LLT V11.docx

LLT V11.docx

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LLTV11

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Introduction1

EnhanceTechnology2

Sagas2

AltruisticLocking2

ApplicationDevelopment5

1.ConTracts5

3.Workflowcomputing6

4.Connectingactivitiesbymessaging6

Work-flowandTask-graph9

Conclusions10

Reference:

11

Introduction

ALong-livedtransaction（LLT）isatransactionthatholdstheresourceofthedatabaseforalongperiodoftimesuchashoursordays.Ithasalongdurationcomparedtoothertransactionsbecauseofaccessingmanydatabaseobjects,computingalongoperationofdatabaseandwaitingforuser’sinput.

Inmostcases,LLTpresentsaseriousproblemoftheperformanceintransactionprocess.Whenitlocksthewholedatabasebeforerunningthetransaction,noothertransactioncanrun.Asaresult,itholdsalotofresourcesfortoolong,delaystheterminationofashorterandcommontransactionsanddecreasesoverallthroughput.Also,LLTincreasesthefrequencyofsystemdeadlockandresultsahigherprobabilityintransactionfailures.Similarly,italsocreateschallengesofconcurrencycontrolandscalability.Forsolvingtheseproblems,therearetwostrategiesintroducedtoalleviateit.OneapproachistocutLLTintoalotofshortsub-transactionssothattheydon’tholdthatmanyresources.AnotherapproachistorequestLLTtodonatetheresourcesinordertoavoidtheresourcesbeingheldtoolong.

IntermsofcuttingLLTintoshortsub-transactions,oneapproachisSagas[1]thatreferstoaLLTthatcanbebrokenupintoacollectionofsub-transactionsthatcanbeinterleavedinanywaywithothertransactions.ToimplementtheSagas,theSagasmanagementsystemisrequiredandsomefunctionsarerequired.TheyareusedtoinformthesystemforrunningSaga.InRecoverystrategy,Sagausesbackwardrecoveryandforwardrecoveryforsolvingthesystemcrashed.

Intermsofdonatingtheresources,oneapproachisAltruisticlocking[2]thatisanextensiontoTwoPhaseLocking（2PL）.ItprovidesaDonateoperationforconcurrencycontrolandallowsLLTtoreleasetheresourcesearly.Donateisusedtoinformtheschedulerthataccesstoanobjectisnolongerrequiredbythelockingtransaction.OthertransactionsthataccessthereleaseddataarecalledtoruninthewakeoftheLLTandspeciallockingrulesmustbefollowed.Altruisticlockingiseasytoimplementandguaranteesserializability.

EnhanceTechnology

Sagas

AsagareferstoaLLTthatisbrokenupintoacollectofsub-transactionssothatitcanbeinterleavedinanywaywithothertransactions.Eachsub-transactioninthiscaseisarealtransactioninthesensethatitpreservesdatabaseconsistency.Asagaislikeanestedtransactionexceptthatitonlypermitstwolevelsofnestingthetoplevelsagaandsimpletransactionsandtheouterlevelfullatomicityisnotproved,sosagasmayviewthepartialresultsofothersagas.

Intheimplementationofasagaprocessingmechanism,therearesomeapproachesthatareuserfacilities,codereliablyandrecovery.Theyareshownasfollows:

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Inuserfacilities,whenanapplicationprogramwishestoinitiateasaga.Itissuesabegan-sagacommandtothesystemthatisfollowedbyaseriesofbegin-transaction,end-transactioncommandsthatindicatetheboundariesofeachtransaction.Inaddition,anabort-transactioncommandcanbeusedforterminatingthecurrenttransaction,butnotthesaga.Similarly,anabort-sagacommandisusedtoabortfirstthecurrentlyexecutingtransactionandsecondtheentiresaga（byrunningcompensatingtransactions）.Finally,anend-sagacommandisusedtocommitthecurrentlyexecutingtransactionandtocompletethesaga.

Incodereliably,themissingtransactionsorthecompensatingtransactionsareneededtoabortthesagaforcompletingarunningsagaafteracrash.Theapplicationcodeisrequiredforsagasbedefinedandupdatedinthesamefashion.Inrecovery,whenafailureinterruptsasaga,therearetwochoicescompensatefortheexecutedtransactionsthattheyarebackwardrecoveryandforwardrecovery.Forbackwardrecovery,thesystemneedscompensatingtransactions,forforwardrecovery,thesystemneedssave-points.

AltruisticLocking

AltruisticLockingwasoriginallypresentedinSalemetal.anditisamodificationtoTwoPhaseLocking（2PL）thatseveraltransactionsmayholdlocksonanobjectsimultaneouslyundercertainconditions.AltruisticlockingprovidesaDonateoperationforconcurrencycontrol.Donateisusedtoinformtheschedulerthataccesstoanobjectisnolongerrequiredbythelockingtransaction.WhenDonateisused,thedonatingtransactionfreelycontinuestoacquirenewlocksbecauseDonateandLockoperationsneednotbetwo-phase.

Donateoperationsarebeneficialbecausetheymaypermitothertransactionstolockthedonatedobjectbeforeitisunlocked.Analtruisticschedulershouldnotallowarbitraryaccesstoanobjectthathasbeendonatedbutnotunlocked.TherearetworulesforgoverningtheDonationoperationshownasfollows:

Rule1:

Twotransactionscannotholdconflictinglocksonthesameobjectsimultaneouslyuntiloneofthetransactionsdonatestheobjectfirst.

Rule2:

IfatransactionT1locksanobjectthathasdonatedbyanothertransactionT2,itcanbecalledinthewakeofthedonatingtransaction.WhenatransactionT1isinthewakeofanothertransactionT2andthenitmustbecompletelyinthewakeofT1untilT2performsitsfirstUnlockoperation.

Thealtruisticlockingrules（1and2）guaranteethatcommittedtransactionsareserialized,buttheydonotguaranteethattransactionswillbefailure-atomic.Toavoidthisproblem,strictaltruisticlockingthattheappropriaterestrictionisthatwritelockscanneitherbedonatednorunlockeduntilthelockingtransactionhascommitted.Altruisticlockingschedulersproduceastrictandserializableschedule.Togetherwithappropriaterecoveryfacilitiessuchasloggingandcheckpointing,strictaltruisticlockingguaranteesfullyatomictransactionexecution.However,itdoesnotbeverybeneficialwhenlong-livedtransactionsconsistprimarilyofupdateoperations.However,itmayprovideidealapproachwhenalongtransactionconsistsprimarilyofreadoperationwithafewupdates.Therearethreealternativestoaltruisticlockingshownasfollows:

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●Onealternativeistouseregularlocking,butdelaythecommitofindebtedtransactionsuntilthedonatingtransaction（s）hascommitted.Althoughthissolutionmayallowmoretransactionconcurrency,itmaystillresultinlengthycommitdelays.Furthermore,itintroducescascadingrollbacks,sinceanabortofonetransactionmaynecessitatetheabortsofothers.

●Anotheralternativeistoimplementsave-pointswithintransactionsthatcreatewakes.Whensuchatransactionpassesasave-point,updatesithasmadesofararecommitted;afutureabortofthetransactionwillonlyrollbackupdatesmadeafterthemostrecentsave-point.Writelocksbehindsave-pointscanbedonatedwithoutfearofcascadingrollbacks.Adrawbackofthistechniqueisthatitcompromisesthefailureatomicityofthetransactioncontainingthesave-points.Afterasavepointatransactioncannotissueaself-abortcommandtorollbacktheentiretransaction.

●Afinalalternativeistoimplementatypeof“conditional”donationofupdatedobjects.Suchconditionaldonationswouldpermitothertransactionstowritethedonatedobjects,butnottoreadthem.Thistechniqueintegratesthenotionofrecoverabilitywithaltruisticlocking.Unfortunately,itisnotacompletesolutiontotheproblemoffailureatomicity.Inaread/writedatabase,onlytransactionsthatperform“blindwritesondonatedobjectscanbenefitfromit.

Intransactionconcurrency,severaltypesofnon-lockingconcurrencycontrolshavealsobeendeveloped.Inparticular,thosebasedonserializationgraphtesting（SGT）canscheduleanyserializablehistory.Despitethisadvantage,lockingprotocolsaremorefrequentlyused.ThismaybebecauselockingprotocolsaresimplertoimplementandmoreefficientthanSGT,whichinvolvesgraphtraversaleachtimeatransactionattemptstocommit.

Locking-basedprotocolsmustperformsomeformofdeadlockdetectionorprevention,whichmayinvolvetraversingawaits-forgraph.However,thewaits-forgraphmustbecheckedmuchlessfrequentlythanaserializationgraph.Inparticular,checksarenotrequiredateachtransaction’scommitpoint.SGTprotocolsmaybeparticularlyinefficientfordistributedschedulersinwhichtheserializationgraphmaybedistributedacrossmultiplesites.

Concurrencycontrolsbasedontimestampordering（TO）havealsobeendeveloped.Unfortunately,TOisnotwellsuitedtolong-livedtransactions.Sincelongtransactionswilltendtohaverelativelyoldtimestamps,theyarepronetoabortbecauseofconflictswithnewtransactionswithlargetimestamps.

Forimprovingthetransactionconcurrency,EnlargingaWakeandAutomaticWakeExpansionareused.InEnlargingaWake,atransactioncanincreasethesizeofitswakeandthenmoretransactionsruninthewakethatcanavoidblocking.InAutomaticWakeExpansion,TheMarkoperationoperatesonasingledatabaseobjectanditprovidestheschedulerwiththenecessaryinformation.Transactionsthatcreatewakesinformtheschedulerwhichdatabaseobjectstheyrequireaccessto.

ApplicationDevelopment

InadditiontocuttingtheLLTintoshortsub-transactionsanddonatingtheresources,therearesomeapproachesimplementedformanagingLLT.TheyareConTracts,Implementingadvancedtransactionsmodelswithobjects,Workflowcomputing,ConnectingactivitiesbymessagingandAgent-orientedModelbasedonWork-flowandTask-graph

1.ConTracts[5]

AConTractisatwo-leveltransac