History

A formal model of views for object-oriented database systems

Description
A formal model of views for object-oriented database systems
Categories
Published
of 28
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Related Documents
Share
Transcript
  AFormalModelofViewsforObject-OrientedDatabase Systems  GiovannaGuerrini*  DipartimentodiInformaticaeScienzedell'Informazione,UniversitadegliStudidiGenova,ViaDodecaneso,35-16146 Genova,Italy.E-mail:guerrini@disi.unige.it  ElisaBertino,BarbaraCatania  DipartimentodiScienzedell'Informazione,UniversitadegliStudidiMilano,ViaComelico39/41-20135Milano,Italy.E-mail:bertino/catania@dsi.unimi.it  JesusGarcia-Molina  y  DepartamentodeInformaticaySistemas,UniversidaddeMurcia,CampusdeEspinardo-30071Espinardo,Murcia,Spain.E-mail:jmolina@fcu.um.es  Thedenitionofaviewmechanismisanimportantissue forobject-orienteddatabasesystems,inordertoprovide anumberoffeaturesthatarecrucialforthedevelopment ofadvancedapplications.Duetothecomplexityofthe datamodel,theobject-orientedparadigmintroducesnew problemsinthedenitionofaviewmechanism.Severalapproacheshavebeendened,eachdeningaparticular viewmechanismtailoredtoasetoffunctionalitiesthatthe viewmechanismshouldsupport.Inparticular,viewscanbe usedasshorthandinqueries,cansupportthedenitionof externalschemas,canbeusedforcontent-dependentau- thorization,and,nally,cansupportsomeformofschema evolution.Inthispaper,weformallyintroduceaview modelforobject-orienteddatabases.Ourviewmodelis comparabletoexistingviewmodelsforwhatconcernsthe supportedfeatures;however,ourmodelistheonlyone forwhichaformaldenitionisgiven.Thisformaldeni- tionofobject-orientedviewmechanismsisusefulbothfor understandingwhatviewsareandasabasisforfurtherin- vestigationsonviewproperties.Thepaperintroducesthe model,discussingallthesupportedfeaturesbothfroma theoreticalandpracticalpointofview.Acomparisonof ourmodelwithothermodelsisalsopresented.  1.Introduction  Theobject-orientedparadigmhasbeenrecognized asasoundbasisforanewgenerationofdatabasesys-  * TheworkofGiovannaGuerrinihasbeenpartiallysupported bytheEECunderESPRITProject6333IDEA. y  TheworkofJesusGarcia-Molinahasbeensupportedby theDGICYT(MinisteriodeEducacionyCiencias,Spain)grant PR94-286.c    (Year)JohnWiley&Sons,Inc. tems.Indeed,itsabilitytomodelcomplexobjects,togetherwithitsmodularityandextensibilityproper- ties,overcomesmostoftheproblemsarisingintheuse ofthesimplerelationalmodel10].Ageneralagree- mentexistsonthefactthatobject-orienteddatabase featuresshouldmeetasmuchaspossiblefunctionalit- iesoftherelationaldatabasesystems5,7,25].Over thelastyears,aconsiderableresearcheorthasbeen devotedtoexplorehowtheparadigmshiftfromthere- lationaldatamodeltoanobject-orientedmodelaects notionssuchasquerylanguages,authorization,index- ing,schemaevolutionandconcurrencycontrol.Animportantrelationalfunctionalityisrepresented byviews.Intherelationalmodel,aviewisavirtual (i.e.notphysicallystored)relation,denedbyaquery ononeormorestoredrelations.Asrelationallanguages areclosed(i.e.theresultofaqueryexpressedinare- lationallanguageisarelation),therelationreturned bysuchaqueryrepresentstheviewcontent.Thus,re- lationalviewscanbeusedin(almost)anycontextin whicharelationmayappear.Moreover,authorizations maybegrantedandrevokedonviewsasonordinaryre- lations.Atthesametime,viewscanbeusedtodene externalschemas,inthatvirtualrelationsaregener- atedbycombiningbaserelations.Viewsareanintegral componentoftheANSIthree-levelschemaarchitecture standardthathasdriventheconstructionanduseof relationaldatabasesystems.Suchaschemaarchitec- tureconsistsofthestorageschemadescribingthestor- agestructuresforadatabase,theconceptualschema describingthelogicalmodelofthedatabase,andthe  THEORYANDPRACTICEOFOBJECTSYSTEMS,Vol.(VolumeNumber)((OptionalIssueNumber)),128(Year)CCC(ccclineinformation)   externalschemadescribingthederivedviewsofthecon- ceptualschemaforparticularusersorgroupofusers.Thedenitionofaviewmechanismhasbeenrecog- nizedtobeafundamentalaspectalsoforthepractical developmentofobject-orientedapplications.Inthis newcontext,viewsshouldbestillusedasshorthandin queries,shouldsupporttheintegrationofheterogeneous databases,shouldbethebasisforcontent-dependent authorizationand,nally,shouldsupportthesimula- tionofschemachanges.Indeed,asithasbeenrecog- nizedbyseveralresearchers8,11,34],viewsallowto dynamicallymodifyadatabaseschemayetretainingits olderversions,averyimportantcapabilityforadvanced applications24].Moreover,afoundationforexternal schemaswillcompletethedevelopmentofathree-level schemaarchitectureforobject-orienteddatabasesys- tems,comparabletothatforrelationaldatabasesys- tems.Unfortunately,thedenitionofanobject-oriented viewmechanismdoesnotcomeforfreefromtherela- tionalapproach.Themainproblemsinthedenitionof anobject-orientedviewmechanismcanbesummarized asfollows: 1  .Theobject-orientedmodelisfarmorecomplex thantherelationalone.Whereasarelational schemaconsistsofasetofindependentrelations,an object-orientedschemaisaclasshierarchy,where classesareconnectedbyinheritancerelationships.Aviewmodelshouldprovideananswertotheques- tion: Howareviewsintegratedintheexistingclass hierarchy?  2  .Objectshaveanidentity.Aview,atthedatalevel,shouldbeaclass.Butwhatareviewinstances? Aretheyvalues,orexistingobjects,ornewlygener- atedobjects?Notethatthisproblemdoesnotarise intherelationalmodel,wherenostrongidentity concept,suchastheobjectidentier,ismodeled. Severalapproacheshavebeenproposedtomodel viewsinobject-orienteddatabasesystems1,8,36,39,42](see30]forasurvey).Theyaddresstheprevious issuesaccordingtodierentapproaches.Ingeneral,be- sidesbeingbasedupondierentdatamodelsandex- ploitingdierentquerylanguagestoexpressviewpop- ulations,theproposalsdierfor:thesetoffunctional- itiessupportedbytheviewmechanism(shorthandin queries,externalschemadenition,schemaevolution,authorization,etc.);theapproachwithrespecttothe placementofviewsintheschema;thepropertiesas- signedtoviewsobjects(i.e.,whetherornotpersist- entobjectidentiersareprovided);theupdateoper- ationsallowedonviews.Anoptimalsolutiontothe viewmechanismdoesnotexists.Rather,some  good  solutionscanbedenedforeachclassofchosenfunc- tionalities.Thus,aviewmechanismdenedtosupport schemaevolutionmaybedierentfromaviewmechan- ismdenedforonlyusingviewsasshorthandinqueries.Theaimofthispaperistheformaldenitionofa viewmechanisminthecontextoftheChimeradata model21].Thoughtheviewmechanismhasbeenpro- posedforaparticularobject-orienteddatamodel,the basicconceptsofourviewmechanismcanbeapplied tootherdatamodelsaswellsuchasO  2  19],GemStone 12],aswellastotheODMGstandard15].Thechoice ofChimeraasreferencedatamodelismainlyduetothe factsthat(  i  )aformalspecicationforChimeraexists;(  ii  )themodelisatthesametimedeductive,activeand object-oriented.Thisallowstoinvestigatenewinsights inthecontextofobject-orientedviewmechanisms,such astheuseoflogicallanguagesasabasisfordening views.Notethatanotherinterestingtopicisrelatedto theuseofotherChimeracapabilities,suchaslogicalin- tegrityconstraintsandtriggers,inviewdenition.This topicishoweverlefttofurtherresearch.Ourviewmechanismiscomparableto(orincludes) existingones1,8,36,39,42]forwhatconcernsthe supportedfeatures;however,ourmodelistheonlyone forwhichaformaldenitionisgiven.Becauseofthe similarityoffeatures,ourdenitioncanbeadaptedto otherobject-orientedviewmechanismsand,thus,has valuebeyondtheparticularviewmechanismwepro- pose.Following8],weagreewiththerequirement thataviewmechanismmustsupportschemaevolution.Moreover,webelievethataviewmechanismshouldal- lowthedenitionofexternalschemas,asabasisfor developingobject-orientedapplications.Thatrequires thataview,atclasslevel,mustbeusableinanycon- textinwhichaclassmayappear.Themainfeaturesof ourapproachthusstrictlydependonthosechoices  1  .In particular:   Indeningaview,theusercanchooseamong  object-preservingviews  , object-generatingviews  or  set-tupleviews  ,dependingonwhethertheviewis populatedwithobjectsextractedfromanexisting class,ortheviewmustbeinstantiatedwithnew objects,ortheviewinstancesdonotrequireper- sistentobjectidentiers.Set-tupleviewsallowto supportrelationsintheobjectdatamodel,thus meetingtherequirementsofrelationalobjectmod- elssuchasUniSQL26]orMatisse2].   Following8],wedonotintegrateviewsintheclass inheritancehierarchy.Rather,viewsareorgan- izedinaseparatehierarchy:theyarerelatedbya viewinheritancerelationshipwhichisanalogousto theinheritancerelationshiponclasses.Moreover,theschemaisextendedwithanewrelationship,called  viewderivation  relationship,connectinga viewwiththeclassesfromwhichitisderived.The viewderivationhierarchyisorthogonaltotheclass inheritancehierarchy.   Twoviewlevelsaredevised:viewsandschema views.Viewsarevirtualclassesandcanbeusedin  2THEORYANDPRACTICEOFOBJECTSYSTEMS  |  (Year)   anycontextinwhichclassescanbeused;schema viewsprovidethecapabilityofrestructuringa schemasothatitmeetstheneedofspecicap- plications.Aschemaviewisavirtualschema,that is,aschemawhichconsistsofviewsratherthanof classes. Thus,ourmodelbasicallyextendstheviewmodel presentedin8]withobject-preservingviews,theview inheritancerelationshipandtheconceptofschema view.Moreover,weanalyzeindepthourapproachto theplacementofviewsinschemas,basedontheview derivationrelationship.Thecontributionofthisworkis,besidesdeninga viewmodelforChimera,thedevelopmentofaformal frameworkwithinwhichthemainissuesconcerningthe denitionofaviewmodelaresystematicallyorgan- izedandformallydescribed.Inouropinion,aformal denitionforaviewmodelisausefulcontribution.It iscrucialinclearlyandunambiguouslyspecifyingthe featuresoftheviewmechanismanditisafounda- tionbasedonwhichpropertiesaboutviews(e.g.,up- datepropagation,viewmaintenance)canbeformally statedand,possibly,betterinvestigated.Inparticular,basedonthismodel,wehaveformallydened:sev- eralnotionsofconsistencyforviewinstancesanddata- bases;well-formednessconditionsforviewinheritance hierarchies;thenotionofviewschemaclosurewithre- specttoaggregationandinheritancehierarchies.Toour knowledge,theabovenotionshaveneverbeenformally dened.Thispaperisorganizedasfollows.Section2briey describestheChimeralanguage,introducingtheChi- meraconceptsrelevanttoourviewmodel.InSection 3,ourdesignchoicesarediscussedandcomparedwith themostrelevantapproachespresentedintheliterat- ure.TheviewdenitionlanguageisdescribedinSec- tion4.Sections5and6presenttheformalspecication oftheviewmodelproposedforChimera;inparticular,Section5introducesviewswhileSection6isdevotedto schemaviews.Finally,Section7presentssomeconclu- sionsandoutlinesfuturework. 2.Chimera  Chimeraintegratesanobject-orienteddatamodel,adeclarativequerylanguagebasedondeductiverules andanactiverulelanguageforreactiveprocessing  2  .In whatfollows,werstintroducethebasicnotionsofthe datamodel,thenpresentitsdeductivequerylanguage. 2.1.Chimeraobject-orienteddatamodel  Chimeraprovidesallconceptscommonlyascribed toobject-orienteddatamodels.Itisworthnotingthe followingfeatures:   Likeotherobject-orienteddatamodels(e.g. O  2  19]),Chimeraprovidesboththenotionsof valuesandtypesandthenotionsofobjectsand classes.Valuesareinstancesoftypesandarema- nipulatedbyprimitiveoperators.Valuescanbe primitiveorcomplex.Eachclassisassociatedwith atypedescribingthestructureoftheclassin- stances.Moreover,inordertotypevariablesthat havetobeinstantiatedwithobjectsinstancesofa givenclass,classnamesareallowedastypes.   Objectattributescanbederived,thatis,dened bydeductiverules.   Theimplementationofmethodsmaybespecied byanupdaterule,thatis,arulecontaininga sequenceofupdateprimitiveswhoseexecutionis constrainedbyadeclarativeformula,ormaybe external,implementedinsomeprogramminglan- guage.   Multipleinheritanceandmultipleclassinstanti- ationaresupported.Thus,anobjectcanbelong toseveralclasses,evenclassesnotrelatedinthe inheritancehierarchy.   Classesareobjects.Thereforeaclassdenitioncan includeclassattributes,methodsandconstraints thatcollectivelyapplytotheclass.   Eachclasshasbothintensionalandextensional nature. Intheremainderofthissection,werecalltheaspects oftheChimeradatamodelrelevanttothiswork.A completeformaldenitionofthemodelcanbefound in21].ThesetofChimeratypes  T  (thatarecollectionof values)isdenedastheunionofvaluetypes(  VT  )and objecttypes(  OT  ).Objecttypesareclassnamesand theirinstancesareobjectidentiers.Valuetypescanbe eitherbasicdomains(integers,reals,booleans,charac- ters,strings)orstructuredtypesbuiltbyapplyingthe set,listorrecordconstructorstovalueorobjecttypes.Objecttypesareclassnames.AChimeraclassden- itionconsistsoftwocomponents:the  signature  ,spe- cifyingalltheinformationthattheusermustknowfor usingtheclass,andthe  implementation  ,providingan implementationforthesignature.Thesignaturecon- sistsofanumberofclauses,includingthenameofthe superclassesandthespecicationofthe  classfeatures  :instanceandclassattributes,instanceandclassopera- tions,instanceandclassconstraints,andtriggers.The signaturealsospeciesforeachattributewhetherthe attributeisderivedornot.Theimplementationofa classmustspecifyanimplementationforallderivedat- tributes,operations,constraints,atinstanceaswellas atclasslevel,andtriggersthatarespeciedinthesig- nature.AChimeraclasssignatureischaracterizedbyastruc- turalandabehavioralcomponent,specifyingthesigna- tureofattributesandmethodsforobjectsinstancesof thatclass.Inaddition,aconstraintcomponentcontains  THEORYANDPRACTICEOFOBJECTSYSTEMS  |  (Year)3   thesignatureoftheconstraintsonclassinstances.Be- ingclassattributessupportedinChimera,aclassisalso characterizedbyatime-varyingstate,whosestructure isspeciedinthecorrespondingmetaclass.Finally,a classischaracterizedbyan  extent  anda  properextent  ,denotingthesetofalltheoidsofmembersoftheclass andtheoidsofinstancesoftheclass,respectively.We recallthat,accordingtotheusualterminology,anob-  jectisan  instance  ofaclassifthatclassisthemost specicone,intheinheritancehierarchy,towhichthe objectbelongs.Wheneveranobject  o  isaninstanceof aclass  c  then  o  isalsoa  member  ofallthesuperclasses of  c  .Inadditiontoasignature,classeshaveanimple- mentation.InaChimeraclassimplementation,derived attributesandconstraintsareimplementedbymeans ofdeductiverulesspecifyingthecomputationofvalues,andtheimplementationofanoperationisanexpression oftheform  3  opname  : condition  !  opcode  where  opname  istheoperationnameappliedtoalistof parameters, condition  isaChimeraformula,specifying adeclarativecontroluponoperationexecution,while  opcode  isasequenceofupdateprimitives(objectcre- ationanddeletion,objectmigrationfromoneclassto anotherandstatechanges).Side-eectfreeoperations canbeexpressedinChimerabyrulesconsistingonlyof aconditionwithout  opcode  part.Theycanbeuseful tocomputederiveddata.Givenatype  T  2T  ,itsextension  T  ]]isdenedas thesetoflegalvaluesforthattype.Theextensionof types,likeclasses,withanexplicittime-varyingextent,isthatextent.Inparticular,foranobjecttype  c  ,  c  ]]isthesetofoidsofmembersofclass  c  .Startingfrom theextensionsofpredenedbasictypes,whicharepos- tulated,theextensionsofothervaluetypesaredened inaquitestraightforwardway21].AChimeraobjectischaracterizedbyanimmutable identierandastate.Thesetofclassestowhichthe objectbelongsasaninstanceisassociatedwitheach object.Eachobjectisrequiredtobeinstanceofone class.Chimeraprovidesmultipleinheritanceandmultiple classinstantiation.Inheritancerelationshipsamong classesaredescribedbyanISAhierarchyestablishedby theuser.ThisISAhierarchyrepresentswhichclasses aresubclassesof(inheritfrom)otherclasses.Asetof conditionsmustbesatisedbytwoclassesrelatedby theISArelationship.Theseconditionsarerelatedto thefactthateachsubclassmustcontainallattributes,operations,constraints(bothontheclassaswellon theinstancelevel)ofallitssuperclasses.Apartfrom theinheritedconcepts,additionalfeaturescanbein- troducedinasubclass.Inheritedconceptsmaybere- dened(overwritten)inasubclassdenitionundera numberofrestrictions.Indeed,inChimerathereden- itionofthesignatureofanattributeispossiblebyspe- cializingthedomainoftheattribute.Theredenition ofthesignatureofanoperationmustverifythe  covari- ancerule  forresultparametersandthe  contravariance rule  fortheinputones.Therefore,resultparameter domainsmaybespecialized,whereasinputparameter domainsmaybegeneralized,inthesubclasssignature oftheoperation.Theimplementationofanattributeor anoperationmayberedenedaswell,introducingadif- ferentimplementationoftherespectiveconcept,which \overrides"theinheriteddenition.Theredenition ofderivedandextensionalattributesisnotallowedif aderivedattributebecomesextensionalorvice-versa.ConstraintredenitionisnotcurrentlyallowedinChi- mera.Wealsorequirethattheextentofasubclassisa subsetoftheextentofallitssuperclasses.Whiletheredenitionofoperationsdoesnothinder thetypesafetyofthelanguage,theredenitionofat- tributesmustbeconsideredcarefully21].Thecov- ariantredenitionofattributes(thedomainofanat- tributemaybespecializedinsubclasses)reectswhat isusuallyneededwhencreatingataxonomyofclasses;indeed,whenspecializingaclassthedesignerusually needstoaddnewattributesortospecializeexisting ones.Theproblemsarisingwhenattributesarere- denedinacovariantwayalongtheinheritancehier- archyhavebeenrstrecognizedbyCardelli14].The approachadoptedinChimeraistoconsiderthedomains ofattributesasintegrityconstraints,thuscheckedrun- time,ratherthandealingwiththemastypeconstraints,tobecheckedstatically.Thus,wheneveravalueisas- signedtoanobjectattributewedynamicallycheckthat thevalueisappropriateforthedomain.Attheintensionallevel(schemalevel)theordering onclassesimposedbytheISAhierarchyissaidtobe well-dened(  int-well-dened  21])ifeachsubclasscon- tainsallthefeaturesofthesuperclasses,possiblyre- denedassketchedpreviously.Attheextensionallevel (instancelevel),theorderingonclassesimposedbythe ISAhierarchyissaidtobewell-dened(  ext-well-dened  21])ifitisconsistentwiththesetinclusionrelationship onclassextents.AChimera  baseschema  isasetofclasses,related byinheritanceandaggregationrelationships,modeling thestructuralandbehavioralaspectsoftheproblem domain.Abaseschemaisthedatabaseinitialschema denedbythesystemadministrator,onwhichtheob-  jectdatabaseiscreated.Anobjectdatabaseisacon- sistentsetofobjects,coupledwithtwofunctions,one,referredtoas  oidassignment  ,handlesclassextents,thatis,mapsobjectstoclasses,whiletheotherone assignsvaluestoclassattributes.Foranobjectdata- basetobeconsistent,eachobjectmustbelongtoaclass denedintheschema,eachobjectstatemustcontain  4THEORYANDPRACTICEOFOBJECTSYSTEMS  |  (Year)   alegalvalueforeachattributeofeachclasstheob-  jectbelongsto,andmustmeeteachconstraintinsuch classes;nally,theISAorderingisrequiredtobeext- well-dened.Givenabaseschema, S  ,theterm  base objectdatabase  willdenoteanobjectdatabasethatis instanceof  S  ;theobjectsinthatdatabasewillbere- ferredtoas  baseobjects  . 2.2.Chimeraformulasandrules  InthissubsectionweintroduceChimerarules,which areameantoexpressdeclarativeconditionsonadata- base.Besidesbeingusedtospecifytheimplementation ofdierentclassfeatures,Chimerarulesareusedto expressqueries.First,weconsiderthesetofChimera terms,whichisinductivelydenedasfollows:   variablesareterms;   values(basicandcomplexones),exceptsoids  4  ,are terms;   pathexpressions(builtmakinguseofthedotnota- tion)areterms;pathexpressionsmaycontainat- tributeaccessesandmethodinvocations,provided thattheinvokedmethodisside-eectfree  5  . Inaddition,anumberoftermsobtainedusingclassical predenedoperatorsforintegers,reals,listsandsets areconsidered.Chimeraatomicformulasarebuiltbyapplyingapre- dicatesymboltoalistofparameterterms.Asstated bythefollowingdenition,weconsiderthreekindsof atomicformulas  6  . Denition1(AtomicFormulas)21].  Chimera atomicformulasaredenedasfollows:    if  t  1  ;t  2  aretermsand  op  2f  <;>;   ;   ; =  ; ==  ; ==  d  g  7  isapredenedpredicate,then  t  1  opt  2  is acomparisonformula;   if  t  1  ;t  2  areterms,orif  t  1  isatermand  t  2  2CI  isa classname,then  t  1  int  2  isamembershipformula;   if  t  isatermand  c  isaclass(ortype)name,then  c  (  t  )isaclassformula. 2  Complexformulas(orsimplyformulas)areobtained fromatomicformulasandnegatedatomicformulasby meansofconjunctions.Allvariablesareassumedtobe implicitlyquantiedasinDatalog16]. Denition2(Formulas)21].  Formulasarein- ductivelydenedasfollows:    allatomicformulasareformulas;   if  F  isanatomiccomparisonormembership formula  8  ,then  :  F  isa(complex)formula;   if  F  1  and  F  2  areformulas,then  F  1  ^  F  2  isa(com- plex)formula. 2  Denition3(Rules)21].  AChimeraruleisanex- pressionoftheform  Head    Body  where  Head  isanatomicformulaand  Body  isanar- bitraryformula,suchthateachvariablein  Head  occurs in  Body  and  Body  containsexactlyoneclassformula foreveryvariableappearingintherule.  2  Theinterestedreadercanndadditionaldetailson Chimerarulesandtheirsemanticsin21]. 3.Dimensionsinviewdesign  Inthissection,wediscussthemaindimensionsin thedesignofaviewmechanism.Foreachdimension,we contrastourchoicewiththeonesmadebymostrelevant viewmodelsintheliterature.Besideschoosingareferencedatamodelandaquery language,themaindesignchoicesconcern:   howviewsareinsertedinthedatabaseschema;   whetherviewsareonlypopulatedwithbaseob-  jects(object-preservingviews),oritispossibleto populateaviewbycreatingnewobjects(object- generatingviews). Thechoicestobetakenwithrespecttothosedimen- sionsarestronglyinuencedbythefunctionalitiestobe supportedbytheviewmechanism.Inwhatfollows,we rstpresentthegoalsoftheproposedmodel,andana- lyzetheirimplicationsontheviewmechanism.Next,weanalyzethetwomaindimensions,showinghowthe chosenobjectivesaectourchoices.Subsection3.4con- cludesthediscussionbysummarizinginTable1the comparisonamongourmodelandotherviewmodels proposedintheliterature. 3.1.Viewfunctionalities  Whereasrelationalviewshavebeenusedforexternal schemadenition,dataprotection(content-basedau- thorizations)andshorthandforqueries,object-oriented viewscanbeexploitedalsoforotherkindsoffunctional- ity,suchassupportingschemaevolutionandintegrating heterogeneousdatabases.Theuseofviewsforinteg- ratingheterogeneousdatabaseschemashasbeencon- sideredin22,27]:aviewdenitionintegratessemantic- allyequivalentclassesbelongingtodierentschemas.Theuseofviewstosimulateschemaevolution,allowing theuserstoexperimentwithschemachangeswithout aectingotherusers,wasrstproposedin8].Views cansupporttheimplementationofaschemaversioning mechanism,suchthatanyobjectstoredinthedata- basecanbeaccessedandmodiedfromanyschema versionincludingaviewoftheobjectclass.Recently,otherviewmodelshaveconsideredthatuseofviews 11,27,34].Thepropertiesofaviewmechanismde- termineswhichschemamodicationscanbesimulated.Forinstance,themodelspresentedin11,34]allowto  THEORYANDPRACTICEOFOBJECTSYSTEMS  |  (Year)5 
Search
Tags
Related Search
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks