1 distributed transaction system based

1、Distrib.Syst.Engng2(1995)5C-64.PrintedintheUKIThedesignandimplementationofa1distributedtransactionsystembasedIonatomicdatatypesZWuts,RJStroudtll,KMoodySTandJBaconl+fDepartmentofComputingScience,UniversityofNewcastleuponTyne,UK$ComputerLaboratory,UniversityofCambridge,UKAbstract.Thecomplexityofpotentialinteractionsamongconcurrentactivitiesandthemultitudeoffailuremodesthatcanoccurindistributedsystemsmakeithardtoreasonaboutdistributedprograms.Transactionsprovidetheprogrammerwithamechanismthatsimpl。

2、ifiesthedevelopmentofconcurrentanddistributedprograms.Inthispaperwepresentthedesignandimplementationofadistributedtransactionsystemthatusesatomicdatatypestoprovidesynchronizationandrkovely.Generallyspeaking,implementinguser-definedatomicdatatypesisadifficulttask.However,unlikeexistingsystems,oursystemrequiresprogrammerstodoverylittleextraworktomakeanobjectatomic.Programmersimplementatomicdatatypesasifforasequentialandreliableenvironmentandspecifytheconcurrentsemanticsofobjectoperationsseparately。

3、inasmall,butexpressivedeclarativelanguage.Appropriatesynchronizationandrecovelycodeforator$datatypesisgeneratedautomaticallybythesystemaccordingtothisinformation.1.IntroductionDistributedcomputingsystemsarepotentiallymorereliablethancentralizedsystemsbecausetheredundancyandautonomypresentinadistributedcomputingenvironmentpermitsfailurestobemaskedorlocalized.However,thecomplexityofpotentialinteractionsamongconcurrentactivitiesandthemultitudeoffailuremodesthatcanoccurindistributedsystemsmakeithard。

4、toreasonaboutdistributedprograms.Transactionshavebeenexploredasawayofresolvingthisissue.Transactionshelpbyallowingtheprogrammertoviewacomplexpieceofcodeasatomic:itappearstohappeninstantaneously,andithappenseithercompletelyornotatall.Inprinciple,transactionsshouldbeusefulforsimplifyingtheconstructionofreliabledistributedsystems.However,thetraditionaltransactionmodel,asdescribedbyGray[l],wasdesignedprimarilyforunderstandingdatabasemanagementapplicationswherethebasicunitofinformationisthetypedrecor。

5、dandtheonlyoperationsonrecordsarereadandwrite.Inmoregeneralsystems,informationisrepresentedbyabstractdatatypeswithtype-specificoperations,andforthisreasonmanyresearchershavesuggestedusingatomicdatatypes[Z]toimplementtransactions.Thisapproachfocusesontheshareddataobjectsthatprogrammersuseinconstructingtransactions.$E-mail:Zhixue.Wu@ncl.ac.ukI/E-mail:R.J.Stroud@ncl.ac.uk7E-mail:km@cl.cam.ac.uk+E-mail:jmb@cl.cam.ac.uk.Atomicdatatypesareabstractdatatypesthatalsohavesynchronization&drecoverypropertie。

6、s.Instancesofatomicdatatypes,calledatomicobjects,areresponsibleforensuringtheirownserializabilityandrecoverability.Ifalloftheshareddataobjectsaccessedbyconcurrenttransactionsareatomic,thenthetransactionscanbemadeserializableandrecoverablewithouttheneedforthetransactionmanagertotakeanyconcurrencycontrolmeasures.Theatomicdatatypeapproachmakesitpossibletopermitgreaterconcurrencyamongactivitiesbyusingtypespecificsemanticsofthesharedobjects.Inaddition,byencapsulatingsynchronizationandrecoverycodeinth。

7、eimplementationofthesharedobjects,modularitycanbeenhanced.Generallyspeaking,implementingatomicdatatypesisadifficuJttask.Thisisbecauseatomicdatatypesdefinethebehaviourofobjectsinaconcurrentandunreliableenvironment.Thus,inadditiontoimplementingtheoperationsofanormalabstractdatatype,anatomicdatatypealsoneedstoimplementsynchronizationgndrecoveryoperations.Therearethreekindsofapproachestoimplementingatomicdsrtatypes:implicit,explicitandhybrid,accordingtowhetherthesystemortheprogrammerisresponsiblefor。

8、implementingthesynchronizationandrecoverycode.Withanimplicitapproach,exceptforspeci@ingtheconcurrentsemanticsofobjectoperations,the,programmerisonlyresponsibleforimplementingthebasicobjectoperationsasifforasequentialreliableenvironmentwithnoconcurrencyorfailure.ThesystemisresponsibleforimpJementingthenecessarysynchronizationandrecoverycodeusingtheknowledgeabouttheobject's0967-1846/95/010050+15$19.5001995TheBritishComputerSociety,TheInstitutionofElectricalEngineersandIOPPublishingLtdDistributedtr。

9、ansactionsbasedonatomictypesPCt+todemonstratethefeasibilityofthisapproach.Theapplicationisconcernedwithmaintainingthedatabaseforanactivebadgesystem.Finally,section6concludesthepaperwithabriefsummaryanddiscussionoffuturework.2.PC++programmingmodelInthissection,wepresentthewayinwhichapplicationprogrammersdefineatomicdatatypesandconstructtransactionsinPCtt.SincePCt+takesanimplicitapproachtoimplementingatomicdatatypes,theonlydifferencebetweendefininganatomicdatatypeanddefininganormalabstractdatatype。

10、fromaprogrammer'spointofviewistheneedtospecifytheconcurrentsemantics.ofoperations.Thiswillbethemaintopicdiscussedinthissection.concurrentsemanticsprovidedbytheprogrammer.Withanexplicitapproach,theprogrammerisresponsibleforimplementingboththebasicobjectoperationsandthesynchronizationandrecoverycode.Withahybridapproach,theworkofimplementingthesynchronizationandrecoverycodeissharedbythesystemandtheprogrammer.Thereareanumberofsystemsthatsupporttransactionsusingatomicdatatypes.ExamplesincludeArgus[Z],。