课程资源-复试数据库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

（隔离性）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（调度）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（串行调度）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（可串行化调度）TheseriesofoperationsisEQUIVALENTtoaSerialscheduleScheduler（调度器）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（冲突动作）TwooperationsXi(A)andYj(B)inahistoryaresaidtoconflict

(i.e.,theordermatters)ifandonlyifthefollowingthreeconditionshold:A

BijOneofthetwooperationsXorYisawrite(W).OthercanbeRorW.2022/12/23UpdateTransaction28Def.10.3.1ConflictingOperations（冲突动作）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)封锁（lock）封锁类型常用的两种类型封锁锁相容矩阵2022/12/23UpdateTransaction393910.4LockingEnsuresSerializability封锁（lock）使用封锁技术的前提在一个事务访问数据库中的数据时，必须先获得被访问的数据对象上的封锁，以保证数据访问操作的正确性和一致性。封锁的作用在一段时间内禁止其它事务在被封锁的数据对象上执行某些类型的操作（由封锁的类型决定）同时也表明：持有该封锁的事务在被封锁的数据对象上将要执行什么类型的操作（由系统所采用的封锁协议来决定）2022/12/23UpdateTransaction4010.4LockingEnsuresSerializability封锁类型常用的封锁类型有两种排它锁(eXclusivelock，简称X锁)又被称为‘写封锁’

(WL–WriteLock)共享锁(Sharinglock，简称S锁)又被称为‘读封锁’

(RL–ReadLock)2022/12/23UpdateTransaction41排它锁（X锁）特性只有当数据对象A没有被其它事务封锁时，事务T才能在数据对象A上施加‘X锁’；如果事务T对数据对象A施加了’X锁’，则其它任何事务都不能在数据对象A上再施加任何类型的封锁。10.4LockingEnsuresSerializability2022/12/23UpdateTransaction42排它锁（cont.）作用如果一个事务T申请在数据对象A上施加‘X锁’并得到满足，则：事务T自身可以对数据对象A作读、写操作，而其它事务则被禁止访问数据对象A这样可以让事务T独占该数据对象A，从而保证了事务T对数据对象A的访问操作的正确性和一致性。缺点：降低了整个系统的并行性10.4LockingEnsuresSerializability‘X锁’必须维持到事务T的执行结束2022/12/23UpdateTransaction43共享锁（S锁）特性如果数据对象A没有被其它事务封锁，或者其它事务仅仅以‘S锁’的方式来封锁数据对象A时，事务T才能在数据对象A上施加‘S锁’；10.4LockingEnsuresSerializability2022/12/23UpdateTransaction44共享锁（cont.）作用如果一个事务T申请在数据对象A上施加‘S锁’并得到满足，则：事务T可以对‘读’数据对象A，但不能‘写’数据对象A不同事务所申请的‘S锁’可以共存于同一个数据对象A上，从而保证了多个事务可以同时‘读’数据对象A，有利于提高整个系统的并发性在持有封锁的事务释放数据对象A上的所有‘S锁’之前，任何事务都不能‘写’数据对象A‘S锁’不必维持到事务T的执行结束(依封锁协议而定)10.4LockingEnsuresSerializability2022/12/23UpdateTransaction45‘排它锁’与‘共享锁’的相互关系可以用如下图所示的‘锁相容矩阵’来表示。当前事务申请的锁其它事务已持有的锁YesYesNoS锁YesNoNoX锁-S锁X锁锁相容矩阵10.4LockingEnsuresSerializability2022/12/23UpdateTransaction46合适(wellformed)事务如果一个事务在访问数据库中的数据对象A之前按照要求申请对A的封锁，在操作结束后释放A上的封锁，这种事务被称为合适事务。‘合适事务’是保证并发事务的正确执行的基本条件。10.4LockingEnsuresSerializability2022/12/23UpdateTransaction4710.4LockingEnsuresSerializabilityDef.10.4.1Two-PhaseLocking(2PL)Lockstakeninreleasedfollowingthreerules.锁申请规则锁申请的等待规则单个事务的锁申请与释放规则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定义当前用户的事务与其它并发执行事务之间的隔离级别(levelofisolation)事务的隔离级别与所采用的封锁策略紧密相关SETTRANSACTIONISOLATIONLEVELMITTED|MITTED|READREPEATABLE|SERIALIZABLE2022/12/23UpdateTransaction5710.5LevelsofIsolationMITTED：未提交读在该方式下，当前事务不需要申请任何类型的封锁，因而可能会‘读’到未提交的修改结果禁止一个事务以该方式去执行对数据的‘写’操作，以避免‘写’冲突现象。MITTED：提交读在‘读’数据对象A之前需要先申请对数据对象A的‘共享性’封锁，在‘读’操作执行结束之后立即释放该封锁。以避免读取未提交的修改结果。2022/12/23UpdateTransaction5810.5LevelsofIsolationREADREPEATABLE：可重复读在‘读’数据对象A之前需要先申请对数据对象A的‘共享性’封锁，并将该封锁维持到当前事务的结束。可以避免其它的并发事务对当前事务正在使用的数据对象的修改。SERIALIZABLE：可序列化(可串行化)并发事务以一种可串行化的调度策略实现其并发执行，以避免它们相互之间的干扰现象。2022/12/23UpdateTransaction5910.5LevelsofIsolation不管采用何种隔离级别，在事务‘写’数据对象A之前需要先申请对数据对象A的‘排它性’封锁(write-lock)，并将该封锁维持到当前事务的结束。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