ArchitectureOverviewJimWaldoAJinisystemisaJavacentricdistributedsyste - PDF document

1 ArchitectureOverviewJimWaldoAJinisystemi
ArchitectureOverviewJimWaldoAJinisystemisaJava-centricdistributedsystemdesignedforsimplicity,flexibility,andfederation.TheJiniarchitectureprovidesmechanismsformachinesorprogramstoenterintoafederationwhereeachmachineorprogramoffersresourcestoothermembersofthefederationandusesresourcesasneeded.ThedesignoftheJiniarchitectureexploitstheabilitytomoveJavalanguagecodefrommachinetomachineandunifiesunderthenotionofaeverythingfromtheuserofaJinisystemtothesoftwareavailableonthemachinestothehardwarecomponentsofthemachinesthemselves. ArchitectureOverview 901 San Antonio Road, Palo Alto, CA 94303 U.S.A.All rights reserved. Copyright in this document is owned by Sun Microsystems, Inc.SunMicrosystems,Inc.(SUN)herebygrantstoyouatnochargeanonexclusive,nontransferable,worldwide,limitedlicense(withouttherighttosublicense)underSUNÕsintellectualpropertyrightsthatareessentialtousethisSpecificationforinternalevaluationpurposesonly.Otherthanthislimitedlicense,youacquirenoright,title,orinterestinortotheSpecificationandyoushallhavenorighttousetheSpecificationforproductiveorUse,duplication,ordisclosurebytheU.S.GovernmentissubjecttorestrictionsofFAR52.227-14(g)(2)(6/87)and FAR 52.227-19(6/87), or DFAR 252.227-7015(b)(6/95) and DFAR 227.7202-1(a).ThissoftwareanddocumentationistheproprietaryinformationofSunMicrosystems,Inc.YoushalluseitonlySUNMAKESNOREPRESENTATIONSORWARRANTIESABOUTTHESUITABILITYOFTHESOFT-WARE,EITHEREXPRESSORIMPLIED,INCLUDINGBUTNOTLIMITEDTOTHEIMPLIEDWAR-RANTIES

2 OFMERCHANTABILITY,FITNESSFORAPARTICULARP
OFMERCHANTABILITY,FITNESSFORAPARTICULARPURPOSE,ORNON-INFRINGEMENT.SUNSHALLNOTBELIABLEFORANYDAMAGESSUFFEREDBYLICENSEEASA RESULT OF USING, MODIFYING OR DISTRIBUTING THIS SOFTWARE OR ITS DERIVATIVES.Sun,theSunlogo,SunMicrosystems,JavaSoft,JavaBeans,JDK,Java,HotJava,HotJavaViews,VisualJava,Solaris,NEO,Joe,Netra,NFS,ONC,ONC+,OpenWindows,PC-NFS,EmbeddedJava,PersonalJava,SNM,SunNetManager,Solarissunburstdesign,Solstice,SunCore,SolarNet,SunWeb,SunWorkstation,TheNet-workIsTheComputer,ToolTalk,Ultra,Ultracomputing,Ultraserver,WhereTheNetworkIsGoing,SunWorkShop,XView,JavaWorkShop,theJavaCoffeeCuplogo,andVisualJavaaretrademarksorregisteredTHISPUBLICATIONISPROVIDEDÒASISÓWITHOUTWARRANTYOFANYKIND,EITHEREXPRESSORIMPLIED,INCLUDING,BUTNOTLIMITEDTO,THEIMPLIEDWARRANTIESOFMERCHANTABILITY,FITNESSFORAPARTICULARPURPOSE,ORNON-INFRINGEMENT.THISPUBLICATIONCOULDINCLUDETECHNICALINACCURACIESORTYPOGRAPHICALERRORS.CHANGESAREPERIODICALLYADDEDTOTHEINFORMATIONHEREIN;THESECHANGESWILLBEINCORPORATEDINNEWEDITIONSOFTHEPUBLICATION.SUNMICROSYSTEMS,INC.MAYMAKEIMPROVEMENTSAND/ORCHANGESINTHEPRODUCT(S)AND/ORTHEPRO-GRAM(S) DESCRIBED IN THIS PUBLICATION AT ANY TIME. Page3 The technology disclosed herein may be covered by patents or patents pending IntroductionThisdocumentdescribesthehighlevelarchitectureofaJinisoftwaresystem,definesthedifferentcomponentsthatmakeupthesystem,characterizestheuseofthosecomponents,anddiscussessomeofthecomponentinteractions.Thisdocumentidentifiesthosepartsofthesyst

3 emthatarenecessaryinfrastructure,thoseth
emthatarenecessaryinfrastructure,thosethatarepartoftheprogrammingmodel,andthosethatareoptionalserviceswhichcanlivewithinthesystem.Thisdocumentalsodiscussesthereasonsbehindparticulardesignchoices.1.1GoalsoftheSystemAJinisystemisadistributedsystembasedontheideaoffederatinggroupsofusersandtheresourcesrequiredbythoseusers.Theoverallgoalistoturnthenetworkintoaflexible,easilyadministeredtoolonwhichresourcescanbefoundbyhumanandcomputationalclients.Resourcescanbeimplementedaseitherhardwaredevices,softwareprograms,oracombinationofthetwo.Thefocusofthesystemistomakethenetworkamoredynamicentitythatbetterreflectsthedynamicnatureoftheworkgroupbyenablingtheabilitytoaddanddeleteservicesflexibly.AJinisystemconsistsofthefollowingparts:asetofcomponentsthatprovideaninfrastructureforfederatingservicesinadistributedsystemaprogrammingmodelthatsupportsandencouragestheproductionofreliabledistributedservices Page4ArchitectureOverview servicesthatcanbemadepartofaJinifederationandwhichofferfunctionalitytoanyothermemberofthefederationWhilethesepiecesareseparableanddistinct,theyareinterrelated,whichcanblurthedistinctioninpractice.ThecomponentsthatmakeuptheJiniinfrastructuremakeuseoftheJiniprogrammingmodel;servicesthatresidewithintheinfrastructurealsousethatmodel;andtheprogrammingmodeliswellsupportedbycomponentsintheinfrastructure.Theendgoalsofthesystemspananumberofdifferentaudiences;thesegoalsincludethefollowing:enablinguserstoshareservicesandresourcesoveranetworkprovidinguser

4 seasyaccesstoresourcesanywhereonthenetwo
seasyaccesstoresourcesanywhereonthenetworkwhileallowingthenetworklocationoftheusertochangeprovidingprogrammerswithtoolsandprogrammingpatternswhichallowthedevelopmentofrobustandsecuredistributedsystemssimplifyingthetaskofbuilding,maintaining,andalteringanetworkofdevices,software,andusers.TheJinisystemextendstheJavaapplicationenvironmentfromasinglevirtualmachinetoanetworkofmachines.TheJavaapplicationenvironmentprovidesagoodcomputingplatformfordistributedcomputingbecausebothcodeanddatacanmovefrommachinetomachine.Theenvironmenthasbuilt-insecuritythatallowstheconfidencetoruncodedownloadedfromanothermachine.StrongtypingintheJavaapplicationenvironmentenablesidentifyingtheclassofanobjecttoberunonavirtualmachineevenwhentheobjectdidnotoriginateonthatmachine.Theresultisasysteminwhichthenetworksupportsafluidconfigurationofobjectswhichcanmovefromplacetoplaceasneededandcancallanypartofthenetworktoperformoperations.TheJiniarchitectureexploitsthesecharacteristicsoftheJavaapplicationenvironmenttosimplifytheconstructionofadistributedsystem.TheJiniarchitectureaddsmechanismsthatallowfluidityofallcomponentsinadistributedsystem,extendingtheeasymovementofobjectstotheentirenetworkedsystem. Page5 TheJiniinfrastructureprovidesmechanismsfordevices,services,anduserstojoinanddetachfromanetwork.JoiningintoandleavingaJinigroupingisaneasyandnatural,oftenautomatic,occurrence.Jinigroupsarefarmoredynamicthaniscurrentlypossibleinnetworkedgroupswhereconfiguringanetworkisacentralizedf

5 unctiondonebyhand.1.2EnvironmentalAssump
unctiondonebyhand.1.2EnvironmentalAssumptionsTheJinisystemfederatescomputersandcomputingdevicesintowhatappearstotheuserasasinglesystem.ItreliesontheexistenceofanetworkofreasonablespeedconnectingthosecomputersanddevicesÑ10mbpsinthegeneralcase.SomedevicesrequiremuchhigherbandwidthandotherscandowithmuchlessÑdisplaysandprintersareexamplesofextremepoints.Weassumethelatencyofthenetworkisreasonable,measured,atmost,insecondsratherthanminutes.WeassumethateachJini-connecteddevicehassomememoryandprocessingpower.DeviceswithoutprocessingpowerormemorymaybeconnectedtoaJinisystem,butthosedevicesarecontrolledbyanotherpieceofhardwareand/orsoftware,calledaproxy,thatpresentsthedevicetotheJinisystemandwhichitselfcontainsbothprocessingpowerandmemory.ThearchitecturefordevicesnotequippedwithaJavavirtualmachineisdiscussedmorefullyinaseparatedocument.TheJinisystemisJava-technologycentered.TheJiniarchitecturegainsmuchofitssimplicityfromassumingthattheJavaprogramminglanguageistheimplementationlanguageforcomponents.TheabilitytodynamicallydownloadandruncodeiscentraltoanumberofthefeaturesoftheJiniarchitecture.However,theJava-centricnatureoftheJiniarchitecturedependsontheJavaapplicationenvironmentratherthanontheJavaprogramminglanguage.AnyprogramminglanguagecanbesupportedbyaJinisystemifithasacompilerthatproducescompliantbytecodesfortheJavaprogramming1.3RelatedDocumentsThisdocumentdoesnotprovideafullspecificationoftheJinisystem.EachoftheJinicomponentsisspecifiedinacompaniondocum

6 ent.Inparticular,thereaderisdirectedtoth
ent.Inparticular,thereaderisdirectedtothefollowingdocuments(note:Pleasecheckforavailability): Page6ArchitectureOverview TheJavaRemoteMethodInvocationSpecificationTheJavaObjectSerializationSpecificationTheJiniDiscoveryandJoinSpecificationTheDistributedEventSpecificationTheDistributedLeasingSpecificationTheTransactionSpecificationTheJavaSpacesªSpecificationTheJiniLookupServiceSpecificationTheJiniDeviceArchitectureSpecification Page7 The technology disclosed herein may be covered by patents or patents pending SystemOverview2.1KeyConceptsThepurposeoftheJiniarchitectureistogroupsofdevicesandsoftwarecomponentsintoasingle,dynamicdistributedsystem.Theresultingfederationprovidesthesimplicityofaccess,easeofadministration,andsupportforsharingthatareprovidedbyalargemonolithicsystemwhileretainingtheflexibility,uniformresponse,andcontrolprovidedbyapersonalcomputerorworkstation.ThearchitectureofasingleJinisystemistargetedtotheworkgroup,whichcanrangeinsizefromtwoorthreeusersinsmallofficesorhomestogroupsofupto1,000.Membersofthefederationareassumedtoagreeonbasicnotionsoftrust,administration,identification,andpolicy.ItispossibletofederateJinisystemsthemselvesforlargerorganizations.2.1.1ServicesThemostimportantconceptwithintheJiniarchitectureisthatofaserviceisanentitythatcanbeusedbyaperson,aprogram,oranotherservice.Aservicemaybeacomputation,storage,acommunicationchanneltoanotheruser,asoftwarefilter,ahardwaredevice,oranotheruser.Aservicemaybe Page8ArchitectureOvervie

7 w morespecificthanthis,forexampleprovidi
w morespecificthanthis,forexampleprovidingatranslationfromoneword-processorformattosomeotherortranslatingadocumentfromonelanguagetosomeother.MembersofaJinisystemfederateinordertoshareaccesstoservices.AJinifederationshouldnotbethoughtofassetsofclientsandservers,orusersandprograms,orevenprogramsandfiles.Instead,aJinifederationconsistsofservicesthatcanbecomposedtogetherfortheperformanceofaparticulartask.Servicesmaymakeuseofotherservices,andaclientofoneservicemayitselfbeaservicewithclientsofitsown.ThedynamicnatureofaJinisystemallowsservicestobeaddedorwithdrawnfromafederationatanytimeaccordingtodemand,need,orthechangingrequirementsoftheworkgroupusingit.Jinisystemsprovidemechanismsforserviceconstruction,lookup,communication,anduseinadistributedsystem.Examplesofservicesincludedevicessuchasprinters,displays,ordisks;softwaresuchasapplicationsorutilities;informationsuchasdatabasesandfiles;andusersofthesystem.ServicesinaJinisystemcommunicatewitheachotherbyusingaprotocol,whichisasetofJavainterfaces.Thesetofsuchprotocolsisopenended.ThebaseJinisystemdefinesasmallnumberofsuchprotocolswhichdefinecriticalserviceinteractions.Forsomeoftheseprotocolswewillprovideexampleimplementationsofservicesthatusethem.2.1.2LookupServiceServicesarefoundandresolvedbyalookupservice.Thelookupserviceisthecentralbootstrappingmechanismforthesystemandprovidesthemajorpointofcontactbetweenthesystemandusersofthesystem.Inpreciseterms,alookupservicemapsinterfacesindicatingthefunctionalityprov

8 idedbyaservicetosetsofobjectsthatimpleme
idedbyaservicetosetsofobjectsthatimplementtheservice.Inaddition,descriptiveentriesassociatedwithaserviceallowmorefine-grainedselectionofservicesbasedonpropertiesunderstandabletopeople.Objectsinalookupservicemayincludeotherlookupservices;thisprovideshierarchicallookup.Further,alookupservicemaycontainobjectsthatencapsulateothernamingordirectoryservices,providingawayforbridgestobebuiltbetweenaJinilookupserviceandotherformsoflookupservice.Of Page9 course,referencestoaJinilookupservicemaybeplacedintheseothernaminganddirectoryservices,providingameansforclientsofthoseservicestogainaccesstoaJinifederation.Aserviceisaddedtoalookupservicebyaprocesscalled,indicatingthatthepresenceoftheserviceisdiscoveredbytheJinisystem.2.1.3JavaRemoteMethodInvocation(RMICommunicationbetweenservicesisaccomplishedusingJavaRemoteMethod).Theinfrastructuretosupportcommunicationbetweenservicesisnotitselfaservicethatisdiscoveredandusedbutis,rather,apartoftheJiniinfrastructure.RMIprovidesmechanismstofind,activate,andgarbagecollectobjectgroups.RMIalsoprovidestheinfrastructureformulticast,replication,andthemechanismsforbasicsecurityandconfidentiality.Fundamentally,RMIisaJava-programming-language-enabledextensiontotraditionalremoteprocedurecallmechanisms.RMIallowsnotonlydatatobepassedfromobjecttoobjectaroundthenetworkbutfullobjects,includingcode.MuchofthesimplicityoftheJinisystemisenabledbythisabilitytomovecodearoundthenetworkinaformthatisencapsulatedasanobject.2.1.4SecurityTheJinisecu

9 ritymodelisbuiltonthetwinnotionsofaandan
ritymodelisbuiltonthetwinnotionsofaandancontrollist.JiniservicesareaccessedonbehalfofsomeentityÑtheprincipalÑwhichgenerallytracesbacktoaparticularuserofthesystem.Servicesthemselvesmayrequestaccesstootherservicesbasedontheidentityoftheobjectthatimplementstheservice.Whetheraccesstoaserviceisalloweddependsonthecontentsofanaccesscontrollistthatisassociatedwiththe2.1.5LeasingAccesstomanyoftheservicesintheJinienvironmentisbased.Aleaseisagrantofguaranteedaccessoveratimeperiod.Eachleaseisnegotiatedbetweentheuseroftheserviceandtheprovideroftheserviceaspartoftheserviceprotocol:Aserviceisrequestedforsomeperiod;accessisgrantedforsomeperiod,presumablytakingtherequestperiodintoaccount.Ifaleaseis Page10ArchitectureOverview notrenewedbeforeitisfreedÑeitherbecausetheresourceisnolongerneeded,theclientornetworkfails,ortheleaseisnotpermittedtoberenewedÑthenboththeuserandtheprovideroftheresourcecanconcludetheresourcecanbefreed.Leasesareeitherexclusiveornon-exclusive.Exclusiveleasesinsurethatnooneelsemayaccesstheresourceduringtheperiodofthelease;non-exclusiveleasesallowmultipleuserstosharearesource.2.1.6TransactionsAseriesofoperations,eitherwithinasingleserviceorspanningmultipleservices,canbewrappedina.TheJinitransactioninterfacessupplyaserviceprotocolneededtocoordinateatwo-phasecommit.HowtransactionsareimplementedÑandindeed,theverysemanticsofthenotionofatransactionÑisleftuptotheserviceusingtheinterfaces.2.1.7EventsTheJiniarchitecturesupportsdistributed.Anobjectm

10 ayallowotherobjectstoregisterinterestine
ayallowotherobjectstoregisterinterestineventsintheobjectandreceiveanotificationoftheoccurrenceofsuchanevent.Thisenablesdistributedevent-basedprogramstobewrittenwithavarietyofreliabilityandscalabilityguarantees.2.2ComponentOverviewThecomponentsoftheJinisystemcanbesegmentedintothreecategories:infrastructureprogrammingmodel,and.TheinfrastructureisthesetofcomponentsthatenablebuildingaJinifederation,whiletheservicesaretheentitieswithinthefederation.Theprogrammingmodelisasetofinterfacesthatenabletheconstructionofreliableservices,includingthosethatarepartoftheinfrastructureandthosethatjoinintothefederation.Thesethreecategories,thoughdistinctandseparable,areentangledtosuchanextentthatthedistinctionbetweenthemcanseemblurred.Moreover,itispossibletobuildJini-likesystemswithvariantsonthecategoriesorwithoutallthreeofthem.ButaJinisystemgainsitsfullpowerbecauseitisawiththeparticularinfrastructureandprogrammingmodelsdescribed,basedonthenotionofaservice.Decouplingthesegmentswithinthearchitecture Page11 allowslegacycodetobechangedminimallytotakepartinaJinifederation.Nevertheless,thefullpowerofaJinisystemwillbeavailableonlytonewservicesthatareconstructedusingtheintegratedmodel.AJinisystemcanbeseenasanetworkextensionoftheinfrastructure,programmingmodel,andservicesthatmadeJavatechnologysuccessfulinthesingle-machinecase.ThesecategoriesalongwiththecorrespondingcomponentsinthefamiliarJavaapplicationenvironmentareshowninFigure1.Figure1:JiniArchitectureSegmentation2.2.1Inf

11 rastructureTheJiniinfrastructuredefinest
rastructureTheJiniinfrastructuredefinestheminimalJinicore.Theinfrastructureincludesthefollowing:anextendedversionoftheJavaRemoteMethodInvocationsystem(RMI)ÑthisisthebasicmechanismofcommunicationbetweencomponentsinaJinisystemadistributedsecuritysystem,integratedintoRMI,whichextendstheJavaplatformÕssecuritymodeltotheworldofdistributedsystemsthediscoveryprotocol,aserviceprotocolthatallowsservices(bothhardwareandsoftware)todiscover,becomepartof,andadvertisesuppliedservicestotheothermembersofthefederationInfrastructureProgramming ModelServices Java VMRMIJava APIsJavaBeansJNDIEnterprise Beans... Two Phase CommitTwo Phase Commit Manager Page12ArchitectureOverview thelookupservice,whichservesasarepositoryofservices.EntriesinthelookupserviceareobjectsintheJavaprogramminglanguage;theseobjectscanbedownloadedaspartofalookupoperationandactaslocalproxiestotheservicethatplacedthecodeintothelookupserviceThediscoveryprotocoldefinesthewayaserviceofanykindbecomespartofaJinifederation;extendedRMIdefinesthebaselanguagewithinwhichtheJinicomponentscommunicate;thedistributedsecuritymodelanditsimplementationdefinehowentitiesareidentifiedandhowtheygettherightstoperformactionsontheirownbehalfandonthebehalfofothers;andthelookupservicereflectsthecurrentmembersofthefederationandactsasthecentralmarketplaceforofferingandfindingservicesbymembersofthe2.2.2ProgrammingModelTheinfrastructurebothenablestheprogrammingmodelandmakesuseofit.Entriesinthelookupserviceareleased,allowingthe

12 lookupservicetoreflectaccuratelythesetof
lookupservicetoreflectaccuratelythesetofavailableservicescurrently.Whenservicesjoinorleavealookupservice,eventsaresignaled,andobjectsthathaveregisteredinterestinsucheventsgetnotificationswhennewservicesbecomeavailableoroldservicesceasetobeactive.Theprogrammingmodelrestsontheabilitytomovecode,whichissupportedbythebaseinfrastructure.BoththeinfrastructureandtheservicesthatusethatinfrastructurearecomputationalentitiesthatexistinthephysicalenvironmentoftheJinisystem.However,servicesalsoconstituteasetofinterfaceswhichdefinecommunicationprotocolsthatcanbeusedbytheservicesandtheinfrastructuretocommunicatebetweenthemselves.Theseinterfaces,takentogether,makeupthedistributedextensionofthestandardJavaprogramminglanguagemodelthatconstitutetheJiniprogrammingmodel.AmongtheinterfacesthatmakeuptheJiniprogrammingmodelarethefollowing:theleasinginterface,whichdefinesawayofallocatingandfreeingresourcesusingarenewable,duration-basedmodeltheeventandnotificationinterface,whichisanextensionoftheJavaBeansªeventmodeltothedistributedenvironmentthatenablesevent-basedcommunicationbetweenJiniservices Page13 thetwo-phasecommit(ortransaction)interfaces,whichenableentitiestocooperateinsuchawaythateitherallofthechangesmadetothegroupoccuratomicallyornoneofthemoccurTheleaseinterfaceextendstheJavaprogramminglanguagemodelbyaddingtimetothenotionofholdingareferencetoaresource,enablingreferencestobereclaimedsafelyinthefaceofnetworkfailures.TheeventandnotificationinterfacesextendtheJavaBe

13 ansªandstandardJavaeventmodeltothedistr
ansªandstandardJavaeventmodeltothedistributedcase,enablingeventstobehandledbythirdpartyobjectswhilemakingvariousdeliveryandtimelinessguarantees.ThemodelalsorecognizesthatthedeliveryofadistributednotificationmaybeThetwo-phasecommitinterfacesintroducealight-weight,object-orientedprotocolenablingJiniobjectstocoordinatestatechanges.Theprotocoldiffersfrommosttransactioninterfacesinthatitdoesnotassumethatthetransactionsoccurinatransactionprocessingsystem.Suchsystemsdefinemechanismsandprogrammingrequirementsthatguaranteethecorrectimplementationofaparticulartransactionsemantics.TheJinitwo-phasecommitprotocoltakesamoretraditionalobject-orientedview,leavingthecorrectimplementationofthedesiredtransactionsemanticsuptotheimplementoroftheparticularobjectsthatareinvolvedinthetransaction.Thegoalofthetwo-phasecommitprotocolistodefinetheinteractionsthatsuchobjectsmusthavetocoordinatesuchgroupsofoperations.TheinterfacesthatdefinetheJiniprogrammingmodelareusedbytheinfrastructurecomponentswhereappropriateandbytheinitialJiniservices.Forexample,thelookupservicemakesuseoftheleasingandeventinterfaces:Leasinginsuresthatservicesregisteredcontinuetobeavailable,andeventshelpadministratorsdiscoverproblemsanddevicesneedingconfiguration.JavaSpacesutilizeleasingandevents,andalsosupportthetwophasecommitprotocol.Thetwo-phasecommitmanagercanbeusedtocoordinatethevotingphaseofatransactionforthoseobjectsthatsupportthetwo-phasecommitprotocol.Itisnotrequiredthattheimplementationofaservi

14 ceusetheJiniprogrammingmodel,butsuchserv
ceusetheJiniprogrammingmodel,butsuchservicesneedtousethatmodelfortheirinteractionwiththeJiniinfrastructure.Forexample,everyserviceinteractswiththeJinilookupservicebyusingtheprogrammingmodel;andwhetheraserviceoffersresourcesonaleasedbasisornot,theserviceÕsregistrationwiththelookupservicewillbeleasedandwillneedtobeperiodicallyrenewed. Page14ArchitectureOverview ThebindingoftheprogrammingmodeltotheservicesandtheinfrastructureiswhatmakesaJinifederationasystemasopposedtoacollectionofservicesandprotocols.Thecombinationofinfrastructure,service,andprogrammingmodel,alldesignedtoworktogetherandconstructedusingeachother,simplifiestheoverallsystemandunifiesitinawaythatmakesiteasierto2.2.3ServicesTheJiniinfrastructureandprogrammingmodelarebuilttoenableservicestobeofferedandfoundinthenetworkfederation.Theseservicesmakeuseoftheinfrastructuretomakecallstoeachother,todiscovereachother,andtoannouncetheirpresencetootherservicesandusers.ServicesappearprogrammaticallyasobjectswrittenintheJavaprogramminglanguage,perhapsmadeupofotherobjects.Aservicehasaninterfacewhichdefinestheoperationsthatcanberequestedofthatservice.Someoftheseinterfacesareintendedtobeusedbyprograms,whileothersareintendedtoberunbythereceiversothattheservicecaninteractwithauser.Thetypeoftheservicedeterminestheinterfacesthatmakeupthatserviceandalsodefinethesetofmethodsthatcanbeusedtoaccesstheservice.Asingleservicemaybeimplementedbyusingotherservices.TheinitialJiniservicesincludethefollowing:JavaSpaces,

15 whichcanbeusedforsimplecommunicationandf
whichcanbeusedforsimplecommunicationandforstorageofrelatedgroupsofJavaobjectsatwo-phasecommitmanager,whichenablesgroupsofobjectstoparticipateinthetwophasecommitprotocoldefinedbytheprogrammingmodel2.3ServiceArchitectureServicesformtheinteractivebasisforaJinisystem,bothattheprogramminganduserinterfacelevels.Thedetailsoftheservicearchitecturearebestunderstoodoncethediscoveryandlookupprotocolsarepresented. Page15 2.3.1DiscoveryandLookupProtocolsTheheartoftheJinisystemisapairprotocolscalledwhichoccuratdifferenttimes.DiscoveryoccurswhenaservicejoinsaJinilookupservice,forexample,whenadeviceispluggedin;lookupoccurswhenaclientoruserneedstolocateandinvokeaservicedescribedbyitsattributesandpossibly,otherattributes.Thefollowingdiagramoutlinesthediscoveryprocess.DiscoveryistheprocessofaddingaservicetoaJinisystem.AserviceprovideristheoriginatoroftheserviceÑadeviceorsoftware,forexample.First,theserviceproviderlocatesalookupservicebybroadcastingapresenceannouncement.Then,aproxyfortheserviceisloadedintothelookupservice.Thisproxycontainstheinterfacefortheservicealongwithanyother ProviderClientLookupService Proxy Figure 2: DiscoveryA service provider registerswith a lookup service, placingproxy code in the service Page16ArchitectureOverview descriptiveattributes.Servicesmustbeabletofindalookupservice;however,thisrequirementmaybedelegatedtoathirdparty.Theserviceisnowreadytobelookedupandused,asshowninthefollowingdiagram.AclientlocatesanappropriateservicebyitstypeÑth

16 atis,byitsJavainterfaceÑalongwithdescri
atis,byitsJavainterfaceÑalongwithdescriptiveattributeswhichareusedinauserinterfaceforthelookupservice.Thefinalstageistoinvoketheservice,asshowninthefollowingdiagram.Theproxycodeisloadedintotheclient.Theproxycodemayimplementaprivateprotocolbetweenitselfandtheoriginalserviceprovider.Differentimplementationsofthesameserviceinterfacecanusecompletelydifferentinteractionprotocols. Proxy ProviderClientLookupService Figure 3: First Stage of LookupA client requests a serviceby Java type and, perhaps,other service attributes Proxy ProviderClientLookupService Proxy Figure 4: Invoking a serviceA copy of the proxy is moved tothe client and used by the clientto talk to the service Page17 Theabilitytomovecodefromtheserviceprovidertothelookupserviceandfromtheretotheclientoftheservicegivestheserviceprovidergreatfreedominthecommunicationpatternsbetweentheserviceanditsclients.Thiscodemovementalsoensuresthattheproxyheldbytheclientandtheserviceforwhichitisaproxyarealwayssynchronized,becausetheproxyissuppliedbytheserviceitself.TheclientonlyknowsthatitisdealingwithanimplementationofaninterfacewrittenintheJavaprogramminglanguage,sothecodethatimplementstheinterfacecandowhateverisneededtoprovidetheservice.Becausethiscodecameoriginallyfromtheserviceitself,thecodecantakeadvantageofimplementationdetailsoftheserviceknownonlytotheTheclientinteractswithaserviceviaasetofwell-definedinterfaceswrittenintheJavaprogramminglanguage.Theseinterfacesdefinethesetofmethodsthatcanbeusedtoi

17 nteractwiththeservice.Ataroughlevel,thes
nteractwiththeservice.Ataroughlevel,theseinterfacescanbebrokendownintotwodistincttypes:programmaticinterfaces,whichallowothersoftwaretointeractwiththeservicebymakingdirectmethodcallsontheserviceuserinterfaces,whichpresentaninteractionmechanismtoend-usersoftheProgrammaticinterfacesareidentifiedbythetypesystemoftheJavaprogramminglanguage,andservicescanbefoundinalookupservicebyaskingforthosethatsupportaparticularinterface.Findingaservicethiswayensuresthattheprogramlookingfortheservicewillknowhowtousethatservice,becausethatuseisdefinedbythesetofmethodsthataredefinedbythetype.ProgrammaticinterfacesmaybeimplementedeitherasRMIreferencestotheremoteobjectthatimplementstheservice,aslocalobjectsthatprovidealloftheservicelocally,orassomecombination.Suchcombinations,calledproxies,implementsomeofthefunctionsofaservicelocallyandtheremainderthroughremotecallstoacentralizedimplementationoftheservice.Auserinterfacecanalsobestoredinthelookupservice.SuchaninterfacewillbeidentifiedbytheJavaprogramminglanguagetypewhichallowstheinterfacetobedisplayedbybrowsersorotheruser-interfacetools.AuserinterfacestoredinthelookupservicebyaJiniserviceisanimplementationthatallowstheservicetobedirectlymanipulatedbyauserofthesystem. Page18ArchitectureOverview Ineffect,auserinterfaceforaserviceisaspecializedformoftheserviceproxythatenablesaprogram,suchasabrowser,tostepoutofthewayandletthehumanuserinteractdirectlywithaservice.Analternativeformoflookupisthepeerlookupservice.Apeerlookupserv

18 icecanbeusedwhenaclientcannotfindalookup
icecanbeusedwhenaclientcannotfindalookupservice.Insuchsituations,theclientcansendoutthesameidentificationpacketusedbyalookupservicetorequestserviceproviderstoregister.Serviceproviderswillthenattempttoregisterwiththeclientasthoughitwerealookupservice.Theclientcanselectthoseservicesitneedsfromtheregistrationrequestsitreceivesinresponseanddroporrefusetherest.Suchanapproachworksbestinsmall,isolated2.3.2ServiceImplementationObjectsthatimplementaservicemaybedesignedtoruninasingleaddressspacewithother,helperobjects,especiallywhentherearecertainlocationorsecurity-basedrequirements.Suchobjectsmakeupanobjectgroup,Anobjectgroupisguaranteedtoalwaysresideinasingleaddressspace/virtualmachinewhenthoseobjectsarerunning.Objectsthatarenotinthesameobjectgroupareisolatedfromeachother,typicallybyrunningtheminadifferentvirtualmachineoraddressspace.Aservicemaybeimplementeddirectlyorindirectlybyspecializedhardware.SuchdevicescanbecontactedbythecodeassociatedwiththeinterfacefortheFromtheserviceclientÕspointofview,thereisnodistinctionbetweenservicesthatareimplementbyobjectsonadifferentmachine,servicesthataredownloadedintothelocaladdressspace,andservicesthatareimplementedinhardware.Alloftheseserviceswillappeartobeavailableonthenetwork,willappeartobeobjectswrittenintheJavaprogramminglanguage,andonekindofimplementationcanbereplacedbyanotherkindofimplementationwithoutchangeorknowledgebytheclient.2.4ForMoreInformationFurtherdetailsontheJiniarchitectureareavailableinthedetailed