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1、StatefulMulticastServicesforSupportingCollaborativeApplicationsHyongSopShim,RobertW.Hall,RaduLitiuandAtulPrakashDepartmentofElectricalEngineeringandComputerScience,UniversityofMichigan,AnnArbor,MI48109-2122USAE-mail:hyongsop,rhall,radu,aprakash@eecs.um
2、ich.eduAbstractCollaborative,multi-userapplicationsrequiregroupmulticastserviceswithorderingguaranteesformain-tainingconsistencyofreplicatedsharedstateamongcollaboratingprocesses.Intraditionalgroupmulticastservices(suchasIsis),thegroup'ssharedstateisma
3、intainedonlybytheclientsandthemulticastservicefacilitatesatomicstatetransferfromexistingclientstonewmembers.Inthispaper,wearguethat,inordertosupportcollaborativeapplicationsinInternet-typeenvironments,acopyofthegroup'sstateshouldalsobemaintainedbythemu
4、lticastservice.Weshowthatbymaintainingacopyofgroup'sstate,themulticastservicecanprovideconsistentlyfastgroup-joinandstatetransfertimewhenbothslowandfastclientsarepresentorwhenclientsareunreliable
5、acrucialrequirementincollaborative,multi-userapplication
6、swhereusersmaydynamicallyjoinandleaveacollaborativesessionandexpectpredictablejointimesandinteractiveresponsetimeeveninthepresenceofsloworunreliableclients.Weshowthattheoverheadsincurredbyamulticastserviceinmanagingeachgroup'ssharedstatecanbemademinima
7、landthatthemulticastservicedoesnothavetobeawareofthesemanticsofthegroup'sstate.Wepresentthedesignofsuchamulticastservice,presentperformanceresults,anddiscusshowitmeetsthevariousneedsofcollaborativeapplications.1IntroductionAswithtraditionalsystemsfordi
8、stributedcomputingsuchasdistributedoperatinganddatabasesystems,computer-supportedcollaborativesystemsenablemultipleuserstosharedata,bothsynchronouslyandasyn-chronously.However,traditionaldistributedsystemsstrivetoprovidetheillusionofworkingalone,andthu
9、sthedatasharingisprovidedastransparentlyaspossible[7].Thegoalofacollaborativesystemistoempowergeo-graphicallydisperseduserstoe ectivelyworktogetheroverdistance.Thusthesharingofdataismadeapparentincollaborativesystems,andthemechanicsofda
