数据库-研究生课件：第24章 高级事务处理

第24章高级事务处理Transaction-ProcessingMonitorsTransactionalWorkflowsHigh-PerformanceTransactionSystemsMainmemorydatabasesReal-TimeTransactionSystemsLong-DurationTransactionsTransactionmanagementinmultidatabasesystems1TransactionProcessingMonitorsTPmonitorsinitiallydevelopedasmultithreadedserverstosupportlargenumbersofterminalsfromasingleprocess.Provideinfrastructureforbuildingandadministeringcomplextransactionprocessingsystemswithalargenumberofclientsandmultipleservers.Provideservicessuchas:PresentationfacilitiestosimplifycreatinguserinterfacesPersistentqueuingofclientrequestsandserverresponsesRoutingofclientmessagestoserversCoordinationoftwo-phasecommitwhentransactionsaccessmultipleservers.SomecommercialTPmonitors:CICSfromIBM,PathwayfromTandem,TopEndfromNCR,andEncinafromTransarc2TPMonitorArchitectures3TPMonitorArchitecturesProcessperclientmodel-insteadofindividualloginsessionperterminal,serverprocesscommunicateswiththeterminal,handlesauthentication,andexecutesactions.MemoryrequirementsarehighMultitasking-highCPUoverheadforcontextswitchingbetweenprocessesSingleprocessmodel-allremoteterminalsconnecttoasingleserverprocess.Usedinclient-serverenvironmentsServerprocessismulti-threaded;lowcostforthreadswitchingNoprotectionbetweenapplicationsNotsuitedforparallelordistributeddatabases4TPMonitorArchitecturesMany-serversingle-routermodel-multipleapplicationserverprocessesaccessacommondatabase;clientscommunicatewiththeapplicationthroughasinglecommunicationprocessthatroutesrequests.IndependentserverprocessesformultipleapplicationsMultithreadserverprocessRunonparallelordistributeddatabaseManyservermany-routermodel-multipleprocessescommunicatewithclients.Clientcommunicationprocessesinteractwithrouterprocessesthatroutetheirrequeststotheappropriateserver.Controllerprocessstartsupandsupervisesotherprocesses.5DetailedStructureofaTPMonitor6DetailedStructureofaTPMonitorQueuemanagerhandlesincomingmessagesSomequeuemanagersprovidepersistentordurablemessagequeueingcontentsofqueuearesafeevenifsystemsfails.Durablequeueingofoutgoingmessagesisimportantapplicationserverwritesmessagetodurablequeueaspartofatransactiononcethetransactioncommits,theTPmonitorguaranteesmessageiseventuallydelevered,regardlessofcrashes.ACIDpropertiesarethusprovidedevenformessagessentoutsidethedatabaseManyTPmonitorsprovidelocking,loggingandrecoveryservices,toenableapplicationserverstoimplementACIDpropertiesbythemselves.7ApplicationCoordinationUsingTPMonitorsATPmonitortreatseachsubsystemasaresourcemanagerthatprovidestransactionalaccesstosomesetofresources.TheinterfacebetweentheTPmonitorandtheresourcemanagerisdefinedbyasetoftransactionprimitivesTheresourcemanagerinterfaceisdefinedbytheX/OpenDistributedTransactionProcessingstandard.TPmonitorsystemsprovideatransactionalremoteprocedurecall(transactionalRPC)interfacetotheirserviceTransactionalRPCprovidescallstoencloseaseriesofRPCcallswithinatransaction.UpdatesperformedbyanRPCarecarriedoutwithinthescopeofthetransaction,andcanberolledbackifthereisanyfailure.8WorkflowSystems9Copyright:Silberschatz,KorthandSudarshanTransactionalWorkflowsWorkflowsareactivitiesthatinvolvethecoordinatedexecutionofmultipletasksperformedbydifferentprocessingentities.Withthegrowthofnetworks,andtheexistenceofmultipleautonomousdatabasesystems,workflowsprovideaconvenientwayofcarryingouttasksthatinvolvemultiplesystems.Exampleofaworkflowdeliveryofanemailmessage,whichgoesthroughseveralmailssystemstoreachdestination.Eachmailerperformsatasks:forwardingofthemailtothenextmailer.Ifamailercannotdelivermail,failuremustbehandledsemantically(deliveryfailuremessage).Workflowsusuallyinvolvehumans:e.g.loanprocessing,orpurchaseorderprocessing.10ExamplesofWorkflows11LoanProcessingWorkflowInthepast,workflowswerehandledbycreatingandforwardingpaperformsComputerizedworkflowsaimtoautomatemanyofthetasks.Butthehumansstillplayrolee.g.inapprovingloans.12TransactionalWorkflowsMustaddressfollowingissuestocomputerizeaworkflow.Specificationofworkflows-detailingthetasksthatmustbecarriedoutanddefiningtheexecutionrequirements.Executionofworkflows-executetransactionsspecifiedintheworkflowwhilealsoprovidingtraditionaldatabasesafeguardsrelatedtothecorrectnessofcomputations,dataintegrity,anddurability.E.g.:Loanapplicationshouldnotgetlostevenifsystemfails.Extendtransactionconceptstothecontextofworkflows.Stateofaworkflow-consistsofthecollectionofstatesofitsconstituenttasks,andthestates(i.e.values)ofallvariablesintheexecutionplan.13WorkflowSpecificationStaticspecificationoftaskcoordination:Tasksanddependenciesamongthemaredefinedbeforetheexecutionoftheworkflowstarts.Canestablishpreconditionsforexecutionofeachtask:tasksareexecutedonlywhentheirpreconditionsaresatisfied.Definedpreconditionsthroughdependencies:Executionstatesofothertasks.

“taskticannotstartuntiltasktj

hasended”Outputvaluesofothertasks. “taskticanstartiftasktjreturnsavaluegreaterthan25”Externalvariables,thataremodifiedbyexternalevents.

“tasktimustbestartedwithin24hoursofthecompletionoftasktj”14Workflow

Specification(Cont.)Dynamictaskcoordination

E.g.Electronicmailroutingsysteminwhichthetexttobescheduleforagivenmailmessagedependsonthedestinationaddressandonwhichintermediateroutersarefunctioning.15Failure-AutomicityRequirementsUsualACIDtransactionalrequirementsaretoostrong/unimplementableforworkflowapplications.However,workflowsmustsatisfysomelimitedtransactionalpropertiesthatguaranteeaprocessisnotleftinaninconsistentstate.Acceptableterminationstates-everyexecutionofaworkflowwillterminateinastatethatsatisfiesthefailure-atomicityrequirementsdefinedbythedesigner.Committed-objectivesofaworkflowhavebeenachieved.Aborted-validterminationstateinwhichaworkflowhasfailedtoachieveitsobjectives.Aworkflowmustreachanacceptableterminationstateeveninthepresenceofsystemfailures.16ExecutionofWorkflowsWorkflowmanagementsystemsinclude:Scheduler-programthatprocessworkflowsbysubmittingvarioustasksforexecution,monitoringvariousevents,andevaluationconditionsrelatedtointertaskdependenciesTaskagents-controltheexecutionofataskbyaprocessingentity.Mechanismtoquerytostateoftheworkflowsystem.17WorkflowManagementSystemArchitecturesCentralized-asingleschedulerschedulesthetasksforallconcurrentlyexecutingworkflows.usedinworkflowsystemswherethedataisstoredinacentraldatabase.easiertokeeptrackofthestateofaworkflow.Partiallydistributed-hasone(instanceofa)schedulerforeachworkflow.Fullydistributed-hasnoscheduler,butthetaskagentscoordinatetheirexecutionbycommunicatingwitheachothertosatisfytaskdependenciesandotherworkflowexecutionrequirements.usedinsimplestworkflowexecutionsystemsbasedonelectronicmail18WorkflowSchedulerIdeallyschedulershouldexecuteaworkflowonlyafterensuringthatitwillterminateinanacceptablestate.ConsideraworkflowconsistingoftwotasksS1andS2.Letthefailure-atomicityrequirementbethateitherbothorneitherofthesubtransactionsshouldbecommitted.SupposesystemsexecutingS1andS2donotprovideprepared-to-commitstatesandS1orS2donothavecompensatingtransactions.Itisthenpossibletoreachastatewhereonesubtransactioniscommittedandtheotheraborted.Bothcannotthenbebroughttothesamestate.Workflowspecificationisunsafe,andshouldberejected.Determinationofsafetybytheschedulerisnotpossibleingeneral,andisusuallylefttothedesigneroftheworkflow.19RecoveryofaWorkflowEnsurethatisafailureoccursinanyoftheworkflow-processingcomponents,theworkfloweventuallyreachesanacceptableterminationstate.Failure-recoveryroutinesneedtorestorethestateinformationofthescheduleratthetimeoffailure,includingtheinformationabouttheexecutionstatesofeachtask.Logstatusinformationonstablestorage.Handoffoftasksbetweenagentsshouldoccurexactlyonceinspiteoffailure.Problem:Repeatinghandoffonrecoverymayleadtoduplicateexecutionoftask;notrepeatinghandoffmayleadtotasknotbeingexecuted.Solution:Persistentmessagingsystems20RecoveryofaWorkflow(Cont.)Persistentmessages:messagesarestoredinpermanentmessagequeueandthereforenotlostincaseoffailure.DescribedindetailinChapter19(DistributedDatabases)Beforeanagentcommits,itwritestothepersistentmessagequeuewhatevermessagesneedtobesentout.Thepersistentmessagesystemmustmakesurethemessagesgetdeliveredeventuallyifandonlyifthetransactioncommits.Themessagesystemneedstoresendamessagewhenthesiterecovers,ifthemessageisnotknowntohavereacheditsdestination.Messagesmustbeloggedinstablestorageatthereceivingendtodetectmultiplereceiptsofamessage.21HighPerformance

TransactionSystems22Copyright:Silberschatz,KorthandSudarshanHigh-PerformanceTransactionSystemsHigh-performancehardwareandparallelismhelpimprovetherateoftransactionprocessing,butareinsufficienttoobtainhighperformance:DiskI/Oisabottleneck—I/Otime(10milliseconds)hasnodecreasedataratecomparabletotheincreaseinprocessorspeeds.Paralleltransactionsmayattempttoreadorwritethesamedataitem,resultingindataconflictsthatreduceeffectiveparallelismWecanreducethedegreetowhichadatabasesystemisdiskboundbyincreasingthesizeofthedatabasebuffer.23Main-MemoryDatabaseCommercial64-bitsystemscansupportmainmemoriesoftensofgigabytes.Memoryresidentdataallowsfasterprocessingoftransactions.Disk-relatedlimitations:Loggingisabottleneckwhentransactionrateishigh.Usegroup-committoreducenumberofoutputoperations(Willstudytwoslidesahead.)Iftheupdaterateformodifiedbufferblocksishigh,thediskdata-transferratecouldbecomeabottleneck.Ifthesystemcrashes,allofmainmemoryislost.24Main-MemoryDatabaseOptimizationsToreducespaceoverheads,main-memorydatabasescanusestructureswithpointerscrossingmultiplepages.Indiskdatabases,theI/Ocosttotraversemultiplepageswouldbeexcessivelyhigh.Noneedtopinbufferpagesinmemorybeforedataareaccessed,sincebufferpageswillneverbereplaced.Designquery-processingtechniquestominimizespaceoverhead-avoidexceedingmainmemorylimitsduringqueryevaluation.Improveimplementationofoperationssuchaslockingandlatching,sotheydonotbecomebottlenecks.Optimizerecoveryalgorithms,sincepagesrarelyneedtobewrittenouttomakespaceforotherpages.25GroupCommitIdea:Insteadofperformingoutputoflogrecordstostablestorageassoonasatransactionisreadytocommit,waituntillogbufferblockisfull,oratransactionhasbeenwaitingsufficientlylongafterbeingreadytocommitResultsinfeweroutputoperationspercommittedtransaction,andcorrespondinglyahigherthroughput.However,commitsaredelayeduntilasufficientlylargegroupoftransactionsarereadytocommit,oratransactionhasbeenwaitinglongenough-leadstoslightlyincreasedresponsetime.Abovedelayacceptableinhigh-performancetransactionsystemssincelogbufferblockswillfillupquickly.26Real-TimeTransactionSystemsInsystemswithreal-timeconstraints,correctnessofexecutioninvolvesbothdatabaseconsistencyandthesatisfactionofdeadlines.Harddeadline–SeriousproblemsmayoccuriftaskisnotcompletedwithindeadlineFirmdeadline-Thetaskhaszerovalueifitcompletedafterthedeadline.Softdeadline-Thetaskhasdiminishingvalueifitiscompletedafterthedeadline.Thewidevarianceofexecutiontimesforreadandwriteoperationsondiskscomplicatesthetransactionmanagementproblemfortime-constrainedsystemsmain-memorydatabasesarethusoftenusedWaitsforlocks,transactionaborts,contentionforresourcesremainasproblemsevenifdataisinmainmemoryDesignofareal-timesysteminvolvesensuringthatenoughprocessingpowerexiststomeetdeadlinewithoutrequiringexcessivehardwareresources.27LongDurationTransactionsTraditionalconcurrencycontroltechniquesdonotworkwellwhenuserinteractionisrequired:Longduration:DesigneditsessionsareverylongExposureofuncommitteddata:E.g.,partialupdatetoadesignSubtasks:supportpartialrollbackRecoverability:oncrashstateshouldberestoredevenforyet-to-becommitteddata,souserworkisnotlost.Performance:fastresponsetimeisessentialsousertimeisnotwasted.28Long-DurationTransactionsRepresentasanestedtransactionatomicdatabaseoperations(read/write)atalowestlevel.Iftransactionfails,onlyactiveshort-durationtransactionsabort.Activelong-durationtransactionsresumeonceanyshortdurationtransactionshaverecovered.Theefficientmanagementoflong-durationwaits,andthepossibilityofaborts.Needalternativestowaitsandaborts;alternativetechniquesmustensurecorrectnesswithoutrequiringserializability.29ConcurrencyControlCorrectnesswithoutserializability:Correctnessdependsonthespecificconsistencyconstraintsforthedatabases.Correctnessdependsonthepropertiesofoperationsperformedbyeachtransaction.Usedatabaseconsistencyconstraintsastosplitthedatabaseintosubdatabasesonwhichconcurrencycanbemanagedseparately.Treatsomeoperationsbesidesreadandwriteasfundamentallow-leveloperationsandextendconcurrencycontroltodealwiththem.30ConcurrencyControl(Cont.)Anon-conflict-serializableschedulethatpreservesthesumofA+B31NestedandMultilevelTransactionsAnestedormultileveltransaction

Tisrepresentedbyaset

T={t1,t2,...,tn}ofsubtransactionsandapartialorderPonT.Asubtransaction

tiinTmayabortwithoutforcingTtoabort.Instead,Tmayeitherrestartti,orsimplychoosenottorunti.If

ticommits,thisactiondoesnotmake

ti,permanent(unlikethesituationinChapter15).Instead,ti,commitstoT,andmaystillabort(orrequirecompensation)ifTaborts.AnexecutionofTmustnotviolatethepartialorderP,i.e.,ifanedgeti

ti

appearsintheprecedencegraph,thentiti

mustnotbeinthetransitiveclosureofP.32NestedandMultilevelTransactions(Cont.)Subtransactionscanthemselvesbenested/multileveltransactions.Lowestlevelofnesting:standardreadandwriteoperations.Nestingcancreatehigher-leveloperationsthatmayenhanceconcurrency.Typesofnested/multileveltransactions:Multileveltransaction:subtransactionofTispermittedtoreleaselocksoncompletion.Saga:multilevellong-durationtransaction.Nestedtransaction:locksheldbyasubtransactiontiofTareautomaticallyassigntoToncompletionofti.33ExampleofNestingRewritetransactionT1usingsubtransactionsTaandTbthatperformincrementordecrementoperations:T1consistsofT1,1,whichsubtracts50fromAT1,2,whichadds50toBRewritetransactionT2usingsubtransactionsTcandTdthatperformincrementordecrementoperations:T2consistsofT2,1,whichsubtracts10fromBT2,2,whichadds10toANoorderingisspecifiedonsubtransactions;anyexecutiongeneratesacorrectresult.34CompensatingTransactionsAlternativetoundooperation;compensatingtransactionsdealwiththeproblemofcascadingrollbacks.Insteadofundoingallchangesmadebythefailedtransaction,actionistakento“compensate”forthefailure.Consideralong-durationtransactionTirepresentingatravelreservation,withsubtransactionsTi,1,whichmakesairlinereservations,Ti,2whichreservesrentalcars,andTi,3whichreservesahotelroom.Hotelcancelsthereservation.InsteadofundoingallofTi,thefailureofTi,3iscompensatedforbydeletingtheoldhotelreservationandmakinganewone.Requiresuseofsemanticsofthefailedtransaction.35ImplementationIssuesForlong-durationtransactionstosurvivesystemcrashes,wemustlognotonlychangestothedatabase,butalsochangestointernalsystemdatapertainingtothesetransactions.Loggingofupdatesismademorecomplexbyphysicallylargedataitems(CADdesign,documenttext);undesirabletostorebotholdandnewvalues.Twoapproachestoreducingtheoverheadofensuringtherecoverabilityoflargedataitems:Operationlogging.Onlytheoperationperformedonthedataitemandthedata-itemnamearestoredinthelog.Loggingandshadowpaging.Useloggingfromsmalldataitems;useshadowpagingforlargedataitems.Onlymodifiedpagesneedtobestoredinduplicate.36TransactionManagementinMultidatabaseSystemsTransactionmanagementiscomplicatedinmultidatabasesystemsbecauseoftheassumptionofautonomyGlobal2PL-eachlocalsiteusesastrict2PL(locksarereleasedattheend);lockssetasaresultofaglobaltransactionarereleasedonlywhenthattransactionreachestheend.GuaranteesglobalserializabilityDuetoautonomyrequirements,sitescannotcooperateandexecuteacommonconcurrencycontrolschemeE.g.nowaytoensurethatalldatabasesfollowstrict2PLSolutions:provideverylowlevelofconcurrentexecution,oruseweakerlevelsofconsistency37TransactionManagementLocaltransactionsareexecutedbyeachlocalDBMS,outsideoftheMDBSsystemcontrol.Globaltransactionsareexecutedundermultidatabasecontrol.Localautonomy-localDBMSscannotcommunicatedirectlytosynchronizeglobaltransactionexecutionandthemultidatabasehasnocontroloverlocaltransactionexecution.localconcurrencycontrolschemeneededtoensurethatDBMS’sscheduleisserializableincaseoflocking,DBMSmustbeabletoguardagainstlocaldeadlocks.needadditionalmechanismstoensureglobalserializability38Two-LevelSerializabilityDBMSensureslocalserializabilityamongitslocaltransactions,includingthosethatarepartofaglobaltransaction.Themultidatabaseensuresserializabilityamongglobaltransactionsalone-ignoringtheorderingsinducedbylocaltransactions.2LSRdoesnotensureglobalserializability,however,itcanfulfillrequirementsforstrongcorrectness.1.Preserveconsistencyasspecifiedbyagivensetofconstraints2.GuaranteethatthesetofdataitemsreadbyeachtransactionisconsistentGlobal-readprotocol

:Globaltransactionscanread,butnotupdate,localdataitems;localtransactionsdonothaveaccesstoglobaldata.Therearenoconsistencyconstraintsbetweenlocalandglobaldataitems.39Two-LevelSerializability(Cont.)Local-readprotocol:Localtransactionshavereadaccesstoglobaldata;disallowsallaccesstolocaldatabyglobaltransactions.Atransactionhasavaluedependencyifthevaluethatitwritestoadataitematonesitedependsonavaluethatitreadforadataitemonanothersite.Forstrongcorrectness:Notransactionmayhaveavaluedependency.Global-read-write/local-readprotocol;Localtransactionshavereadaccesstoglobaldata;globaltransactionsmayreadandwritealldata;Noconsistencyconstraintsbetweenlocalandglobaldataitems.Notransactionmayhavevaluedependency.40GlobalSerializabilityEvenifnoinformationisavailableconcerningthestructu