OCI-based Group Communication Support in

IEEETRANSACTIONSONPARALLELANDDISTRIBUTEDSYSTESMS,20041Abstract—Groupcommunicationisausefulmechanismguaranteeingtheconsistencyamongreplicatedobjects.Theexistingapproachesdonotallowtransparentplug-inofgroupcommunicationprotocolsintoCORBA.TheyeitherrequiremodificationofCORBAorOS,orprovidenoroomforincorporatinggroupcommunicationtransportprotocolsintoCORBA.Weproposeagenericgroupcommunicationframeworkthatallowstransparentplug-inofvariousgroupcommunicationprotocolswithnomodificationofexistingCORBA.WeextendtheOpenCommunicationsInterface(OCI)tosupportinteroperability,reusabilityofexistinggroupcommunication,andindependencyonORBandOS.WedefineGroupCommunicationInter-ORBProtocol(GCIOP)asagroupcommunicationinstantiationofGeneralInter-ORBProtocol(GIOP)thatencapsulatesunderlyinggroupcommunicationprotocols.TheproposedschemecanbeexploitedforfaulttolerantCORBA(FTCORBA).Experimentresultsshowthatgroupobjectinvocationusingtheproposedschemedoesnotincurperformancedegradationasthenumberofmembersinagroupincreases.IndexTerms—Groupcommunication,CORBA,Opencommunicationsinterface,FTCORBA.I.INTRODUCTIONbjectreplicationisatechniquetoenhancefaulttoleranceandhighavailability[7].Anobjectgroupisacollectionofobjectreplicasthatcooperativelyworksforacommontask[10].Hereconsistentstateofanobjectgroupneedstobemaintainedforconstructinghighavailableandfault-tolerantdistributedapplications.Groupcommunicationservice(GCS)isausefulmechanismguaranteeingtheconsistencyofthestatesofallthememberobjects.Itmaintainsaview,alistofthecurrentlyactiveandconnectedmembersinanobjectgroup,andalsoinformstherunningobjectsoftheupdatedviewwheneveritchanges.Theconsistencycanbeguaranteedbyreliabledeliveryofmessagestothemembersinthecurrentview.TherehavebeenseveralapproachesforsupportinggroupcommunicationserviceinCORBA.Ingeneral,theycanbecategorizedintothreeapproaches–integration,service,andinterceptionapproach.AnexampleofeachofthethreeapproachesisElectra[15],ObjectGroupService(OGS)[4],andEternal[19,22],ManuscriptreceivedDecember8,2002.ThisworkwassupportedinpartbytheNationalResearchLaboratoryProgramfundedbyMinistryofScienceandTechnology,theRepublicofKoreaunderGrant2EG1900.Apreliminaryversionofthispaperappearedin[14]and[21].D.Lee,D.Nam,andC.YuarewiththeSchoolofEngineering,InformationandCommunicationsUniversity,58-4Hwaam-dong,Yuseong-gu,Daejeon,305-732Korea.Tel.:[phone],Fax:[phone](e-mail:{dlee,paichu,cyu}@icu.ac.kr).H.Y.YouniswiththeSchoolofElectricalandComputerEngineering,SungkyunkwanUniversity,300Chunchun-dong,Jangan-gu,Suwon440-746Korea.(e-mail:[e-mail]).OCI-basedGroupCommunicationSupportinCORBADongmanLee,Member,IEEEComputerSociety,DukyunNam,HeeYongYoun,SeniorMember,IEEE,andChansuYu,Member,IEEEOIEEETRANSACTIONSONPARALLELANDDISTRIBUTEDSYSTESMS,20042respectively.Intheintegrationapproach,theGCSmoduleisanintegralpartofObjectRequestBroker(ORB),andthusintroducinganewGCSrequiresmodificationofORB.TheserviceapproachprovidesgroupcommunicationasObjectService[5]ontopofCORBA,leveragingtheCORBA’spoint-to-pointcommunications,GeneralInter-ORBProtocol(GIOP).Thatis,noroomexistsforgroupcommunicationtransportprotocolstobeincorporatedintoCORBA.Theinterceptionapproachsupportsthegroupcommunicationservicebyinterceptingthesystemcallsrelatedtogroupcommunicationsusinganinterceptor.ItdoesnotallowCORBAobjectstodirectlyexploittheunderlyinggroupcommunicationservicessincetheinterceptorisnotpartofCORBA.Insummary,theexistingapproachesdonotsupporttransparentplug-inofgroupcommunicationprotocolsintoCORBA,andthusCORBAapplicationprogrammerscannotdirectlyexploittheprotocols.Theremustbeagenericgroupcommunicationframeworkthatallowstransparentplug-inofvariousgroupcommunicationprotocolsviaastandardCORBAinterface.InthispaperweproposeamechanismthatallowssuchframeworkwithnomodificationofexistingCORBA.WeleveragetheOCI(OpenCommunicationsInterface)[1]tosupportinteroperability,reusabilityofexistinggroupcommunication,andindependencyonORBandOS.WeaddnewoperationsintotheexistingOCIbywhichanobjectgroupcanbemanagedandinvocationsaremadetothegroup.WedefineGroupCommunicationInter-ORBProtocol(GCIOP)asagroupcommunicationinstantiationofGeneralInter-ORBProtocol(GIOP)thatencapsulatesunderlyinggroupcommunicationprotocols.WethenaddgroupcommunicationInfoObjecttotheOCIandextendtheinterfacesoftheOCItosupportgroupsemantics.TheexistingOCIconsistsofaConnector,ConnectorFactory,Acceptor,andTransportmodule[1,10].Theexistinggroupcommunicationbasicallyrequiresgroupaddressexpansion,deliveryordering,andstatetransfer[2].Tosatisfytheserequirementsofgroupcommunication,agroupname,orderingtype,andstatearesavedintheInfoobject.WeaddanattributetoConnector’sTransportInfoforsettingtheordering-typeandaddgroupmaintenanceoperationstoAcceptor.TheproposeddesigncanbeusedtowraporembedvariousgroupcommunicationprotocolswithoutanymodificationofORBandtheOCI.WedeviseagroupInteroperableObjectReference(IOR)forGCIOPinordertosupportthenon-FTCORBAcompliantsystemswhichdonotsupportgroupobjectreferences.Theproposedschemecanbeexploitedforthefaulttolerantinfrastruc