL)Lockstakeninreleasedfollowingthreerules.鎖申請規(guī)則鎖申請的等待規(guī)則單個事務(wù)的鎖申請與釋放規(guī)則2022/12/23UpdateTransaction4810.4LockingEnsuresSerializabilityDef.10.4.1Two-PhaseLocking(cont.)BeforeTicanreadadataitem,Ri(A),schedulerattemptstoReadLocktheitemonit'sbehalf,RLi(A);before

Wi(A),tryWriteLock,WLi(A).2022/12/23UpdateTransaction4910.4LockingEnsuresSerializabilityDef.10.4.1Two-PhaseLocking(cont.)2)Ifconflictinglockonitemexists,requestingTx

mustWAIT.Conflictinglockscorrespondingtoconflictingoperations:twolocksonadataitemconflictiftheyareattemptedbydifferentTxsandatleastoneofthemisaWL.2022/12/23UpdateTransaction5010.4LockingEnsuresSerializabilityTherearetwophasestolockingthegrowingphasetheshrinkingphasewhenlocksarereleased:RUi(A);Theschedulermustensurethatcan'tshrink(dropalock)andthengrowagain(takeanewlock).Rule3)impliescanreleaselocksbeforeCommit;MoreusualtoreleasealllocksatonceonCommit,andweshallassumethisinwhatfollows.2022/12/23UpdateTransaction5110.4LockingEnsuresSerializabilityNotethatatransactioncanneverconflictwithitsownlocks!IfTi

holdsRLonA,cangetWLonAsolongasnootherTx

holdsalockonA(mustbeRL).aTx

withaWLdoesn'tneedaRL(WLmorepowerfulthanRL).2022/12/23UpdateTransaction5210.4LockingEnsuresSerializabilityClearlylockingisdefinedtoguaranteethatacircuitinthePrecedenceGraphcanneveroccur.ThefirstTx

tolockanitemforcesanyotherTxthatgetstoitsecondto"comelater"inanySG.ButwhatifotherTxalreadyholdsalockthefirstonenowneeds?Thiswouldmeanacircuit,butintheWAITrulesoflockingitmeansNEITHERTx

CANEVERGOFORWARDAGAIN.ThisisaDEADLOCK.2022/12/23UpdateTransaction53Example10.4.1ScheduleExplanationABRL1(A)50501R1(A,50)5050RL2(A)50502R2(A,50)5050WL2(A)T2WAIT5050RL1(B)50506R1(B,50)50507C1T2cangetWL2(A)50503W2(A,20)2050RL2(B)20504R2(B,50)2050WL2(B)20505W2(B,80)20808C22080H41R1(A)2R2(A)3W2(A)4R2(B)5W2(B)6R1(B)7C18C2non-SRSerializability2022/12/23UpdateTransaction54Example10.4.2ScheduleABT1T2RL1(A)50501R1(A,50)5050RL2(B)50502R2(B,50)5050WL2(B)50503W2(B,80)5080RL2(A)50804R2(A,50)5080WL2(A)5080T2waitingRL1(B)T1waitingH51R1(A)2R2(B)3W2(B)4R2(A)5W2(A)6R1(B)7C18C2non-SRDeadlock2022/12/23UpdateTransaction5510.4LockingEnsuresSerializabilityTheorem.10.4.2LockingTheoremAhistoryoftransactionaloperationsthatfollowsthe2PLdisciplineisSR(Serializability).short-termlocksAlockistakenpriortotheoperation(RorW)andreleasedIMMEDIATELYAFTERWARD.long-termlocksAlockarehelduntilEOT(EndOfTransaction).2022/12/23UpdateTransaction5610.5LevelsofIsolation定義當(dāng)前用戶的事務(wù)與其它并發(fā)執(zhí)行事務(wù)之間的隔離級別(levelofisolation)事務(wù)的隔離級別與所采用的封鎖策略緊密相關(guān)SETTRANSACTIONISOLATIONLEVELMITTED|MITTED|READREPEATABLE|SERIALIZABLE2022/12/23UpdateTransaction5710.5LevelsofIsolationMITTED：未提交讀在該方式下，當(dāng)前事務(wù)不需要申請任何類型的封鎖，因而可能會‘讀’到未提交的修改結(jié)果禁止一個事務(wù)以該方式去執(zhí)行對數(shù)據(jù)的‘寫’操作，以避免‘寫’沖突現(xiàn)象。MITTED：提交讀在‘讀’數(shù)據(jù)對象A之前需要先申請對數(shù)據(jù)對象A的‘共享性’封鎖，在‘讀’操作執(zhí)行結(jié)束之后立即釋放該封鎖。以避免讀取未提交的修改結(jié)果。2022/12/23UpdateTransaction5810.5LevelsofIsolationREADREPEATABLE：可重復(fù)讀在‘讀’數(shù)據(jù)對象A之前需要先申請對數(shù)據(jù)對象A的‘共享性’封鎖，并將該封鎖維持到當(dāng)前事務(wù)的結(jié)束?？梢员苊馄渌牟l(fā)事務(wù)對當(dāng)前事務(wù)正在使用的數(shù)據(jù)對象的修改。SERIALIZABLE：可序列化(可串行化)并發(fā)事務(wù)以一種可串行化的調(diào)度策略實(shí)現(xiàn)其并發(fā)執(zhí)行，以避免它們相互之間的干擾現(xiàn)象。2022/12/23UpdateTransaction5910.5LevelsofIsolation不管采用何種隔離級別，在事務(wù)‘寫’數(shù)據(jù)對象A之前需要先申請對數(shù)據(jù)對象A的‘排它性’封鎖(write-lock)，并將該封鎖維持到當(dāng)前事務(wù)的結(jié)束。2022/12/23UpdateTransaction6010.5LevelsofIsolationlong-termshort-termshort-termNoReadLocks(Predicates)Nolong-termYesSerializableUpdateAnomalylong-termYesRepeatableReadLostUpdateshort-termYesReadCommittedDirtyReadsNoNo(ReadOnly)ReadmittedProblemReadLocks(row)WritelocksLevelsofIsolationTypesofLocks2022/12/23UpdateTransaction6110.6TransactionalRecoveryLogsThelogentriescontain"BeforeImages"and"AfterImages"ofeveryupdatemadebyaTransaction.BeforeImageAfterImage2022/12/23UpdateTransaction6210.6TransactionalRecoveryBeforeImageInrecovery,wecanbackupanupdatethatshouldn'thavegottentodisk(thetransactiondidn'tcommit)byapplyingaBeforeImage.AfterImageInrecovery,wecanapplyAfterImagestocorrectfordiskpagesthatshouldhavegottentodisk(thetransactiondidcommit)butnevermadeit.2022/12/23UpdateTransaction6310.6TransactionalRecoveryThreetypesofLogsUNDOLogsonlyBeforeImageRecoveryforthetransactiondidn’tcommit.REDOLogsonlyAfterImageRecoveryforthetransactiondidcommit.UNDO/REDOLogsBeforeImage&AfterImageRecoveryforalltransactions.2022/12/23UpdateTransaction6410.7RecoveryinDetail:LogFormatsFigure10.13(pg.673)OperationLogentryExplanationR1(A,50)(S,1)StarttransactionT1-nologentryiswrittenforaReadoperation.W1(A,20)(W,1,A,50,20)T1WritelogforupdateofA.balance.Thevalue50istheBeforeImage(BI)forA.balancecolumninrowA,20istheAfterImage(AI)forA.balanceR2(C,100)(S,2)Anotherstarttransactionlogentry.W2(C,50)(W,2,C,100,50)AnotherWritelogentry.C2(C,2)CommitT2logentry.(WriteLogBuffertoLogFile)R1(B,50)NologentryW1(B,80)(W,1,B,50,80)C1(C,1)CommitT1(WriteLogBuffertoLogFile)2022/12/23UpdateTransaction6510.7RecoveryinDetail:LogFormatsAssumethataSystemCrashoccurredimmediatelyaftertheW1(B,80)operation.2022/12/23UpdateTransaction66Logentry5(S,1)4(W,1,A,50,20)3(S,2)2(W,2,C,100,50)1(C,2)FailureROLLBACKACTIONPERFORMEDMakeanotethatT1isnolonger"Active".Nowthatnotransactionswereactive,wecanendtheROLLBACKphase.TransactionT1hasnevercommitted(it'slastoperationwasaWrite).Therefore,thesystemperformsUNDOofthisupdatebyWritingtheBeforeImagevalue(50)intodataitemA.PutT1into"mittedList“.MakeanotethatT2isnolonger"Active"SinceT2ison"CommittedList",wedonothing.PutT2into"CommittedList"2022/12/23UpdateTransaction67LogentryROLLFORWARDACTIONPERFORMED6(S,1)Noactionrequired.7(W,1,A,50,20)T1ismitted—Noactionrequired.8(S,2)Noactionrequired.9(W,2,C,100,50)SinceT2isonCommittedList,weREDOthisupdatebywritingAfterImagevalue(50)intodataitemC.10(C,2)Noactionrequired.11Wenotethatwehaverolledforwar