Abstract. 1IntroductionandMotivation

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1 AScalableArchitectureforAutonomous HeterogeneousDatabaseInteractions SteveMilliner,AthmanBouguettayaandMikePapazoglou QueenslandUniv.ofTechnology Brisbane,QLD4001,Australia SchoolofInformationSystems ing,interestinworldwidedatabaseinteroperabilityhasgainedmomentum.scalability andlanguagesupportforthisnewenvironmentremainopenquestions.weproposea est.datasharingisthenpursued,withanyrelationshipinformationdiscoveredbeing schemewheredatabasenodesaredynamicallyclusteredaroundcurrentareasofinter- fedbackforre{clustering.inordertoachievescalability,theproposedarchitecture Withtheexponentialproliferationofdatabasesandadvancesinwideareanetwork- Abstract Keywords:InteroperableHeterogeneousAutonomousDatabases,DatabaseClustering, DistributedDatabaseLanguage,ScalableArchitectureforInteroperability. sub{dividesboththerelationshipandinformationspaces. Thesesystemstendtobedevelopedinisolation,andthisresultsinstructuralandsemantic beenaproliferationofdatabasesystemstohandleeverincreasingvolumesofinformation. theirdierentplatforms(softwareandhardware).inmanylargeorganizationstherehas Sharinginformationamongautonomousheterogeneousdatabaseshasbeenresearchedextensively.Inessencetheproblemhasbeentomakecomponentdatabasesinteroperabledespite 1IntroductionandMotivation heterogeneity,andrelatedproblems.thepromiseofacommercialcompetitiveedgevia information)issmall.recentadvancesincommunicationstechnologyhaveledtoexpectationsoflargescale,worldwidedatabaseinteroperability.therearevariousfundamental dicultiesassociatedwithlargescaledatabaseinteroperability.theseincludescale,autonomyandheterogeneity[4].thecaseofthefailed31/2year$125millionconfirmproject However,thisisonlyreasonableprovidedthenumberofparticipatingdatabases(andglobal thelogicalintegrationofexistingdatabasesystems,hasattractedintenseinterest.amajor assumptionhasbeenthatcomponentdatabaseshavea-prioriknowledgeofremoteschemas. 1

2 betweenthesystems[21]. reasongivenforfailurewas\technicaldiculty"associatedwithconstructinginterfaces organizationsspanningonlythreegeneralareas:hotel,airlinesandcarrentals.thenal clearlydemonstratesthelimitsofcurrenttechnology.thisprojectentailedthelinkingof andthecomplexityofquerieswhichmaybeformedbasedupontheseinter-relationships. thecomplexityofimplicitinter-relationshipsofinformationitemsbetweendatabasenodes, Therearethustwodistincttypesofspacethatmustbeexploredoneconcernsthesearch priateinformationbecomesparamount.thisisfurthercomplicatedinthedatabasecaseby forappropriateinformation-theinformationspace,theotherconcernsthesearchforrelationshipsbetweennodes-therelationshipspace.topromotescalability,atwolevel Inlargecollectionsofautonomousdistributedresourcesthequestionofndingappro- notsub{dividedexhaustivesearchingmustbeperformed.ifexhaustiveinteractionsbetween searches,andallowforthesharingofdatainatractablemanner. databasenodesarepermitted,thencommunicationsandprocessingbottleneckswilloccur. architectureisintroducedwhichsegmentsbothofthesespaces.iftheinformationspaceis andsharingofinter-relationshipmeta-information[5,18]ispursued.amainobjectiveisto andinformationspaces.thus,inbothlevelsofthearchitecture,anincrementalbuilding tationofdatabasenodesneedstobeintroducedtolterinteractions,accelerateinformation Hence,inthecaseoflargenumbersofautonomousdatabases,anorganizationandsegmen- letdatabasesknowdynamically(asopposedtostatically)howtheyrelatetoremotesites, andwhatthesedatabasescontain.relationshipinformationdiscovery,anddatasharing, mustbedrivenbythestatesoftheindividualdatabasenodesasthesystemexecutes.in particular,theaimhereistoconsiderhowalargecollectionofdatabasenodesmaybe Systemsizeandcomplexityprecludesthestaticapriorisub{divisionoftherelationship theleandoperatingsystems,thedbmsandtransactionmanagementlevels,areoutside thescopeofthispaper. languageanddatabaseapplicationlevels.heterogeneityresolutionoflowerlevelssuchas organizedsothatinformationresourcesmaybeeasilyidentiedandshared.thisworkis ispresented.section3providesanoverviewoftheproposedarchitecture,whilesections4 concernedwithintegrationattheschemaandmodellevels,aswellasinteroperationatthe inthenalsection6. 2RelatedWork and5describetheinitializationandexecutionofthesystem.theconclusionispresented Theorganizationofthispaperisasfollows.Insection2anoverviewofrelatedwork usersknowthespecicinformationofinterestanddatabasesarewillingtoreleaseacertain ateddatabases,thefocushasmainlybeenonsharinginformation.thesesystemsassume amountoftheirautonomy.inmostmultidatabasesystems,sharingisprovidedthrough partialortotalintegration[14,16].infederateddatabases,loosepartialintegrationisthe Relatedworkhasbeencarriedoutinareasincludingmultidatabases,federateddatabases, informationretrieval,anddistributedsystemnaming.inmultidatabasesystemsandfeder- 2

3 typesthatcarrylittleornostructure(orbehavior)inanetworkofcomputersystems[28]. tems. hasmainlyfocusedonaccessingtextdocumentsinacentralizedenvironment.research [23]describesaninterestingsystemfordiscoveringresourcesinanetworkofcomputersys- indistributednamingsystemshasmainlybeengearedtowardsndingsimpleinformation meansusedforsharinginformation[11,24].ontheotherhand,informationretrievalsystemshavetraditionallybeenmoreinterestedinaccessmethods[22].inthisarea,research Multidatabases onemightmapanunderstandingofentitiesfromoneschematoanother[2].inexisting systems,translationsandintegrationaredoneinanadhocfashion.inadditionautonomy 15]whichisusuallyobtainedbyintegratingmultipleschemas.Globalqueriesarethenexecutedagainstthisglobalschema.Themajorproblemassociatedwiththistechniqueisthe Mostmultidatabasesystemsprovideresourcesharingthroughaglobalschema[27,19, translationoflocallanguagesandschemasintoaglobalformat.noautomatictranslation, schemaupdateandintegrationhasbeenperformedsofar.themaindicultyconcernshow issacricedanddecentralizeddecisionmakingisnotachieved.nodesarerequiredtoreveal canbeformed.becauseofthescaleofsystemsbeingconsideredhere.globalschemasare detailsoftheirschemassothatacentralschema,designedbyasingleschemaadministrator, entityintoasetofproperties,eachofwhichisastring.thesearchisusuallyinstancebased GlobalNaming viewsresourcesassimpleentities.thenameserviceisinchargeofmappingthenameofan notconsidered,astheyleadtoseriousdesignbottlenecks. organizationalmanagement.[23]presentsaninterestingmodelforndingresourcesina ratherthantypebased.thedatainvolvedbelongstoasmallsetofbasictypes.hence, tocopewithextensibility[3].thesehierarchiesaremeanttoprovidemeansforbetter littleornosemanticsareattachedtothedata.mostservicesuseonesinglehierarchy Anotherareaofrelatedresearchisglobalnaming[28].Inthisscheme,thesystem Themostinterestingideainthislatterpaperisthestressontheseparationofconcerns theresearchwasconductedfromasystempointofview,databasesissuesweresimplied. betweenresourceprovidersandresourceconsumers. InformationRetrieval networkofcomputersystems.inthisproject,resourcesaretypicallyunstructuredtext.as indexingschemetoecientlyaccessinformationgivensomehintsabouttheresource[22]. Mostofthedistributedinformationretrievalsystemsaredesignedtoworkinahomogeneous environment.therehasbeensomeworktoextendschemestoanetworkofheterogeneous indexcontainsanetworkaddressalongwithasetofcondenseddescriptionscalledskeleton. indexingforndinginformationinanetworkofinformationsystems.eachnodeofthe informationretrievalsystems[26].in[1,25],anapproachisdescribedthatreliesonexternal Inmostinformationretrievalsystems,theemphasisisusuallyonhowtobuildan Resourceprovidersareaddedtotheindexusingknows-aboutrelationships.Thisapproach tendstocentralizethesearchasasingleindexisusedfortheactualresourcediscovery. Potentially(ifusersmakequeriesaboutallexistinginformationspace),allnodeswould 3

4 havethesameindex.itisnotclearfromtheabovereferenceshowthesystembehavesif selectionisperformed. FederatedDatabases severalnodescanansweragivenquery.thereisalsonoreferenceofhowtheactualnode informationsharingoccursthroughimportandexportschemas.alldatabasesareregistered amountofautonomyforindividualdatabasesystemsismaintained.inthisapproach, existingdatabasesandavailableschemas,andsecond,importsallknownschemas(whenever isachievedintwosteps.first,therequestingdatabaseconsultsthefederaldictionaryfor inafederaldictionary.inthecurrentstateoffederateddatabases,locatinginformation interesttoourresearch,isthefederatedapproach.infederateddatabases[11,24],acertain Aworkthatsharessomephilosophicalcommonalitieswithoursandisofparticular Thefederatedapproachasdescribedin[11]doesnotaddresstheissueofhowthefederal dictionaryistobedesignedinpresenceofalargenumberofdatabases.thefederated Inthisframework,noincrementalsharingisimplied.Instead,databasesshareactualdata negotiationisinitiatedwiththeexportingdatabasetoqueryaspecicinformationtype. possible)andbrowsesthroughthemforacertaininformationtype.oncethisisdone,a alargenetworkofdatabaseshastorelyondeningaexiblearchitecture,avariablegrainedinformationsharing,andamoreuserorientedcenteredsharing(versusdatabase approachwasnotspecicallydesignedtoaddresstheissueofnetworksofdatabases.the administrator-centeredsharing).itshouldbeborneinmindthatthefederateddatabases purposehasbeentoprovideabetteralternativetotheglobalintegrationapproachandto AcademicandCommercialInformationRetrievalDatabases alargeextent,hassucceededindoingso. theseproductsarecommercialwhileothersareacademic.thebasicassumptionunderlying mostoftheseproductsisthattheparticipatingdatabaseshavelittleornostructureat toclients.inthecaseofgopherandwww,serversareconnectedinagraph-likefashion. accessfreestyledocuments.thearchitecturecentersaroundserversprovidinginformation Web(orWWW)[9],andGopher[17]useindicesandbrowsingoracombinationthereofto all(i.e.text-based)[20,23].academicproductslikearchie[8],wais[12],worldwide Thereareseveralproductsthatoeraccesstoawidevarietyofdatabases.Someof andinonesinglestep.webelievethatanyviableapproachtosharinginformationin approachimplicitlyassumesthatonlyafewdatabasesparticipateinsharinginformation. InthecaseofWWW,hypertextlinksconstitutethegraph. requiresauniforminterfaceandaccesscanonlybethroughspecializedterminals.all arehookedtogethertoprovideawiderangeofinformationservices.however,thisservice Minitelservice[7].ThisserviceisprovidedbythestaterunPTT.Hundredsofdatabases ofdatabaseautonomyisnotaconcern.inmostcases,databasesareactuallyplaintext databaseshookedtothisservicearetext-based. AninterestingcommercialproductthatissomewhatmoresophisticatedistheFrench les.inthesecases,nointeractionwithadatabasemanagementsystemsisassumed,thus noproblemofdatabaseheterogeneityarises.however,theseissuesarefundamentalto Insummary-aconstantacrossallthesystemsdescribedabove,isthattheproblem 4

5 addressingtheproblemofdatasharingindatabasesystems.datasharingindatabases 3OverviewoftheRelationshipandInformationSpace relationshipsbetweendatabases. requiresthat,beforedatacanbeaccessed,werstneedtounderstandschemasandinter- Inordertosub{dividetherelationshipspace,ahighlevel\contextabstraction"isused. Thisistobeimplementedthroughtheuseofdynamicmetaobjectstermedglobalconcepts (GCs).GCsarebasedupon\descriptions"(metadata)ofeachlocaldatabase'sdomain,and correspondtothecurrentareasofinterestwithintheuniverseofdiscourse(uod).database nodesthenformlinkstoeachgc,andassociateanupdatableweightwitheachofthese. Architectures Thisresultsinaclusteringofdatabasenodes[6]aroundthevariousGCs(asshownin Figure1). implementedviareferencetothelinkweights.whilethisstructuremayappear\at", thelinkweightsactuallyimposeadynamicorderingandstructureuponthegcsandthe Byclusteringdatabasenodes,therelationshipspaceissub{divided,andsearchingis Figure1:DatabaseNodesLinkedtoGlobalConceptsandClusters. databasenodesclusteredaroundthem.whenseveraldatabasenodesalllinkstronglyto thesamegc(eg.weight10/10),adynamicclusteraisformed.however,eachofthese samedatabasenodeswillalsolinklessstrongly(eg.weigh8/10)toothergcswhichwill havetheirownassociatedclustersb,c,dandsoon.databasenodesinclusterawill isbaseduponindividualdatabasenode\states",andthuscontrolremainslogicallydistributed.consequently,theorganizationisinherentlyexible,andavoidstheshortcomings ofstricthierarchicalandobjectorientedorganizationsnotedin[13,23].searchgranularity isdeterminedbythenumberofgcsandthenumberofnodesclusteredaroundthem. thetassililanguagewasdeveloped[5].theconstructsofthislanguagearealsousedto Linkweightsupdatesareperformedlocallyateachdatabasenode.Theorganization Inordertoestablish,andmaintainadynamicsubdivisionoftheinformationspace, thereforeoverlapwithallotherclusterstovariousdegrees. 5 Global Concept A Database Node 1 Global Concept B Database Node 2 Global Concept C Database Node Global Concept n Database Node m Database Node x Global Concept A Database Node p Global Concept B Global Concept C Global Concept n Cluster B Cluster C.... Cluster n n Cluster A.... Database Node f

6 informationspace.linkingofdatabasenodestooneanotherresultsintheformationof queryandextractinformationfromthesystem,aswellasforeducatingusersaboutthe databasenodesintheinformationspace,metalevelrelationshiptypesarespecied.their fortheexchangeofstructuralinformationbetweendatabasenodes,asapreludetodata sharing.inaddition,relationshipsmaybeformedbetweentwoclusters,orbetweena tothoseformedintherelationshipspace.clustersformedintheinformationspaceallow logicalassociations,anddoesnotresultinthesharingofphysicalinformation.bygrouping singledatabasenodeandacluster.intherelationshipspace,clusteringprovideshighlevel databaseclusters.itshouldbenotedthatthese\informationspace"clustersaredierent formationisdrivenbythesystem,andtheyrepresenthowdatabasenodesphysicallyrelate backintotherelationshipspaceforlinkweightandgcadjustment.ameta-information adatabaseandacluster,ortwoclusters. alterationincurrentareasofinterest.hence,informationregardingrelationshipsisfed toeachother.formationandalterationofinformationspacerelationshipsrepresentan fromacluster,andasecondwhichisinstantiatedbyrelationshipsbetweentwodatabases, typeisaunitwhichdescribesthestructureandbehavior,ofsemanticallyrelatedportions ofdatabaseschemas.twobasicformsofmeta{informationtypesareused;onewhicharises CheapRent Database Car Electrical Rentals Housing Machinery... Vehicle Sales Dealers Manufacturers Insurance Vehicle Spare Parts Vehicle Dealers Vehicle Government Vehicle Specifications Insurance Manufacturers Manufacturers Vehicle Wreckers Consumer Organizations.. GC GC GC GC representing representing representing representing Sales Spare Parts Car Registration Insurance Vehicle Insurance House Insurance Insurance 10 Boat Vehicle Insurance Registration relatingtocars.assumethedealerdatabasenoderequirescontinualinformationfrom moreweaklylinked(eg.weightof3/10)toothergcsdealingwiththeareaofinterests stronglylinked(eg.weight10/10)toagcrepresentingtheareaofinterest\sales",and Considerforinstance,asecond{handcardealer'sdatabasenode.Initiallyitwillbe Figure2:APartialViewofaNetworkofRelationshipsandClusters 10 4 > 7 3 > 8 2 > 6 6 Government Government Police Insurance Insurance Vehicle Agents Safty Manufactures Consumer Organisations Space Dealer... Database Database Node Nodes

7 representingthecurrentareasofinterest.databasenodeswhichnowsharingphysicaldata, linkstotheremotegcsisincreased(asshowninthefigure2insert).linkspicturedas linesinthefigure2insert,donotrepresentrelationships,butoverlapsbetweenclusters theinformationattheseremotesitesisfoundtobeappropriate,thenthedealerdatabase nodewillexaminegcsitislessstronglylinkedto,andwilllocatetheseremotesites.if areinitiallystronglylinkedtogcsrepresentingthesethreeareas.thedealerdatabase severalremotesitesregardingcarregistration,carinsuranceandcarsparepartswhich clusteringisimplementedbyhavingthedealerdatabasenodejoinwith\vehiclesales" fortheresultinginformationspacesub{division.astimeprogressesseveninformationspace cluster.informationspacerelationshipsarethenformedbetweenthe\vehiclesales"cluster formaninformationspaceclustering.figure2depictstheinformationspacesub{division, clustersandtheirassociatedrelationshipsareformed. andwillbereferredbacktothroughouttheremainderofthepaper.theinformationspace linkedtothegcrepresentingrentals{andlessstronglyconnectedtogcsrepresentingother andthe\vehiclespareparts",\insurance"and\vehicleregistration"clusters(shownas solidlinesinthegure).inthisway,therelationshipspacesub{divisioncreatesaplatform contactsremotesitesandeventuallyjoinstherentalsinformationspacecluster-thisis depictedinfigure2bythesolidboldlineleadingfromthecheaprentdatabasesymbol tothecollectionofdatabasescomprisingtherentalscluster.cheaprentalsoentersinto areasofinterest(suchascars).afterexplorationoftherelationshipspace,itsdatabase arelationshipwiththevehiclesparepartscluster.thisisdepictedinfigure2bythe ConsidertheCheapRentcarrentalcompanyintheFigure.Itwillinitiallybestrongly comprisingthevehiclesparepartscluster.oncetheseinformationspaceassociationshave beenestablished,thisrelationshipinformationisfedbackintotherelationshipspacelevel brokenboldlinebetweenthecheaprentdatabasesymbolandthecollectionofdatabases architecture,causingthelinkweightupdatingshown. User DB DB however,theseusersmustinteractwiththeinterfaceprocess.co{databases(co-db)are withthelocaldatabase(asindicatedbythebrokenarrow).toaccessremotesystems usersandadministratorsinteractwiththesystem.generalusersarepermittedtointeract Figure3depictshowdatabasenodesmaybeconnectedoveranetwork,andhowgeneral Figure3:DatabaseNodesConnectedOveraNetwork Interface Interface Process Administrator Process CO DB 7 CO DB Node Network

8 objectorienteddatabaseswhereinformationandknowledgeoftherelationshipspaceand informationspacesub{divisionsarestored.generalusersmayalsosuggestupdatestolocal co{databases,however,persistentupdatesareperformedonlybytheadministrators(as indicatedbythearrowsinthegure). interoperabilitylevel,aninteraction/negotiationlevel,andanexplorationlevel.theseare discussedinsection5. theproposedarchitecture.thesearetheinitializationphaseandexecutionphase.during initializationgcsarecreatedandlinkweightstothemareestablished.thisisessentially basisonly.duringexecutionthesystemoperatesonfourlevels:anarchitecturallevel,an apre{clusteringprocess,wheregroupingsofdatabasenodesareestablishedonatentative Therearetwobasicphasesinthelifetimeofadistributedsystemorganizedaround 4.1DatabaseNodeDescriptions descriptions,globalconceptinstantiationandlinkweightinitialization. Theinitializationphaseconsistsofthreebasicactivities:presentationofdatabasenode 4SystemBootstrapping Initialclusteringisbaseduponadescriptionwhicheachdatabasenodeforwardstoacentrallocation.Clusteringbaseduponthesedescriptionsisthenperformed.Thisstepis categoriesaredenedstaticallyatinitialization,thereisnointrinsicreasonwhyfurther description,otherdescriptivecategoriesmaybeincludedtofurtherlterdatabasenodes corporatestructuremaybeincluded,andsoon.althoughitisassumedthatdescriptive performedonlyonce,andshouldbeviewedaspre{clusteringofthesystem.asinformation spaceclustersareformed,inaccuraciesinthepre{clusteringphaseareeliminatedandlink weightadjustmentresultsinre{clusteringofthesystem.inadditiontothedatabasenode 4.2Pre{ClusteringandGlobalConceptInstantiation categoriescannotbeaddedorexistingcategoriesdeleted. involvedintheinformationspaceclusteringprocess.forexample,geographicallocationand The\topic"basedclusteringofdatabasenodes,isdepictedinFigure4.Afterbeingclustered,eachdatabasenodeformsaweightedlinkwiththeotherclusters.InFigure4,each nodeswillbemoststronglyconnected(eg.haveaweightof10/10)totheclustertheywere initiallyassignedto.weightsfortheotherlinksareautomaticallyinitializedbyhavingeach byahumanoperatorisrequiredbeforethesystembecomesuseful.theeventualoutcome nodecalculatetheratioofsimilarityofalldatabasenodesinitsclustertoalldatabasenodes intheremoteclusteritisformingalinkto.thisprocesswilloccurforeachclusterset,to produceaseriesofdistinctfullylogicallyconnectedgroupings.vericationandrenement nodeincluster#1formsaweightedlinkwithcluster#2,cluster#3andsoon.database 8

9 Cluster #1 Cluster #2 Cluster #n "Rentals" "Insurance" Figure4:CreationofLinksAfterInitialClustering.... "Vehicle Spare Parts" DB1 DB 2 DB x DB 3 DB 8 DB y DB p DB q DB z isacollectionofabstractglobalconceptsaroundwhichnodescancluster.thesetofgcs whichformsabasisforrelationshipspaceby: Cluster #1 Cluster #2 Cluster #n "Vehicle "Rentals" "Insurance".... Spare Parts" Initialclusteringmaynotresultinuniformsizegroupings.Hence,anadditionalsub-division ofgroupsmayberequired[10].atuningphasemayalsobeintroducedifnodedescriptions donotprovideanadequatebasisfortheclusteringprocess(eg.whenasmallproportion 1.Sub-dividingthenodestoproduceareasonablyuniformdistribution, 2.Providinganappropriatedegreeofsearchspacegranularity. DB1 DB 2 DB x DB 3 DB 8 DB y DB p DB q DB z ofclusterscontainalargepercentageoftheclustereditems).thissituationcorresponds toanovergeneralizationofkeyfeaturesandcanberesolvedbyemployingasubdivision algorithmwhichrenesanddecomposesovergeneralizedgcs. 5TheExecutionPhase teroperabilitylevel,aninteraction/negotiationlevel,andanexplorationlevel.inthear- chitecturallevelalogicalinterconnectionofnodesispursued,andclustersareformedin therelationshipspace.intheinteraction/negotiationlevel,thegcorganizationisusedas aplatformforinformationspaceclustering.informationconcerningphysicalrelationships formedintheinformationspace,isthenfedbackintotherstlayerforadjustmentofthe relationshipspacestructure(vialinkweightandgcupdates).togetherthesetwolayers alloworganizationofinteractionsbetweenthesystem'sdatabasenodes.remoteinformationmustbeunderstood,thisisachievedintheinteroperabilitylevelthroughtheuseof Duringexecution,activitiesarecharacterizedbyfourlevels:anarchitecturallevel,anin- 9

10 demonstrations.lastly,theappropriateinformationmustbelocated.thisisachievedin Thislevelsuppliesanarchitecturefordatasharingbetweendatabasenodes,usingtheclustersformedinboththerelationshipandinformationspaces.Byformingclusters,ameans Byadjustinglinkweightslocally,eachdatabasenodeitselfdecideswhichrelationshipspace languageandexistinginformationspaceclusters. 5.1TheArchitecturalLevel theexplorationlevelusingrelationshipspacegcclusters,inconjunctionwiththetassili databasenodesimplicitlyagreetoworktogether(andthussacricetheirautonomytosome clustersitwillparticipatein.thus,controlremainsdecentralized.byjoiningacluster isprovidedforthesynchronizingofbothrelationshipformationanddatasharingactivities. dynamicallyupdated,formed,anddissolvedinanincrementalfashion. degree).however,nodesretaincontrollocallyandjoinorleaveclustersbaseduponlocalconsiderations(byfollowingtheappropriateprotocol-describedlater).consequently, maximumlocalautonomyismaintainedandboth,inter{relationshipspaceandinformationspacecontrol,remainsdistributed.inaddition,inter-noderelationshipscontinuetobe interoperabilityproblembecomesintractable. involved.ifthissubdivisionisnotperformedtheinformationspaceremainshugeandthe abilityprocessanddatasharing{thisisrequiredtorestrictthepotentialnumberofnodes anyinformationtypedenedbyaparticularclusteringofnodesispotentiallyaccessible. Onceinter{noderelationshipshavebeeninitiated,aplatformisprovidedfortheinteroper- Thesystemthusallowsthesharingamongdatabasesofallinformationtypesavailable. Becausedatabasenodesarefullylogicallyinterconnected,viaGCsandlinkweights, identied,accesstotheremoteinformationmaybenegotiatedtodeterminewhichtasks viously,apre{clusteringofnodeshasbeenperformed.oncetheappropriatenodesare getdelegatedtowhichremoteservers.thislatterprocessisperformedthroughthetassililanguage,andmayresultinupdatestoboththeinformationspaceclusters,aswellas involvessearchingoftheinter-noderelationshipspacebymeansofsearchheuristics.pre- Thereareseveraldatabasenodeinteractionphases.Therstformofnodeinteraction 5.2TheInteraction/NegotiationLevel whicharestoredinthenodeco{database.inordertoextract/explore(orexport)informationanodemaycontactandinteractwiththerelevantnodesinainformationspace databasecluster.onceagain,thisinteractionisperformedusingtassililanguageprimitives{understandingofremotedataisfacilitatedthroughtheuseofdemonstrationsas relationshipspaceclusters(i.e.gclinkweights,andgcsthemselves). Thesecondformofnodeinteractionresultsinthediscoveryofinteroperabilityfacts describedinsection

11 5.2.1InformationSpaceClusterConstructionandUpdate domaininterest.iftheserequestsaresmallandnotpersistent,amappingtoaremote orientedinformationspaceschema. providedbythetassililanguage,andresultinthecreationandmaintenanceofanobject Theabilitytoform,andcontroljoiningandleavingofclusters,isrestrictedtoselected ensue.thedatabasewilltheneithersharethemeta{informationtyperepresentedbythe users.theprimitivesfortheseinteractionsandsemi{automatednegotiationsessionsare remote\popular"databases,oranewinformationtypewillbeformed. informationtypewillsuce.ifthenumberofrequestsremainshigh,re{clusteringmay Insomeinstances,usersmayaskaboutinformationthatisnotinthelocaldatabase theavailableinformationspaceclusterinformation,thecheaprentadministratorjoinsthe whichremotedatabasesshouldbecontacted.afterexaminingtherelationshipspace,and/or rentalscluster.thatis,afteridentifyingtheappropriate\areaofinterest",joiningofthe informationspaceclusterandsharingofdataensues.intassilithisnegotiationprocessis performedusingthethefollowingprimitive: InquireatGM-Spare-PartsWithMessage\Wishtoestablisharelationship.Whatare IntheCheapRentcarrentalcompanycase,informationretrievalbeginsbydetermining themainattributesofyourresource?" partsclusters.ageneralusercanthusinvestigatethislatterclusterinthehopeofresolving tionregardingdatabasesintheclusteritself.thecheaprentadministratorwillnow,for sparepartsclusterforexample{becauseoftherelationshipbetweenthesalesandspare instance,haveknowledgeofthevehiclesalescluster,andindirectknowledgeofthevehicles relationshipsbetweentherentalsclusterandothercluster/databases,aswellasinforma- fails,adiagnosticisreturned.itsco-databaseisthenloadedwithinformationregarding Amessageissentalongwiththequerytoexplainwhatisexpected.Ifthequery anav-cost-of-partsqueryforinstance.ifthisinvestigationprovesfruitfulashortterminteractionwiththisclustersitewillensue.inordertoobtainremotestructuralinformation, thefollowingtassiliprimitiveisused: Sendqueryissuccessful,nothingisreturned.Ifthequeryfails,adiagnosticofthefailureis renement.thisprocessofnegotiationendswhenevertheinvolvedentitiesdecideso.ifthe servicingentityspecications,aninquirequeryissentbacktothetargetentityforfurther theinformationrequestedtotheservicingdatabaseorcluster.ifthespecicationsmeets theservicingneeds,nofurtheractionistaken.however,ifthespecicationsdonotmeetthe Thisqueryisusedbythetarget(representative)databasetosendinformationabout SendtoCheapRentObjectGM-Spare-Parts.template. therelationshipabstraction,andtoendtherelationship.thisresultsinformationand/or returned.otherprimitivesexisttocreatedatastructuresatlocalco-databasestoimplement existingclustermember.thisupdateisthenpropagatedtotheotherclustermembers. modicationoftheobjectorientedschema. updateoccurs.anewclass(representingtherentalcompanynode)isinstantiatedviaan Whenthecarrentalcompanyjoinstherentalclusterainformationspaceschema 11

12 information.forexample,thecheaprentnodemaywishtoallowaccessto\rental-rate", Thesechangesareachievedusingtheprimitive: \model"and\year"attributeswhicharecontainedwithinitsdatabase.thisisachievedby addingmethodstoitsclassviathetassiliprimitive: Therentaldatabasemanagermaythenchoosetoallowremoteaccesstocertainlocal InstantiateClassRentalsWithObjectCarWithName=CheapRent. AddMethodRental-priceWithBody ifdate.month>=octanddate.month<=janthen elsereturn(base-price) ToClassRentals. OtherTassiliprimitivesexisttoremovemethodsandobjects(ie.whenanoderelinquishesaccesstolocalinformationorleavesacluster),andtoaltermethodsorobjects. Therearealsomorebasicprimitiveswhichareusedtoestablishacluster,andpropagate administrators. andvalidatechanges.eachoperationhastobevalidatedbytheparticipatingdatabase itself.theadministratorofthatclusterwillthendecidehowtheinformationspaceschema providesomeinformationaboutthedataitwouldliketoshareaswellasinformationabout setbythatcluster.ifadatabasewouldliketobeamemberofacertaincluster,ithasto istobeaugmentedifthedatabaseisacceptedasanewmember.duringthisinformal exchange,manyparametersneedtobeset.forinstance,athresholdfortheminimumand maximumnumberofclustermembersisnegotiatedandset.likewise,athresholdonthe Foranydatabasetoenterorleaveacertaincluster,ithastofullltherequirements return(1.2*base-price) minimumandmaximumnumberofrelationshipswithdatabasesandclusterisalsoset.in theexampleshowninfigure2,thecheaprentadministratorhasdecidedthatlongterm tothecreatoroftheobject.basedonthisfeedback,thecreatorwilldecidewhetherto thisisdone,therootoftheschemaissenttoeveryparticipatingdatabaseforvalidation. intowiththevehiclesparepartscluster'snodes(ie.informationowfromthevehiclespare Iftheoperationisnotvalidated,therejectingnodesendsaneditedversionoftheobject partsclustertothecheaprentdatabase). connectionisrequiredwiththevehiclesparepartscluster.thus,arelationshipisentered changetheobjectornot.thisprocesswillcontinueuntilthereisaconsensus.changesare onlymadeatasinglesiteuntilconsensusisachieved-atwhichtimethechangeismade persistentandpropagatedtotheappropriatedatabasenodes. Initially,adatabaseadministratorcreatestherootclassoftheclusterschema.Once \owns"it.priortoanychanges,thedatabaseownertellseveryparticipatingdatabase tolocktheobjecttobechanged.afteranacknowledgmentfromthosedatabases,the Ifexistingclasses/methodsaretobeupdatedresponsibilitylieswiththedatabasethat 12

13 localdatabaseadministratorproceedswithimplementingthechanges,andpropagatesthe update. everyparticipatingdatabaseschema,alongwithallobjectsbelongingtotheclassesofthat cluster.allclusterswithwhichthereisarelationshiparenotiedthattheclusternolonger exists.localschemasareupdatedbytheiradministrators.thedecisiontodismantlea thesameasdenedforclusters.theonlydierencebeingthatchangesintheclustercase inthecluster.theupdateofco-databasesresultingfromrelationshipschangesispractically clusterisreachedbyaconsensusofparticipatingdatabases,orwhenonlyonenoderemains Adatabaseclusterisdismantledbydeletingthewholecorrespondingsubschemain Rentjoinstherentalsclusterco-databasesmustbeupdatedtoreectthenewinter-node relationshipstructure.inthiswaycheaprentbuildsupitslocalknowledgeofremotesites bothitandtheclustermembersmustupdatetheirco-database.similarly,whencheap- lteringtheamountofinformationwhichmustbeassimilatedatthecarrentalnode.fur- obeyastrictersetofrules. therrelationshipsandclustersarecontactedbycheaprentusersastheyattempttoac- cessrequiredinformation.therentaldatabaseadministratormaythenchoosetoenter toexploretheinformationspaceusingtheunderlyinginformationspacegcsub{division. andformsappropriaterelationshipswiththosecontainingapplicableinformation.thereby AfterCheapRentcreatesarelationshipconnectionwiththevehiclesparepartscluster, into/createfurtherrelationshipsandclustersbaseduponusersneeds.whennoinformation spacemeta{typesexist,orwhenthereareahugenumber,itbecomesnecessaryforusers Inalargescalesystem,interpretationofremoteinformationisamajorproblem,evenif underlyingrelationshipspacesub{divisionmaybeutilizedtorestrictthesearchspace[18]. 5.3TheInteroperabilityLevel Thatis,whentherearenoinformationspaceclusters,orwhentherearetoomany,the thedatamodelisthesameacrossdatabases.beforeinformationcanbeaccessedinterderstandingofremoteinformationishandledthroughtheintroductionofdocumentationsrelationshipsbetweendatabasesmustbeorganizedandschemasmustbeunderstood.un- arisewheninteractingwithdatabasesinthevehiclesparepartsclusterbecauseofdierences remotenodemethodsadvertisedinthevariousclustersitmustinteractwith.confusionmay localusageswillvaryanddierentbehaviorsmaybeexhibited. Thesearesamplebehaviorsorstructuresofcertainclassesofinformation,whichareattachedtoeachinformationtypethatissharedwiththeoutsideworld.Theiraimisto inpricelistings(eg.onedatabasemayincludesalestax,othersmaynot),dierencesin domains(eg.adatabasemayonlystockpartsforonetypeofcar),dierencesinthetypes explain/denethatinformationtype.eveniftwodatabasescontainthesameinformation, ofbusinesses(eg.acarwreckermayonlydisplaypartavailability,notprice)andsoon. Demonstrationsmayalsoprovidedetailsofinformationneededbytheremotenodetoaccess Intheexample(Figure2)documentationisrequiredbyCheapRenttomakesenseof therequireddata.forexample,avehicle'syearandmakewillberequiredtoobtainaspare part'sprice. 13

14 ifacarisdamagedbeyondrepair,andsoon.inthevehiclesalesclusterdatabasesmay policywithanother.forexample,onecompanymayprovidecoverforboththevehicleand asimplepolicystatement,orincludegraphical/video/audiodatacomparingonecompanies driver,othersmaynot.dierentcompaniesmayalsopayoutdierentmarketvalueprices listpricesofmodels.videoandaudiodemonstrationsmaybeincludedtohighlightvehicle beincludedtodierentiatepoliciesoeredbyvariousinsuranceagents.thismayinclude Similarly,inthecaseofinterpretinginsuranceclustermembers,demonstrationsmay vehiclesafetyclustertoexplainattributesinasafetystandardsdatabase. features.asimilardemonstrationformatmaybeincludedbyagovernmentdatabaseinthe objectstousers'screens.thisassumesusershaveaccesstohighresolutionworkstations. system,andmayincludeseveralofthefollowingfeatures: graphicaldescriptionofdatabaseinformation. GraphicalInterface:Thedocumentingdatabaseischargedwiththesendingbinary StructuredText:ThisissimilartothemanpagesdenedinUNIXandoersanon- Documentationisoeredbytheprovidingdatabasesandisnotanintegralpartofthe ofoperatingsystemstochoosefrom.lastandnotleast,thedocumentationrunsona ofuserscanusethedocumentation.databasesarealsoencouragedtoprovidedocumentationthatmayrununderdierentoperatingsystems.theuserispromptedforachoice areencouragedtoprovidemanyplatformsofimplementationsothatthemaximumnumber theirdisposal,databasesmaydocumentinformationwithsoundcapability. AudioCapability:Ifusershaveaudiocapability,andadequateamountofstorageat multitudeofhardware. Demonstrationsthusprovideawayofconvertingdata(eg.priceofpartsversusprice SystemDependencies:Ifthedocumentationreliesonaprogrammingtool,databases translatingdata(eg.demonstrationsmaybeoeredinseveraldierentlanguages),and withsalestax),evaluatingdata(eg.the\best"insurancepolicy-costbenetanalysis), featuresinagovernmentvehiclesafetydatabase(meantforexpertconsumption).thus, interpretingdata(eg.whatisthedenitionofa\safe"car).notealsothatdemonstrations onthesametopicwillvaryfromdatabasetodatabase.forinstance,safetyfeaturesin amanufacturer'sdatabase(meantforpublicconsumption)maynotequatewithsafety UoDmaybepursuedviatherelationshiporinformationspaceorganizationrepresentations. Onceanumberofinformationspaceclustershavebeenestablished,explorationofthe 5.4TheExplorationLevel thelocal\meaning"ofattributes/objectsisdened,throughthetassiliquerylanguage. spacebasedsearches.thesyntaxspecicationsoftassiliqueriesprovideconstructsto educateusersabouttheavailableinformationspaceorganization,aswellasconnecting Althoughanexplosioninthenumberofinformationspaceclustersmaylimitinformation andgraphicalrepresentationareusedasahandleforidentifyingtheappropriateinformation databasesandperformingremotequeries.theinformationtypename,structure,behavior, 14

15 spacequeries(localandremote)aredirected. resources.nodeco-databasesmaintainschemasanditistoco-databasesthatallinformation Co-database Root Class Relationships Root Class Cluster Root Class Cluster 1 Root Class Cluster n Root Class..... Eachsubschemarepresentseitheraclusterorarelationshipandcontainsalatticeofclasses. Cluster Relationships Root Class Relationships Root Class Eachclassrepresentsasetofdatabasesthatcananswerqueriesaboutaspecictypeof information(e.g.queriesaboutcarparts).agraphicalinterfacehasbeenimplementedso usersmaynavigatethroughtheinformationspace.therelationshipsubschema(leftsideof Figure5:ASkeletonofaTypicalCo{DatabaseSchema Figure5)consistsoftwosubschemas,therstdepictsrelationshipsthatclusters(itismemberof)havewithotherdatabasesandclusters.Thesecondisasubschemaofrelationships Aco-databaseinformationschemaconsistsoftwosubschemasasdepictedinFigure5. Cluster-Database Class thedatabaseinquestionhaswithotherdatabasesandclusters.eachofthesesubschemas, Cluster-Cluster Class Database-Cluster Class Database-Database Class whichrepresentsaclusterthedatabaseinquestionisamemberof. tactwiththoseclustersthatareinvolvedinarelationshipwith.otherdescriptionsprovide inturn,consistsoftwosubclassesthatdescriberelationshipswithdatabasesandclusters. Theclustersubschema(rightsideofFigure5)consistsofoneormoresubschemas,eachof informationtolocaldatabasessothebestpointofcontactcanbechosen.itshouldbe notedthatthesubschemarepresentingthesetofrelationshipsprovidingclusterswillbethe sameforalldatabasesthataremembersoftheprovidingcluster. Clusterrelationshipdescriptionsincludeinformationaboutpointsofentriesandcon- etc.adescriptionoftheinformationtypeincludesitsgeneralstructureandbehavior informationtype(s)theycontain.someattributesdescribetheinformationtypewhilethe intheschema.everyclasscontainsadescriptionoftheparticipatingdatabasesandthe descriptionsincludeinformationaboutthedatamodel,operatingsystem,querylanguage, remainingattributesdescribethedatabasesthatcontainthisinformationtype.database Asetofdatabases,containingacertaininformationtypeisrepresentedbyaclass 15

16 thisstructuralinformationandvarioustassiliqueryprimitives,auseratthecheaprent anexampleofaclassdescriptionshowingsomedetailsoftheclassroot-cluster.using fromdemonstrationsinthattheyonlyoergeneralstructuralinformation.followingis company(figure2)canbegintoresolvecomplexqueries.thecheaprentco-databaseis onlythecommonpartsoftheviewarerepresentedintheclass.thesedescriptionsdier (ifapplicable).sincedatabasesmayhavedierentviewsonthesameinformationtype, BecauseoftheserelationshipsCheapRenthasknowledgeofthevehiclesalesclusterstored amemberoftherentalsclusterandhasarelationshipwiththevehiclesparepartscluster. initsco-database.thisinformationisaccessedbyauserthroughthetassiliprimitives: ThisprovidesalistofclustersCheapRentisamemberof. DisplayClustersofCheapRent. CheapRentuserscandiscovertheclusters:vehiclespecications,insuranceandvehicle with(eg.vehiclesalesandvehiclespareparts).asimilarquerymaybeusedtoexamine registration.lastly,byexaminingtherelationshiplinkstotheinsurancecluster,thevehicle therelationshiplinksofthevehiclesparepartsandvehiclesalesclusters.inthisway safetyclustercanbefound. ThisprovidesalistofclusterstheCheapRentdatabaseandrentalsclusterhasarelationship DisplayClusterRelationshipsRentals. nulllistisreturned.inthiscaseausermayeithersubmitanotherinformationnameterm, attempttonavigatethroughthesystemanddiscovertheinformation\manually",ormake requiresthatnodesmaintainanappropriatelistofthesaurusterms.ifnomatchismadea connectionstoremoteclusters/databaserelationships.usersmayutilizeotherprimitives andstipulatean\informationname"-forexample\partprices"-whichreturnsalistof possiblyremoteclusters/relationshipsthathaveacorrespondingclassname.thisprocess Primitivesalsoexistwhichdisplaytheclassesofrelationshipsandclusters,andallow sparepartsmayhavebeenreturnedbystipulatingtheinformationnamesparepartsorsimply useoftheunderlyingrelationshipspacesearchstructure.inthelastcasethreebasicsearch heuristicshavebeenproposedin[18],basedonlinkweightrelaxation. bylistingrelationships.onceanappropriatecluster/relationshiphasbeenfound,tassili providesprimitivesforinvestigatingitsclasses.forexample: InthecarrentalcasetherelationshipbetweentheCheapRentdatabaseandthevehicle Thequery: willdisplayallthesubclassesofthevehiclesparepartscluster. DisplaySubClassesvehicle-spare-parts spareparts. Lastly,thequery: willdisplayinformationassociatedwiththesubclasspartpriceswithintheclustervehicle FindInformationpart-prices 16

17 \1990". willdisplayactualinformation.clearlythesequeriesrequireincreasinglygreaterknowledge oftheclusterschema.thusbothexpertandnoviceusersarecateredfor. thersttwoofthethreeclassqueriesabove).forexample,the\displaysubclass"query FindInformationWithAttributesparts:\exhaust";model:\JaguarXJS";year: willreturnthelist:cars,trucks,motorbikesandsoon.theuserthenissuesthequery: displayed.similarly,tassiliallowsattributesofinstancestobedisplayed. Byleavingthe\instance"termblankallinstancesassociatedwiththesub-class\Car"are FurtherTassiliprimitivesareavailableforexploringobjectsmorefully(asrequiredin Finally,Tassiliallowsforfurtherunderstandingofaninformationtypeusingtheprimitive: DisplayDocumentationofInstanceFord-spare-partsOfInformationpart-price. DisplayInstancesByAttributeOfInformationCar. providesomeinformationabouttheenvironmentcurrentlyused.thelistofinformationthat mayberequestedispresentedinaprecedingsection.ifthereisnobehavioralcapabilityor nosuitableenvironmentispresent,thequeryfails.otherwise,documentationisdisplayed. mayrequireacertaintypeofhardwareand/orsoftwaretoberun.theuserispromptedto Theanswercanbeintextualorgraphicalform(asnotedpreviously).Thequeryitself disparatedatabasenodes.theproposedformofinteractionresultsinthecreationofdynamicclustersofdatabasescenteredaroundsubjectareasofinterestorexpertisecalletiallyrepresentcentroidsofadatabaseclusterpertainingtoanareaofinterest.database nodesthenformweightedlinkstoglobalconceptsinawaywhichreectstheirowninterests. globalconcepts.globalconceptsarehigh-levelabstractmeta-schemaobjectswhichessen- anarchitecturalframeworkfororganizinginteractionsamongalargenumberofautonomous 6Conclusion Thispaperaddressesissuesrelatingtolarge-scaleinteroperation.Inparticular,wepropose drivenbythestatesoftheindividualdatabasenodesasthesystemexecutes. degreeofexibilityasitallowsrelationshipinformationdiscoveryanddatasharingtobe systemprovidesfacilitiesforqueryingandextractinginformationregardingtheinter-cluster lishandmaintainthesubdivisionoftheinter-databaseinformationspace.moreover,the relationshipsandtheinformationspaceingeneral.theproposedapproachallowsforahigh Thustheyorganizetheinter-databaseinformationandrelationshipsearchspace. Theproposedapproachfulllsthreefundamentaltasks:itprovidesacommonframework(theglobalconcepts)towhichparticipantdatabasescontribute;itspeciesarelatively Linguistictoolsarealsoprovidedinordertoallowapplicationprogrammerstoestab- clustersanddatabasenodes.othervirtuesofthisapproachareitssimplicity,dynamic informationsearches;andimplicitlyprovideslocalnodeswithanabstractmodelofother smallsetofdatabases/nodesforinteraction(viz.databaseclusters)therebyaccelerating 17

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