Ieee Computer 2004

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COVERFEATURE

PhilipK.

McKinleySeyedMasoudSadjadi

EricP.

KastenBettyH.C.Cheng

MichiganStateUniversity

ComposingAdaptiveSoftware

Compositionaladaptationenablessoftwaretomodifyitsstructureandbehaviordynamicallyinresponsetochangesinitsexecutionenvironment.

Areviewofcurrenttechnologycompareshow,when,andwhererecompositionoccurs.IInterestinadaptivecomputingsystemshasincreaseddramaticallyinthepastfewyears,andavarietyoftechniquesnowallowsoftwaretoadaptdynamicallytoitsenvironment.Tworevolutionsinthecomputingfieldaredrivingthisdevelopment.Firstistheemergenceofubiquitouscomputing,1whichfocusesondissolvingtraditionalboundariesforhow,when,andwherehumansandcomputersinteract.Forexample,

mobilecomputingdevicesmustadapttovariableconditionsonwirelessnetworksandconservelimitedbatterylife.Secondisthegrowingdemandforautonomiccomputing,2whichfocusesondevelopingsystemsthatcanmanageandprotectthemselveswithonlyhigh-levelhumanguidance.Thiscapabilityisespeciallyimportanttosystemssuchasfinancialnetworksandpowergridsthatmustsurvivehardwarecomponentfailuresandsecurityattacks.

MiddlewareCompositionaladaptation

SeparationofconcernsComputationalreflectionComponent-baseddesignFigure1.Maintechnologiessupportingcompositionaladaptation:separationofconcerns,computationalreflection,andcomponent-baseddesign.

Therearetwogeneralapproachestoimplementing


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softwareadaptation.Parameteradaptationmodifiesprogramvariablesthatdeterminebehavior.TheInternetsTransmissionControlProtocolisanoften-citedexample:TCPadjustsitsbehaviorbychangingvaluesthatcontrolwindowmanagementandretransmissionsinresponsetoapparentnetworkcongestion.Butparameteradaptationhasaninherentweakness.Itdoesnotallownewalgorithmsandcomponentstobeaddedtoanapplicationaftertheoriginaldesignandconstruction.Itcantuneparametersordirectanapplicationtouseadifferentexistingstrategy,butitcannotadoptnewstrategies.

Bycontrast,compositionaladaptationexchangesalgorithmicorstructuralsystemcomponentswithothersthatimproveaprogramsfittoitscurrentenvironment.Withcompositionaladaptation,anapplicationcanadoptnewalgorithmsforaddressingconcernsthatwereunforeseenduringdevelopment.Thisflexibilitysupportsmorethansimpletuningofprogramvariablesorstrategyselection.Itenablesdynamicrecompositionofthesoftwareduringexecutionforexample,toswitchprogram

componentsinandoutofamemory-limiteddeviceortoaddnewbehaviortodeployedsystems.

Dynamicrecompositionofsoftwaredatesbacktotheearliestdaysofcomputing,whenself-modifyingcodesupportedruntimeprogramoptimizationandexplicitmanagementofphysicalmemory.However,suchprogramsweredifficulttowriteanddebug.Severalnewsoftwaretoolsandtechnologiesnowhelpaddresstheseproblems.Giventheincreasingpaceofresearchincompositionaladaptation,weofferareviewofthesupportingtechnologies,proposedsolutions,andareasthatrequirefurther

study.

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MiddlewareandAdaptation

Muchrecentresearchinadaptivesoftwarefocusesonmiddlewarethelayersofservicesseparatingapplicationsfromoperatingsystemsandnetworkprotocols.

DouglasSchmidtdecomposesmiddlewareintofourlayers,1showninFigureA:

Host-infrastructuremiddlewareresidesatoptheoperatingsystemandprovidesahigh-levelAPIthathidestheheterogeneityofhardwaredevices,operatingsystems,andtosomeextentnetworkprotocols.Distributionmiddlewareprovidesahigh-levelprogrammingabstraction,suchasremoteobjects,enablingdeveloperstowritedistributedapplicationsinawaysimilartostand-aloneprograms.Corba,DCOM,andJavaRMIallfitinthislayer.Commonmiddlewareservicesincludefaulttolerance,security,persistence,andtransactionsinvolvingentitiessuchasremoteobjects.Domain-specificmiddlewareservicesaretailoredtomatchaparticularclassofapplications.

Mostadaptivemiddlewareisbasedonanobject-orientedprogrammingparadigmandderivedfrompopularmiddlewareplatformssuchasCorba,JavaRMI,andDCOM/.NET.

Manyadaptivemiddlewareapproachesworkbyinterceptingandmodifyingmessages.FigureBshowstheflowofarequest-replysequenceinasimplifiedCorbaclient-serverapplication.Thisapplicationcomprisestwoautonomousprogramshostedontwocomputersconnectedbyanetwork.

AssumethattheclienthasavalidCorbareferencetotheservantobject.Theclientrequesttotheservantgoesfirsttothestub,whichrepresentstheCorbaobjectontheclientside.The

stubmarshalstherequestandsendsittotheclientobjectrequestbroker.TheclientORBsendstherequesttotheserverORB,whereaskeletonunmarshalstherequestanddeliversittotheservant.Theservantrepliestotherequest,bywayoftheserverORBandskeleton.TheclientORBwillreceivethereplyanddispatchittotheclient.

Inrecentyears,numerousstudieshaveaddressedtheissueofhowmiddlewarecanadapttodynamic,heterogeneousenvironmentstobetterserveapplications.2,3Middlewaretraditionallyhidesresourcedistributionandplatformheterogeneityfromtheapplicationbusinesslogic.Thusitisalogicalplacetoputadaptivebehaviorthatisrelatedtocrosscuttingconcernssuchas

QoS,energymanagement,faulttolerance,andsecuritypolicy.

References

1.D.C.Schmidt,MiddlewareforReal-TimeandEmbeddedSystems,Comm.ACM,June2002,pp.43-48.2.Comm.ACM,specialissueonadaptivemiddleware,June2002,pp.30-64.


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3.IEEEDistributedSystemsOnline,specialissueonreflectivemiddleware,June2001;http://dsonline.computer.org/0105/features/gei0105.htm.ApplicationsDomain-specificmiddlewareservicesCommonmiddlewareservicesDistributionmiddlewareHost-infrastructuremiddlewareOperatingsystemsandprotocolsHardwaredevices

FigureA.Four-layerdecompositionofmiddlewaretobridgethegapbetweenanapplicationprogramandtheunderlyingoperatingsystems,networkprotocols,andhardwaredevices.

ClientapplicationServerapplication

ClientApplicationsServantDomainservicesCommonservicesStubSkeletonDistribution

ClientORBServerORBHostinfrastructureSystemplatform

NetworkRequestflow

Replyflow

FigureB.Corbacallsequenceforasimplifiedclient-serverapplication.

ENABLINGTECHNOLOGIESInaddition,thewidespreaduseofmiddleware

Atthecoreofallapproachestocompositionalindistributedcomputinghasbeenacatalystforadaptationisalevelofindirectionforinterceptingcompositionaladaptationresearch.Middlewareandredirectinginteractionsamongprogramenti-providesanaturalplacetolocatemanytypesties.Figure1showsthreetechnologiessepara-ofadaptivebehavior,astheMiddlewareandtionofconcerns,computationalreflection,andAdaptationsidebardescribes.

component-baseddesignthatweconsideraskeytoreconfigurablesoftwaredesign.ProgrammersSeparationofconcernscanusethesetechnologiestoconstructself-adap-Separationofconcerns4enablestheseparatetivesystemsinasystematicandprincipledasdevelopmentofanapplicationsfunctionalbehavopposedtoadhocmanner.3iorthatis,itsbusinesslogicandthecodefor

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BaselevelApplicationMOPsObjectsMetalevelStructuralBehavioralIntrospectionIntercessionIntercessionIntrospectionBaselevelApplicationMOPsObjectsMetalevelStructuralBehavioralIntrospectionIntercessionIntercessionIntrospectionFigure2.Metalevelunderstandingcollectedintometaobjectprotocols.

crosscuttingconcerns,suchasqualityofservice(QoS),energyconsumption,faulttolerance,andsecurity.Anapplicationcannotimplementacrosscutting

concernatasingleprogramlocation;instead,itmustaddthecodeatmanyplaces.Separatingcrosscuttingconcernsfromfunctionalbehaviorsimplifiesdevelopmentandmaintenance,whilepromotingsoftwarereuse.

Separationofconcernshasbecomeanimportantprincipleinsoftwareengineering.5Presently,themostwidelyusedapproachappearstobeaspect-orientedprogramming.6AOPprovidesabstractiontechniquesandlanguageconstructstomanagecrosscuttingconcerns.Thecodeimplementingtheseconcerns,calledaspects,isdevelopedseparately

fromotherpartsofthesystem.InAOP,pointcutsaresetsoflocationsinthecodewherethedevelopercanweaveinaspects.Pointcutsaretypicallyidentifiedduringdevelopment.Later,forexampleduringcompilation,thedeveloperusesaspecializedcompiler,calledanaspectweaver,tocombinedifferentaspectswithanapplicationsbusinesslogictocreateaprogramwithnewbehavior.AnexampleistheAspectJcompiler.AOPproponentsarguethatdisentanglingcrosscuttingconcernsleadstosimplersoftwaredevelopment,maintenance,andevolution.

AOPisimportanttodynamicrecompositionbecausemostadaptationsarerelativetosomecrosscuttingconcern,suchasQoS.AOPenablestheseconcernstobeisolatedfromtherestoftheprogram.However,intraditionalAOPthecompiledprogramisstilltangled.Tosupportdynamicrecomposition,theprogrammerneedsawaytomaintainthisseparationatruntime.

Computationalreflection


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Computationalreflectionreferstoaprogramsabilitytoreasonabout,andpossiblyalter,itsownbehavior.7Reflectionenablesasystemtorevealselecteddetailsofitsimplementationwithoutcompromisingportability.

Reflectioncomprisestwoactivities:introspection

toletanapplicationobserveitsownbehavior,and

intercessiontoletasystemorapplicationacton

theseobservationsandmodifyitsownbehavior.Inaself-auditingdistributedapplication,forexample,softwaresensorscoulduseintrospectiontoobserveandreportusagepatternsforvariouscomponents.Intercessionwouldallowthesystemtoinsertnewtypesofsensors,aswellascomponentsthatimplementcorrectiveaction,atruntime.

AsFigure2shows,areflectivesystem(representedasbase-levelobjects)anditsself-representation(representedasmetalevelobjects)are

causallyconnected,meaningthatmodificationstoeitheronewillbereflectedintheother.

Ametaobjectprotocol(MOP)isaninterfacethatenablessystematicintrospectionandintercessionofthebase-levelobjects.MOPssupporteitherstructuralorbehavioralreflection.3Structuralreflectionaddressesissuesrelatedtoclasshierarchy,objectinterconnection,anddatatypes.Asanexample,ametalevelobjectcanexamineabase-levelobjecttodeterminewhatmethodsareavailableforinvocation.Conversely,behavioralreflectionfocusesontheapplicationscomputational

semantics.Forinstance,adistributedapplicationcanusebehavioralreflectiontoselectandloadacommunicationprotocolwellsuitedtocurrentnetworkconditions.

AdevelopercanusereflectiveservicesthatareeithernativetoaprogramminglanguagesuchasCommonLispObjectSystem(CLOS),Python,orvariousJavaderivativesorprovidedbyamiddlewareplatform.WhencombinedwithAOP,reflectionenablesaMOPtoweavecodeforcrosscuttingconcernsintoanapplicationatruntime.However,dynamicallyloadingandunloading

adaptivecoderequiresthetargetsoftwaremodulestoexhibitaplug-and-playcapability.

Component-baseddesign

Thethirdmajortechnologysupportingcompositionaladaptationiscomponent-baseddesign.Softwarecomponentsaresoftwareunitsthatthirdpartiescanindependentlydevelop,deploy,andcompose.9Popularcomponent-basedplatforms


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includeCOM/DCOM,.NET,EnterpriseJavaBeans,andtheCorbaComponentModel.

Component-baseddesignsupportstwotypesofcomposition.Instaticcomposition,adevelopercancombineseveralcomponentsatcompiletimetoproduceanapplication.Indynamiccomposition,thedevelopercanadd,remove,orreconfigurecomponentswithinanapplicationatruntime.Toprovidedynamicrecomposition,acomponent-basedframeworkmustsupportlatebinding,whichenablescouplingofcompatiblecomponentsatrun

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Table1.Exampleresearchprojects,commercialpackages,andstandardspecificationsthatprovidecompositionaladaptation.ProjectInstitution/Organization

Language-basedprojects

AspectJXeroxPaloAltoResearchCenterCompositionfiltersUniversiteitTwente,TheNetherlandsProgramControlLanguage(PCL)UniversityofIllinoisOpenJavaIBMResearchR-JavaUniversityFederaldeSoCarlos,BrazilKavaUniversityofNewcastle,UKAdaptiveJavaMichiganStateUniversityTransparentReflectiveAspectProgramminginJava(TRAP/J)MichiganStateUniversityMiddleware-basedprojectsDomain-specificserviceslayer:BoeingBoldStroke(BBS)BoeingCommonserviceslayer:CorbaServicesObjectManagementGroupQualityobjects(QuO)BBNTechnologiesAdaptiveCorbaTemplate(ACT)MichiganStateUniversity

InteroperableReplicationLogic(IRL)UniversityofRome,ItalyDistributionlayer:.NETremotingMicrosoftOpenORBandOpenCOMLancasterUniversity,UKTheACEORB(TAO)andComponentIntegratedACEORB(CIAO)DistributedObjectComputingGroupDynamicTAOandUniversallyInteroperableCore(UIC)UniversityofIllinoisOrbix,Orbix/E,andORBacusIonaTechnologiesSquirrelUniversityofKaiserslautern,GermanyAspectIXFriedrich-AlexanderUniversity,GermanyHostinfrastructurelayer:

Javavirtualmachine(JVM)

CommonLanguageRuntime(CLR)SunMicrosystemsMicrosoftIguana/JTrinityCollege,DublinProseSwissFederalInstituteofTechnologyAdaptiveCommunicationEnvironment(ACE)EnsembleDistributedObjectComputingGroupCornellUniversityCross-layerprojectsDistributedExtensibleOpenSystems(DEOS)GraceGeorgiaInstituteofTechnology

UniversityofIllinois

timethroughwell-definedinterfacesusedascontracts.Inaddition,toprovideconsistencywithotherapplications,acomponent-basedframeworkmustsupportcoexistenceofmultipleversionsofcomponents.

Byenablingtheassemblyofoff-the-shelfcomponentsfromdifferentvendors,component-based


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designpromotessoftwarereuse.Moreover,mechanismsformaintainingaprogramscomponentstructureaftertheinitialdeployment,whencombinedwithlatebinding,facilitatecompositionaladaptation.

Middlewareandotherfactors

Inadditiontothethreemaintechnologiessupportingdynamicrecomposition,manyotherfactorshavecontributedtothegrowthinthisarea.Perhapsthemostimportantismiddlewares

increasingroleindistributedcomputing.Middle-wareprovidesalayerthatdeveloperscanexploittoimplementadaptivebehavior.Indeed,manyapproachestocompositionaladaptationarerealizedinvariousmiddlewarelayers.

Othertechnologiesimportanttoadaptivesoftwaredesignincludesoftwaredesignpatterns,mobileagents,generativeprogramming,adaptiveprogramming,andintentionalprogramming.5

COMPOSITIONALADAPTATIONTAXONOMY

Researchersanddevelopershaveproposedawidevarietyofmethodsforsupportingcompositionaladaptation.Table1listsseveralresearchprojects,commercialsoftwarepackages,andstandardspecificationsthatsupportsomeformofcompositionaladaptation.Thelistisbynomeansexhaustive.Rather,itincludesprojectsthatexemplifythe

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Table2.Softwarerecompositiontechniques.

TechniqueDescriptionExamplesFunctionpointersApplicationexecutionpathisdynamicallyVtablesinCOM,delegatesandeventsinredirectedthroughmodificationoffunction.NET,callbackfunctionsinCorbapointers.WrappersObjectsaresubclassedorencapsulatedbyotherACE,R-Java,PCL,QuO,TRAP/Jobjects(wrappers),enablingthewrappertocontrolmethodexecution.ProxiesSurrogates(proxies)areusedinplaceofobjects,ACT,AspectIXenablingthesurrogatetoredirectmethodcallstodifferentobjectimplementations.StrategypatternEachalgorithmimplementationisencapsulated,DynamicTAOandUICenablingtransparentreplacementofoneimplementationwithanother.VirtualcomponentpatternComponentplaceholders(virtualcomponents)areACEandTAOinsertedintotheobjectgraphandreplacedasneededduringprogramexecution.MetaobjectprotocolMechanismssupportingintercessionandOpenJava,Kava,TRAP

/J,OpenORB,introspectionenablemodificationofprogramOpenCOM,Iguana/Jbehavior.AspectweavingCodefragments(aspects)thatimplementaAspectJ,CompositionFilters,TRAP/J,crosscuttingconcernarewovenintoanapplicationAspectIX,Iguana/J,Prosedynamically.MiddlewareinterceptionMethodcallsandresponsespassingthroughaACT,IRL,Prosemiddlewarelayerareinterceptedandredirected.IntegratedmiddlewareAnapplicationmakesexplicitcallstoadaptiveAdaptiveJava,Orbix,Orbix/E,ORBacus,servicesprovidedbyamiddlewarelayer.BBS,CIAO,Iguana/J,Ensemble

distinctionsinataxonomywehavedevelopedbasedonhow,when,andwheresoftwarecompositiontakesplace.Wehaveappliedthetaxonomytomanyadditionalprojects.10

Howtocompose

Thefirstdimensionofourtaxonomyaddressesthespecificsoftwaremechanismsthatenablecompositionaladaptation.Table2listsseveralkeytechniqueswithbriefdescriptionsandexamples.MehmetAksitandZidChoukair8provideanexcellentdiscussionofsuchmethods.

AllofthetechniquesinTable2createalevelofindirectionintheinteractionsbetweenprogramentities.Sometechniquesusespecificsoftwaredesignpatternstorealizethisindirection,whereasothersuseAOP,reflection,orboth.Thetwomiddlewaretechniquesbothmodifyinteractionbetweentheapplicationandmiddlewareservices,buttheydifferinthefollowingway:Middlewareinterceptionisnotvisibletotheapplication,


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whereasintegratedmiddlewareprovidesadaptiveservicesinvokedexplicitlybytheapplication.

Weusethetermcomposertorefertotheentitythatusesthesetechniquestoadaptanapplication.Thecomposermightbeahumanasoftwaredeveloperoranadministratorinteractingwitharunningprogramthroughagraphicaluserinterfaceorapieceofsoftwareanaspectweaver,acomponent

loader,aruntimesystem,orametaobject.Indeed,autonomiccomputingpromisesthat,increasingly,composerswillbesoftwarecomponents.

Whenandwherethecomposermodifiestheprogramdeterminesthetransparencyoftherecomposition.Transparencyreferstowhetheranapplicationorsystemisawareoftheinfrastructureneededforrecomposition.Forexample,amiddlewareapproachtoadaptationistransparentwithrespecttotheapplicationsourcecodeiftheapplicationdoesnotneedtobemodifiedtotakeadvantageoftheadaptivefeatures.Differentdegreesoftransparency(withrespecttoapplication

source,virtualmachine,middlewaresource,andsoon)determineboththeproposedsolutionsportabilityacrossplatformsandhoweasilyitcanaddnewadaptivebehaviortoexistingprograms.10

Whentocompose

Second,wedifferentiateapproachesaccordingtowhentheadaptivebehavioriscomposedwiththebusinesslogic.Generallyspeaking,latercompositiontimesupportsmorepowerfuladaptationmethods,butitalsocomplicatestheproblemofensuringconsistencyintheadaptedprogram.For

example,whencompositionoccursatdevelopment,compile,orloadtime,dynamismislimitedbutitiseasiertoensurethattheadaptationwillnotproduceanomalousbehavior.Ontheother

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Applicationtype

Figure3.Classification

Mutable

forsoftware

Dynamic

compositionusing

composition

Tunable

thetimeofcompositionor

Configurable

recompositionasaclassification

Static

Customizable

compositionmetric.

Hardwired

Development

Compile/link

Load

Runtime

timetime

time

Increasingdynamism

hand,whileruntimecompositionisverypowerful,itisdifficulttousetraditionaltestingandformalverificationtechniquestochecksafetyandothercorrectnessproperties.Figure3illustratestheuseofcompositiontimeastheclassificationmetricforadaptiveapplications.Theverticalaxislistsapplicationtypesthatimplementeitherstaticordynamiccomposition.Staticcompositionmethodstakeplaceatdevelopment,compile,orloadtime,whereasdynamiccompositionreferstomethods


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thatacomposercanapplyatruntime.

Staticcomposition.Ifanadaptiveprogramiscomposedatdevelopmenttime,thenanyadaptivebehaviorishardwiredintotheprogramandcannotbechangedwithoutrecoding.

Alternatively,adeveloperorusercanimplementalimitedformofadaptationatcompiletimeorlinktimebyconfiguringtheapplicationforaparticularenvironment.Forexample,aspect-orientedprogramminglanguagessuchasAspectJenableweavingofaspectsintoprogramsduringcompilation.Aspectsmightimplementanenvironment-specificsecurityorfault-tolerancepolicy.Suchcustomizableapplicationsrequireonlyrecompilationorrelinkingtofittoanewenvironment.

Configurableapplicationsdelaythefinaldecisiononthealgorithmicunitstouseinthecurrentenvironmentuntilarunningapplicationloadsthecorrespondingcomponent.Forexample,theJavavirtualmachine(JVM)loadsclasseswhenaJavaapplicationfirstusesthem.Althoughweconsider

load-timecompositionatypeofstaticcomposition,itoffersmoredynamismthanotherstaticmethods.Whentheapplicationrequeststheloadingofanewcomponent,decisionlogicmightselectfromalistofcomponentswithdifferentcapabilitiesorimplementations,choosingtheonethatmostcloselymatchesthecurrentneeds.Forexample,ifauserstartsanapplicationonahandheldcomputer,theruntimesystemmightloadaminimaldisplaycomponenttoguaranteeproperpresentation.

Otherload-timeapproachesworkbydynamicallymodifyingtheclassitselfasitisloaded.For

example,toprovideruntimemonitoringanddebuggingcapabilities,KavaenablestheJVMtomodifythebytecodeasitloadsaclass.

Dynamiccomposition.Themostflexibleapproachestocompositionaladaptationimplementitatruntime.Acomposercanreplaceorextendalgorithmicandstructuralunitsduringexecutionwithouthaltingandrestartingtheprogram.Wedifferentiatetwotypesofapproachesaccordingtowhetherornotthecomposercanmodifytheapplicationsbusinesslogic.

Tunablesoftwareprohibitsmodificationofcodeforthebusinesslogic.Instead,itsupportsfine-tuningofcrosscuttingconcernsinresponsetochangingenvironmentalconditions,suchasdynamicconditionsencounteredinmobilecomputingenvironments.AnexampleisthefragmentobjectmodelusedinAspectIX,whichenablesruntimetuningofaCorbaapplicationsdistributionbehavior.


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Incontrast,mutablesoftwareallowsthecomposertochangeeventheprogramsimperativefunction,enablingdynamicrecompositionofarunningprogramintoonethatisfunctionallydifferent.Forexample,intheOpenORBmiddlewareplatform,allobjectsinthemiddlewareandapplicationcodehavereflectiveinterfaces,soatruntimethereflectiveapplicationcanchangevirtuallyanyobjectinanyway,includingmodifyingitsinterfaceandinternalimplementation.Whileverypowerful,inmostcasesthedevelopermustconstrainthisflexibilitytoensurethesystemsintegrityacrossadaptations.

Wheretocompose

Thefinaldimensioninwhichwecompareapproachestocompositionaladaptationcentersonwhereinthesystemthecomposerinsertstheadaptivecode.Thepossibilitiesincludeoneofthemiddlewarelayers(seeFigureAintheMiddlewareandAdaptationsidebar)ortheapplicationcodeitself.Inthissurvey,wedonotdiscusschangestotheoperatingsystem;however,wenotethatoperatingsystemextensibilityisanactiveresearcharea.

Moreover,adaptationsincross-layerframeworks

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suchasDEOSandGraceinvolvethecoop-

Introducingadaptivebehaviorinhighermiddlewarelayersenablesportabilityacrossvirtualmachines.

erationoftheoperatingsystem,middleware,andapplication.

Middlewarelayers.Projectsinvolvingcompositionaladaptationatthehost-infrastructuremiddlewarelayergenerallyfallinoneoftwogroups.Oneapproachistoconstructalayerofadaptablecommunicationservices.ACEisanearlyexamplethatusedservicewrappersandC++dynamicbindingtosupportadaptableinterprocesscommunication

andeventhandlingservices.Ensemblepro

videsalayeredarchitecturethatenablesadistributedapplicationtoselectaparticularcommunicationprotocol.

Thesecondapproachistoprovideavirtualmachinewithfacilitiestointerceptandredirectinteractionsinthefunctionalcode.Forexample,JVMandcommonlanguageruntime(CLR)facilitatedynamicrecompositionthroughreflectionfacilitiesprovidedbytheJavalanguageand.NETplatform,respectively.R-Javasupportsmetaobjects

byaddinganewinstructiontotheJavainterpreter,whileProseandIguana/JuseaspectweavingtoaddbehavioralreflectiontothestandardJVM.Ingeneral,approachesinthiscategoryareveryflexiblewithrespecttodynamicreconfigurationinthattheyallownewcodetobeintroducedatruntime.However,theyusecustomizedvirtualmachinestoprovidetransparencytotheapplication,whichmayreduceportability.

Introducingadaptivebehaviorinhighermiddlewarelayersdistribution,commonservices,anddomain-specificservicesenablesportability

acrossvirtualmachines.Theseapproachestypicallyinvolvemiddlewarecomponentsthatinterceptmessagesassociatedwithremotemethodinvocationsandredirectormodifytheminamannerthataccountsforcurrentconditions.Forsomeframeworks,theapplicationdeveloperconstructsexplicitcallstoadaptivemiddlewareservices.ExamplesincludeOrbix,Orbix/E,ORBacus,CIAO,andBoeingBoldStroke.QuOuseswrappersaroundCorbastubsandskeletonstogaincontrol


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ofthecallsequence,whereasIRLandACTuseCorbaportableinterceptorstodoso.Portableinterceptorsserveasgenerichooksthatacomposercanuseatruntimetoloadothertypesofinterceptors.Sinceausercanloadaportableinterceptorusingacommand-lineparameter,thisapproachenablesthecomposertointegrateadaptivecomponentsintotheprogramwithoutmodifyingeithertheapplicationorthemiddlewarecode.

Applicationcode.Althoughmiddlewareapproachessupporttransparentadaptation,theyapplyonlytoprogramsthatarewrittenagainstaspecificmiddlewareplatform.Amoregeneralapproachisfordeveloperstoimplementcompositionaladaptationintheapplicationprogramitself.

Twomaintechniquesareavailable.Thefirstistoprogramallorpartoftheapplicationcodeusingalanguagethatdirectlysupportsdynamicrecomposition.Somelanguages,suchasCLOSorPython,providesupportinherently,whileothershavebeenextendedtosupportadaptation.Forexample,Open

Java,R-Java,Handi-Wrap,PCL,andAdaptiveJavaallextendJavatoincludenewkeywordsandconstructsthatenhancetheadaptivecodesexpressiveness.However,thisapproachrequiresthedevelopertousethesefeaturesexplicitlyinconstructingtheprogram.

Thesecondtechniqueistoweavetheadaptivecodeintothefunctionalcode.AspectJandCompositionFiltersweaveadaptivebehaviorintoexistingapplicationsatcompiletime.Incontrast,toolssuchasTRAP/Juseatwo-stepapproachtoenabledynamicrecomposition.Inthefirststep,an

aspectweaverinsertsgenericinterceptionhooks,inthiscaseimplementedasaspects,intotheapplicationcodeatcompiletime.Inthesecondstep,acomposerdynamicallyweavesnewadaptivecomponentsintotheapplicationatruntime,andametaobjectprotocolusesreflectiontoforwardinterceptedoperationstotheadaptivecomponents.Thisapproachoffersawaytoaddadaptivebehaviortoexistingapplicationstransparentlywithrespecttotheoriginalcode.Suchacapabilityisimportantasusersexpectlegacyapplicationstoexecuteeffectivelyacrossanincreasinglydiversecomputinginfrastructure.

KEYCHALLENGES

Despitemanyadvancesinmechanismstosupportcompositionaladaptation,thefullpotentialofdynamicallyrecomposablesoftwaresystemsdependsonfundamentaladvancesonfourotherfronts.

Assurance


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Recomposablesoftwaredesignrequiresaprogrammingparadigmthatsupportsautomatedcheckingofbothfunctionalandnonfunctionalsystemproperties.11

Tohelpensuretheadaptedsystemscorrectness,developersmustfirstcertifyallcomponentsforcorrectnesswithrespecttotheirspecifications.Theycanobtainthiscertificationeitherbyselectingcom

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ponentsthathavealreadybeenverifiedandvalidatedofflineusingtraditionaltechniques,suchastesting,inspection,andmodelchecking,orbygeneratingcodeautomaticallyfromspecifications.Thecertificationcanincludenonfunctionalrequirements,suchassecurityandperformance,aswellasfunctionalrequirements.

Second,techniquesareneededtoensurethatthesystemstillexecutesinanacceptable,orsafe,mannerduringtheadaptationprocess.Ourgroupandothersareusingdependencyanalysistoaddressthisproblem.Inaddition,developerscanusehigh-levelcontracts12andinvariantstomonitorsystemcorrectnessbefore,during,andafteradaptation.

Security

Whereasassurancedealsprimarilywithsystemintegrity,securityaddressesprotectionfrommaliciousentitiespreventingwould-beattackersfromexploitingtheadaptationmechanisms.Inadditiontoverifyingcomponentsources,anadaptivesoftware

systemmustprotectitscorefromattackers.Variouswell-studiedsecuritymechanismsareavailable,suchasstrongencryptiontoensuretheconfidentialityandauthenticityofmessagesrelatedtoadaptation.

However,thesystemmustalsohideadaptationmanagementfromwould-beintrudersandpreventthemfromimpedingorcorruptingtheadaptationprocess.Acomprehensiveapproachtothisproblemmustensuretheintegrityofthedatausedindecision-makingandconcealtheadaptiveactions,perhapsbyobscuringthemwithinothersystemactivities.

Interoperability

Distributedsystemsthatcanadapttotheirenvironmentmustbothadaptindividualcomponentsandcoordinateadaptationacrosssystemlayersandplatforms.Softwarecomponentsarelikelytocomefromdifferentvendors,sothedevelopermayneedtointegratedifferentadaptivemechanismstomeetanapplicationsrequirements.Theproblemiscomplicatedbythediversityofadaptivesoftwareapproachesatdifferentsystemlayers.Evensolutionswithinthesamelayerareoftennotcompatible.

Developersneedtoolsandmethodstointegratetheoperationofadaptivecomponentsacrossthelayersofasinglesystem,amongmultiplecomputingsystems,andbetweendifferentadaptiveframeworks.

Decisionmaking

Adaptivesystemsrespondtoadynamicphysical


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world.Theymustactautonomously,modifying

softwarecompositiontobetterfitthecurrent

environmentwhilepreventingdamageorlossofservice.Decision-makingsoftwareusesinputfromsoftwareandhardwaresensorstodecidehow,when,andwheretoadaptthesystem.Interactivesystemsmayevenrequirethedecisionmakertolearnaboutandadapttouserbehavior.

Someresearchershaveconstructedsoft

waredecisionmakersusingrule-basedapproachesorcontroltheory.Othershavedesigneddecisionmakerswhoseactionsareinspiredbybiologicalprocesses,suchasthehumannervoussystemandemergentbehaviorininsectspeciesthatformcolonies.

Theseapproachestodecisionmakinginadaptivesoftwarehavebeeneffectiveincertaindomains,but

environmentaldynamicsandsoftwarecomplexityhavelimitedtheirgeneralapplication.Moreextensiveresearchindecisionmakingforadaptivesoftwareisneeded.Futuresystemsmustaccommodatehigh-dimensionalsensorydata,continuetolearnfromnewexperience,andtakeadvantageofnewadaptationsastheybecomeavailable.

MManyofthemechanismsforcompositionaladaptationareavailablenow,andweexpecttheirusetoincreaseasprogrammersbecome

morefamiliarwithadaptivesoftwaretechnologiesandsocietycomestoexpectcomputersystemstomanagethemselves.Thereisapotentialdownside,however,inthelackofsupportingdevelopmentenvironments.Compositionaladaptationispowerful,butwithoutappropriatetoolstoautomaticallygenerateandverifycode,itsusecannegativelyimpactratherthanimprovesystemintegrityandsecurity.

Thecomputersciencecommunitymustbuilddevelopmenttechnologiesandtools,wellgroundedinrigoroussoftwareengineering,tosupportcompositionaladaptation.Thisfoundationwillraise

thenextgenerationofcomputingtonewlevelsofflexibility,autonomy,andmaintainabilitywithoutsacrificingassuranceandsecurity.

Acknowledgments

Weexpressourgratitudetothemanyindividualswhohavecontributedtothisemergingareaofstudy.DiscussionswithresearchersassociatedwithmanyoftheprojectslistedinTable1havegreatly


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improvedourunderstandingofthisarea.WealsothankthefacultyandstudentsintheSoftwareEngineeringandNetworkSystemsLaboratoryat

Thesystemmustalsohideadaptationmanagementfromwould-beintruders.

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MichiganStateUniversityfortheircontributionstoRAPIDware,Meridian,andrelatedprojects.

ThisworkwassupportedinpartbyNationalScienceFoundationgrantsCCR-9901017,CCR9912407,EIA-0000433,EIA-0130724,andITR0313142,andbytheUSDepartmentoftheNavy,OfficeofNavalResearch,undergrantno.N0001401-1-0744.

Furtherinformation

Ourgroupisparticipatingincompositionaladaptationresearchthroughtwoprojects:RAPIDware(www.cse.msu.edu/rapidware)addressesadaptivesoftwareforprotectingcriticalinfrastructures,andMeridian(www.cse.msu.edu/meridian)addressesautomatedsoftwareengineeringformobilecomputing.Amongotherartifacts,theseprojectsproducedACT,AdaptiveJava,andTRAP/J.ThetechnicalreportonourtaxonomyisalivingdocumentavailablethroughtheRAPIDwareURL.

References

1.M.Weiser,HotTopics:UbiquitousComputing,Computer,Oct.1993,pp.71-72.2.J.O.KephartandD.M.Chess,TheVisionofAutonomicComputing,Computer,Jan.2003,pp.41-50.3.G.S.Blairetal.,AnArchitectureforNext-GenerationMiddleware,Proc.IFIPIntlConf.DistributedSystemsPlatformsandOpenDistributedProcessing(Middleware98),Springer,1998,pp.191-206.

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SystemsintoModules,Comm.ACM,Dec.1972,pp.1053-1058.5.K.CzarneckiandU.Eisenecker,GenerativeProgramming,Addison-Wesley,2000.6.G.Kiczalesetal.,Aspect-OrientedProgramming,Proc.EuropeanConf.Object-OrientedProgramming(ECOOP),LNCS1241,Springer-Verlag,1997,pp.220-242.

7.P.Maes,ConceptsandExperimentsinComputationalReflection,Proc.ACMConf.Object-OrientedProgrammingSystems,Languages,andApplications(OOPSLA),ACMPress,1987,pp.147-155.

8.M.AksitandZ.Choukair,Dynamic,Adaptive,andReconfigurableSystemsOverviewandProspectiveVision,Proc.23rdIntlConf.DistributedComputingSystemsWorkshops(ICDCSW03),IEEECSPress,May2003,pp.84-89.9.C.Szyperski,ComponentSoftware:BeyondObject-OrientedProgramming,2nded.,Addison-Wesley,2002.10.P.K.McKinleyetal.,ATaxonomyofCompositional


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Adaptation,tech.reportMSU-CSE-04-17,Dept.ComputerScienceandEngineering,MichiganStateUniv.,2004.11.N.Venkatasubramanian,SafeComposabilityofMiddlewareServices,Comm.ACM,June2002,pp.49-52.12.A.Beugnardetal.,MakingComponentsContractAware,Computer,July1999,pp.38-45.PhilipK.McKinleyisaprofessorintheDepartmentofComputerScienceandEngineeringatMichiganStateUniversity.Hisresearchinterestsincludeadaptivemiddleware,mobilecomputing,pervasivecomputing,distributedsystems,andgroupcommunication.McKinleyreceivedaPhDincomputersciencefromtheUniversityofIllinoisatUrbana-Champaign.HeisamemberoftheIEEEComputerSocietyandtheACM.Contacthimatmckinley@cse.msu.edu.

SayedMasoudSadjadiisaPhDcandidateintheDepartmentofComputerScienceandEngineeringatMichiganStateUniversity.Hisresearchinterestsincludeadaptivesoftware,middleware,pervasivecomputing,autonomiccomputing,andsensornetworks.

SadjadireceivedanMSinsoftwareengineeringfromAzadUniversityatTehran.HeisastudentmemberoftheIEEEComputerSocietyandtheACM.Contacthimatsadjadis@cse.msu.edu.

EricP.KastenisaPhDcandidateintheDepartmentofComputerScienceandEngineeringandasoftwaredeveloperintheNationalSuperconductingCyclotronLaboratory,bothatMichiganStateUniversity.Hisresearchinterestsincludeautonomiccomputingandlearningalgorithmsforadaptablesoftware.KastenreceivedanMSincomputersciencefromMichiganStateUniversity.Heisamember

[email protected].

BettyH.C.ChengisaprofessorintheDepartmentofComputerScienceandEngineeringatMichiganStateUniversity.Herresearchinterestsincludeformalmethodsforsoftwareengineering,component-basedsoftwaredevelopment,object-orientedanalysisanddesign,embeddedsystemsdevelopment,andvisualization.ChengreceivedaPhDincomputersciencefromtheUniversityofIllinoisatUrbana-Champaign.SheisaseniormemberoftheIEEEComputerSocietyandamemberofthe

[email protected].

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