CORBA A Platform for Distributed Object Computing

CORBA:APlatformforDistributedObjectComputing(AState-of-the-ArtReportonOMG/CORBA)ZhonghuaYangandKeithDuddyCRCforDistributedSystemsTechnology(DSTC)Level7GehrmannLaboratoriesUniversityofQueenslandAustralia4072email:fyang,[e-mail]putinghasmadesignicantadvancesinthepastfewyearsaswitnessedbythereleaseofOSF’sDistributedComputingEnvironment(DCE)andtheadoptionofasetofspecicationsfordistributedobjectcomputingbytheObjectManagementGroup(OMG).Inthisreport,wediscussOMG’sworkindistributedcomputingbasedonobjecttechnology.WeattempttoprovideacomprehensivecoverageofOMG’stechnology,includinganoverviewofObjectManagementArchitecture,therationalebehindOMG’swork,atechnicaltourofCORBA,andanoverviewofObjectServicesandCommonFacilities.AtechnicalcomparisonbetweenDCEandCORBAisprovided.UserperspectiveandthefuturepotentialofOMG’stechnologyarediscussedintheconclusion.1IntroductionTheObjectManagementGroup(OMG)isaninternationalindustryconsortiumthatpromotesthetheoryandpracticeofobjectoriented(OO)softwaredevelopment.Theirgoalistoprovideacommonarchitecturalframework,acrossheterogeneoushardwareplatformsandoperatingsystems,forinter-communicationofapplicationobjects.Theadoptionofanobject-orientedapproachismotivatedbythedesireforsoftwaredevelop-mentwithreusablecomponentsthatinteractwithoneanotherthroughwelldenedinterfaces.Industrytrendsindicatethatobjecttechnologyisgainingwideacceptance,withmostvendorsnowprovidingtoolsandenvironmentstosupportOOsoftwaredevelopment.Justasstructuredmodularprogrammingbecamestandardsoftwareengineeringpracticeinthe1980sitisanticipatedthatthiswillgivewaytoadominanceoftheobject-orientedparadigmwithinafewyears.TheOMGwasfoundedinMay1989,byeightcompanies:3ComCorporation,AmericanAir-lines,CanonInc.,DataGeneral,Hewlett-Packard,PhilipsTelecommunicationsN.V.,SunMi-crosystemsandUnisysCorporation.TheOMGbeganindependentoperationsasanon-protcorporationinOctober1989[8].Itnowboastsover500membercompaniesinternationally,withalmosteverysignicantcomputervendorrepresented.Theorganisationperformsnodevelopmentwork,relyingratheronexistingtechnologiesoeredbymembercompanies.ItsapproachistoissueRequestsforProposals(RFPs)onallaspectsofobjecttechnology,whichsolicitspecicationsofcomponentstotintoabroadObjectManage-mentArchitecture(OMA).Membersmaythenproposeaspecicationthatisaccompaniedbyanimplementationprovingtheconceptsdetailedtherein.Areviewandvotingprocessiscon-ductedandonceaspecicationisaccepted,anyvendorthatconformstoitmayoeralternativeimplementationstomarket.2OverviewofOMGOMAFromtheoutset,theOMGrealizedthatoneofthemainproblemsfacingdistributedcomputingissoftwarecomponentintegration,andthereisnosingle,commerciallyavailable,widelyrecognisedandstandardisedapproachandframeworkforachievingthisintegration.InitsbookObjectMan-agementArchitectureGuide[8,page30],theOMGoereditsvisionandapproachtodistributedcomputing:Toadoptinterfaceandprotocolspecicationsthatdeneanobjectmanagementarchi-tecturesupportinginteroperableapplicationsbasedondistributedinteroperableobjects.ThespecicationsaretobebasedonexistingtechnologythatcanbedemonstratedtosatisfyOMG’sTechnicalObjectives.TheOMGhasdevelopedaconceptualmodel,knownasthecoreobjectmodel,andareferencearchitecture,calledtheObjectManagementArchitecture(OMA)uponwhichapplicationscanbeconstructed.TheOMGOMAattemptstodene,atahighlevelofabstraction,thevariousfacilitiesnecessaryfordistributedobject-orientedcomputing.TheOMGOMApartitionstheOMGproblemspaceintopractical,high-levelarchitecturalcomponentsthatcanbeaddressedbytechnologyproposers.Itconsistsoffourcomponents:ObjectRequestBroker(ORB),ObjectServices(OS),Commonfacilities(CF),andApplicationObjects(AO)(Figure1).Thesecomponentsdenethecompositionofobjectsandtheirinterfaces.ObjectsarecategorisedintoObjectServices,CommonFacilities,andApplicationObjectstoestablishthestandardisationstrategyfortheOMG.ThecoreoftheOMAistheObjectRequestBroker(ORB)whichisacommoncommunicationbusforobjects.ThetechnologyadoptedforORBsisknownastheCommonObjectRequestBrokerArchitecture(CORBA)[29,27,43],whichspeciesaframeworkfortransparentcommunicationbetweenapplicationobjects.CORBAistherstspecicationadoptedbytheOMG.ThelatestversionisCORBA2[27],adoptedattheendof1994.ObjectServicesiscomprisedofasetinterfacestolowerlevelimportantinfrastructureobjects.TheOMGObjectServicesspecicationsdenesetsofobjectswhichperformfundamentalfunctionssuchasnaming,lifecycleservices,andtransactions[24].Sofar,theOMGhasadoptedtwovolumesofObjectServicesspecications,theCommonObjectServicesSpecication1and2(COSS1[15,20];COSS2[16,20]).CommonFacilities(CF)[13]arethemostrecentareaofeortsbytheOMG.OMAdenesCommonFacilitiesashavinganapplicationfocusandmostlyusedbydevelopersworkinginadis-tributedenvironment.CommonFacilitiesprovidestandardisedinterfacestocommonapplicationinterfaces.TheApplicationObjectscomponentreectstheneedforindependentlydevelopedapplicationinterfaces.WhilethisisoneofthecomponentsoftheOMA,theOMGwillprobablyneverdevelopstandardsforit.2OBJECTREQUESTBROKER(ORB)OBJECTSSERVICE