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1 MobileCodeParadigmsandTechnologies: CarloGhezziandGiovanniVigna ACaseStudy Dip.ElettronicaeInformazione,PolitecnicodiMilano P.zaL.DaVinci23,20100Milano,Italy Abstract.TheopportunitiesoeredbytheInternetareencouraging [ghezzi researchaimedatthecreationofacomputationalinfrastructurethat exploitsthewidespreadcommunicationinfrastructure.themobilecomputationparadigmisaproposaltobuildacomputationalinfrastructure dynamicityandexibility.despitethepromisingrststeps,thereisstill thatgoesbeyondthewell-knownclient-serverparadigmandincreases confusionontheroleofparadigmsandtechnologyinthedevelopment onapplicationsbasedonthemobilecomputationparadigm.wepresent acasestudyinwhichwedevelopseveralversionsofanapplicationusingdierentparadigmsanddierenttechnologiesinordertoshowwhen 1Introduction theseconceptscomeintoplayandwhicharetheirrelationships. Keywords:mobilecode,designparadigms,casestudy largerbandwidthbymeansofnewnetworktechnologiesandprotocols,thecomputationalinfrastructureisstillratherprimitiveandonlyrecentlyhasbecome thefocusofsystematicresearchactivities.bycomputationalinfrastructurewe network-centricapplicationsandthedesignparadigmsaccordingtowhichsuch meanboththetechnologiesavailabletoimplementandsupportexecutionof applicationscanbestructured.althoughapplicationdevelopmentsareliterally andevolvingtechnologies,andverylittleisknownregardingdesignparadigms, besidestheconventionalclient-servermodel. explodingontheinternet,suchapplicationsaremostlydevelopedusingunstable TheInternethasnowbecomethelargestdistributedsystemeverbuilt.While thedeployedcommunicationinfrastructureisevolvingatafastpaceproviding ofdesignparadigms.infact,inordertollthisgap,theconceptofmobile work,thecomputationalinfrastructureiscomposedofa(possiblyworld-wide) researchisneededbothatthelevelofenablingtechnologiesandatthelevel computationshasbecomethefocusofmuchrecentresearch[1].inthisframe- Toconsolidatetheexplosionofnetwork-centricapplications,webelievethat distributedenvironmentwithseveralcomputationalenvironments(ces)that supportthecomputationsofexecutingunits(eus).sucheusmaychangetheir computationalenvironmentandeventhecodetheyexecutedynamically.several

2 paradigm(shortly,mobilecodeapplicationsormcas)lacksamethodological technologies,liketelescript[16],messengers[15],andjava[10]havebeenproposed.still,thedevelopmentofapplicationsbasedonthemobilecomputation background. foranyspecicapplication.thesoftwarearchitecturedesignedaccordingto usingthetechnology(languagesandtheirsupporttools)thatbesttthechosen theselecteddesignparadigmshouldthenbemappedontoanimplementation withmethodsthatallowthemtoselectthemostappropriatedesignparadigm Weenvisionanidealizedprocessbywhichsoftwaredesignersareequipped paradigm. identifythespecicissuesthatcharacterizemobilecodeapplicationsatallstages isoftenblurredandnotwellunderstood.thefocusofourworkisexactly oftheirdevelopmentprocess.inparticular,inthispaperweaddresstheissue onunderstandingtheconceptualfoundationsofmobilecomputations,tryingto Unfortunately,thedistinctionbetweenparadigms[4]andtechnologies[5] thataresuitableforseveralkindsofmobilecodeapplicationsandwediscuss ofdesignparadigmsandtechnologies.weidentifyarepertoireofparadigms theirrelationshipswithtechnologies.wewillshowhowthedierentparadigms aresult,onemightdevelopguidelinesthathelptochoosethemostappropriate canbeimplementedusingdierenttechnologies.wewillalsoshowthatcertain paradigmsaresupportedmoreeasilyandecientlybycertaintechnologies.as directionoftheabovementionedidealizedprocess. technologiesforeachparadigm.thismaybeviewedasaninitialstepinthe andimplementationissues.thecasestudyisthesearchofinformationinadistributeddatabase.insection3weidentifyaninitialrepertoireofdierentdesign paradigmsformobileapplications.basedonsuchparadigms,section4presents Thepaperisorganizedasfollows.InSection2weintroduceacasestudy thatwillbeusedthroughoutthepapertodiscussbothpossibledesignparadigms entimplementationsofourapplication.insection9wediscussimplementation classesoftechnologies,whileinsections6,7,8,weshortlypresentthedier- issues.section10drawssomeconclusionsandillustratesfuturework. dierentdesignalternativesforourcasestudy.insection5wepresentsome 2ACaseStudy asearchengine.dbmssaredistributedoverasetofcomputationalenvironments.theyareidentiedbyalocation,i.e.,thecomputationalenvironment theyarerunningon,andasymbolicnamethatuniquelyidentiesaparticular DBMSinthecorrespondingCE.EachDBMSmanagesasingletablecomposed oftwoelds:keywordanddata.thekeywordisanidentierofanentity.the dataeldmayeitherbesomekindofinformationrelatedtotheentitydenoted bythekeywordortheidentierofanotherdatabasethatmaycontainadditional cationiscomposedofseveraldatabasemanagementsystems(dbmss)andof Ourstudyapplicationisadistributedinformationretrievalsystem.Theappli- informationaboutthecorrespondingkeyword.

3 associatedwithaparticularkeyword,givenaninitialsetofdmbss.ifduring thequeryoftheseinitialdbmsssomereferencestootherdbmssarefound, thesearchenginecontinuesthesearchonthosedbmss. TheSearchEngine'staskistogatherthelargestamountofinformation 3MobileCodeParadigms totakeintoaccounttheseconceptsatthedesignlevel.weidentiedthree mainparadigmsformobilecomputations:remoteevaluation(rev),codeon fromthepointofviewofbothcodeandlocationandthereforeitisnecessary ThedesignofMCAsisachallengingtask.Suchapplicationsarehighlydynamic Demand(COD),andMobileAgent(MA)[4].Althoughwedon'tclaimthat suchparadigmscoverallpossibledesignstructuringstylesfornetwork-centric applications,theycanbeviewedasthemosttypicalrepresentatives. thatexecutesthecodeandholdsthestateofthecomputation,aredistributed puts/outputsofthecomputation,andtheprocessor,i.e.,theabstractmachine theseparadigmsdierinhowtheknow-howoftheapplication,i.e.,thecode thatisnecessarytoaccomplishthecomputation,theresources,i.e.,thein- GiventwointeractingcomponentsAandBofadistributedarchitecture, betweenthecomponents.inordertoletthiscomputationtotakeplace,allthe tothecapabilitiesdescribedabove.thetablealsoliststheclient-server(cs) paradigm.althoughconceptuallycscannotbeviewedasaparadigmformobilecomputations(nomobilitytakesplace),weincludeditbecauseitisahighly usedparadigmonnetwork-centricapplications. interestedinitseects.table1showshowtheparadigmsbehavewithrespect WeassumeAtobetheentitythatcausestheinteractionandtheonethatis threecapabilitiesabovehavetobepresentatonelocationatthesametime. client-server Paradigm Aside { know-how Bside evaluationknow-howresources demand*resources codeon remote mobile agentknow-how processorknow-how processor* Table1.Mobilecodeparadigms.Thistableshowsthelocationofthecapabilitiesjustbeforetheinteractiontakesplace.Thestar(*)indicateswherethe processorresources* capabilitiesmeet,i.e.,wherecomputationassociatedwiththeinteractiontakes

4 services.thecodethatimplementssuchservicesisownedbytheservercomponent,thus,wesaythattheserverholdstheknow-how.itistheserveritself intable1)isinterestedinaccessingsomeentitymanagedbytheserver,and thatexecutestheservice,thusithastheprocessorcapability.theclient(a IntheCSparadigm,aservercomponent(BinTable1)exportsasetof thereforeitistheserverthathastheresources. power(theprocessor)anditsresources,butdoesnotprovideany\specic" bybinitslocation. service.itisathatsendstheservicecode(theknow-how)thatwillbeexecuted IntheREVparadigm,theexecutorcomponent(B)oersitscomputational A,thecomputationiscarriedoutonA'slocation,thus,Aholdstheprocessor holdstheresources. ItisBthatprovidesthecode(i.e.,theknow-how).Oncethecodeisreceivedby capability.thecomputationinvolvesonlylocallesandlocaldevices;thus,a IntheCODparadigm,componentAinitiallyisunabletoexecuteitstask. theresourcesinvolvedreside. computationassociatedwiththeinteractiontakesplaceonb'slocationwhere IntheMAparadigm,Ahastheknow-howandprocessorcapabilities.The 4DesigninganApplication Usually,anapplication(orpartsthereof)maybedesignedfollowingdierent paradigms.inourcasestudy,wehavedesignedourapplicationusingthewellknownclient-serverparadigmandtwomobilecodeparadigms,namely,remote 4.1Client-ServerDesign AccordingtotheCSparadigm,DBMSsplaytheroleofserverswhiletheSearch Engineisasinglecomponentthatactsasaclient.EachDBMSoersaquery operationthat,givenaparticularkeywordandanindexn,returnstheinformationcontainedinthenthtuplecontainingthespeciedkeywordoranerror ifsuchtupledoesnotexist.thesearchenginequeriesthedbmsscontained initsinitialsetandthenupdatessuchsetwithpossiblenewdatabaseswhose EvaluationandMobileAgent. 4.2RemoteEvaluationDesign arenomoreunquerieddbmss. identiershavebeenfoundbypreviousqueries.thisprocessgoesonuntilthere querytheirlocaldbmssusingthesameprimitivesthatdbmssexportinthe AccordingtotheREVparadigm,theSearchEnginecomponentsendsacode evaluatedontheremotelocationswheredbmssarelocated,i.e.,newcomponentsarecreatedtoexecutethecodeonthedestinationsites.suchcomponents fragmenttothelocationofeachdbmssithastoquery.suchfragmentsare CScase.Whenaremotecomponentterminatesitstask,itsendsbacktothe

5 SearchEnginetheresultsofqueries.Iftheresultscontainsomereferencesto otherdbmssnewrevinteractionswiththeselocationsarestarted. FollowingtheMAparadigm,theSearchEngineismodeledasaseriesofcom- 4.3MobileAgentDesign issenttothelocationofeachdbmsofthegivendbmssset.eachcomponent hasaninitiallyempty\to-be-visited"listcontainingthedbmssthathavetobe mationaboutthegivenkeyword.initially,asinthecaseofrev,acomponent ponentsthatroamthroughthelocationsofthedbmssinordertocollectinfor- searchedandanempty\visited"listcontainingthedbmssthathavealready eachvisitingcomponentupdateswiththelistofitsvisiteddbmss. visited"list.there,itperformsthefollowingactions: beenvisited.oneachlocationthathostsadbmsthereisa\history"listthat 2.itupdatesthelocal\history"withits\visited"list; 1.itchecksthelocal\history"andprunesits\to-be-visited"listaccordingly; AcomponentAmigratestothelocationoftherstDBMSinits\to-be- 3.itchecksifanothercomponentBispresentlysearchingthelocalDBMS.If (a)aprovidesbwithitsown\visited"list,sothatbcanupdateits\tobe-visited"listaccordingly; itisso,ainteractswithbfollowingthesesteps: 4.ifits\to-be-visited"listdoesnotcontainthecurrentDBMSanymore,A (b)abalancesits\to-be-visited"listwiththeb'sone; (c)aremovesthecurrentdbmsfromits\to-be-visited"list; 7.migratestothelocationoftherstDBMSofthe\to-be-visited"listifany; 5.iftheliststillcontainsthecurrentDBMS,itsearchestheDBMS,possibly 6.addsthevisitedDBMStothelocal\history"; skipstostep7; otherwiseitgoesbacktothestartinglocationandreportsitsresults. enrichingits\to-be-visited"list; 5ImplementationTechnologies HavingdesignedaMCAaccordingtosomeparadigm,onehastochooseatechnologytoimplementit.Givenaparticularparadigm,whichtechnologyshould beused? Message-basedThesetechnologiesenablethecommunicationbetweenremote WeaklymobileThesetechnologiesprovidemechanismsthatenableanEUto Weidentifythreeclassesoftechnologies[7]: sendcodetobeexecutedinaremotecetogetherwithsomeinitialization EUsintheformofmessageexchange.AtypicalexampleisRPC[2]. dataortobinddynamicallycodedownloadedfromaremoteeu.examples ofsuchtechnologiesarethershfacilityinunix,languageslikem0[14]and Obliq[3],orsystemslikeTACOMA[9]andMole[13].

6 StronglymobileThesetechnologiesenableEUstomovewiththeircodeand typesoftechnologies.wechosetcl-dp[12],obliq[3],andagenttcl[8]. Weimplementedthedierentarchitecturesofourapplicationusingdierent executionstatetoadierentce.anexampleisrepresentedbythetelescript Tcl-DPisanextensionoftheTcllanguage[11]fordistributedprogramming. technology. Tcl-DPprovidessupportforTCP/IPandRPCprogramming.Itisamessagebasedtechnology. supportsremotemethodinvocationandremoteevaluationofcode.thereforeit isbothamessage-basedandaweaklymobiletechnology. AnexecutingTclscriptcanmovefromonehosttoanotherwithasinglejump Obliqisanuntyped,object-based,lexicallyscoped,interpretedlanguage.It instruction.ajumpfreezestheprogramexecutioncontextandtransmitsitto adierenthostwhichresumesthescriptexecutionfromtheinstructionthat followsthejump.thereforeagenttclcanbeviewedasastronglymobile AgentTclprovidesaTclinterpreterextendedwithsupportforEUmigration. technology. designsbasedonthecs,rev,andmaparadigmscanbeimplementedusing Tcl-DP,Obliq,andAgentTcl. Sections6through8provideaninformalandsuccinctdescriptionofhowthe Inthisversionoftheapplication,weusedTcl-DPtoimplementtheCSarchitecture.TheDBMScomponentisimplementedasaprocessrunningaTcl-DP 6.1Message-basedTechnology interpreteractingasanrpcserverthatexportsasinglequeryservicewith thecharacteristicsdescribedinsection2.thesearchenginecomponentis implementedasanotherprocessrunningatcl-dpinterpreterthatactsasan RPCclient.TheclientqueriestheserverusingRPCprimitives.TheCEsare 6.2WeaklyMobileTechnology representedbyunixhosts. 6ImplementingtheClient-ServerArchitecture tousethemechanismsforremoteevaluationofcodeoeredbythelanguage, tecture.evenifobliqenablestheremoteinvocationofmethods,wedecided WeusedtheObliqlanguagetoimplementthisversionoftheapplicationarchiuationofcodesupportstheCSparadigm.Wehaveimplementedlocationsas sinceourgoalherewastoevaluatehowatechnologybasedontheremoteeval- Obliqinterpretersthatexportengines(i.e.,executionservices),andDBMSsas objectsthatenclosethedatabasedataandprovideaquerymethod.thesearch interpretersinordertoevaluatesegmentsofcoderemotely.atrun-timethe SearchEnginechoosesaDBMSandsendsapieceofcodecontainingasingle Engineisathreadinaninterpreterthatusestheenginesexportedbyother

7 querymethodinvocationtotheremoteengineoftheinterpretercontainingthe 6.3StronglyMobileTechnology createdthreadandthentheresultsaredeliveredback. DBMS.Thesingleinvocationisperformedintheremoteinterpreterbyanewly WeusedAgentTcltoimplementtheClient-Serverarchitecture.Inthissetting thedbmssarestationaryagentsthatacceptqueriesfromagentslocatedonthe samesite(representedbyaunixhostextendedwiththeagenttclrun-time toitsstartingsite. support).thesearchengineisamovingagentthat,inordertoqueryadbms, 7ImplementingtheRemoteEvaluationArchitecture jumpstothecorrespondingsite,performsasinglequery,andthenjumpsback WeusedtheTcl-DPlanguagetoimplementtheREVarchitecture.Inthis 7.1Message-BasedTechnology case,theremotesiteisextendedwithaprocessrunningthetcl-dpinterpreter thatperformsasetofqueries,itsendstotheremoteexecutoraservicerequest thatactsasacodeexecutor.itisanrpcserverthatexportstheservice containingthequeryscript.inturn,theexecutorinterpretsthescriptthat localprocesses.whenthesearchenginewantstoevaluateremotelythecode DPinterpretersthatexportthequeryserviceasintheCScase,butonlyto execute.suchservicetakesasaparameteratcl-dpscript.dbmssaretclqueriesthedbmsandthenreturnstheresultsback. WeusedObliqtoimplementtheREVarchitecture.Inthisimplementation,the SearchEnginethreadrequeststheexecutionofthequerycodetotheremote 7.2WeaklyMobileTechnology thecode.thenewlycreatedthreadperformsalltheneededqueriestothelocal enginecorrespondingtoaparticulardbms.anewthreadiscreatedtoexecute DBMS,which,asinthecaseofCS,isanobjectownedbytheremoteinterpreter. 7.3StronglyMobileTechnology Then,theresultsaredeliveredbacktothesourcesite. WeusedAgentTcltoimplementtheREVarchitecture.Inthisimplementation \squad"ofagentsissenttothesitesofthedbmsswhoseidentiershavebeen totheoriginalsitewiththeresults.suchresultsareanalyzedandthenanew foundduringtheexecutionofpreviousqueryprocess. intheinitialset,performthesetofqueryonthelocaldbmss,andthenreturn thesearchenginecreatesasetofagentsthatjumptothesitesofeachdbms

8 WeusedTcl-DPtoimplementtheMAarchitecture.Inthiscase,theDBMSs 8.1Message-BasedTechnology 8ImplementingtheMobileAgentArchitecture computation)intoamessageandthensendthemessagetoanexecutorthat \visited"listsandthevaluesofsomestatevariablesthatkeeptraceoftheir arerpcserversthatacceptqueriesfromlocalprocesseswhilesearchengine unpacksthemessageandcreatesanewcomponent.thenewcomponentuses mobilecomponentspacktheircodeandstate(i.e.,their\to-be-visited"and mobilecomponentsaretcl-dpscripts.inordertomovefromsitetosite,such theinformationstoredinthestatepartofthemessageinordertorestoreits executionow.afterdispatchingthemessagetotheremoteexecutor,thesender terminates. migratefromoneinterpretertoanother,whenasearchenginecomponentmust WeusedObliqtoimplementtheMAarchitecture.SinceObliqthreadscannot migratetoanotherinterpreteritcreatesacopyofitselfontheremotesiteand 8.2WeaklyMobileTechnology thenterminates.inordertokeeptrackoftheexecution,anobjectisusedto maintainthestateofthecomponent.aftereach\jump",thenewlycreated threadmustcreatealocalcopyoftheobjectrepresentingthestateandperform someoperationsbasedonitscontentsinordertore-establishthestateofthe computation. 8.3StronglyMobileTechnology WeusedAgentTcltoimplementtheMAarchitecture.TheSearchEngineis composedofasetofagents.atstartup,suchagentsmigratetothelocationsof i.e.,theyquerythelocaldbmsand,ifreferencestootherdbmssarefound, theymigratetosuchlocations,untiltheir\to-be-visited"listbecomesempty. thegivensetofdbmss.there,theyperformthestepsdescribedinsection4.3, Eventually,theyjumpbacktotheirstartingsiteandreporttheirndings. 9Discussion Thecasestudydescribedintheprevioussectionsshowsthatparadigmsandtechnologiesarenotcompletelyorthogonal.Inprinciple,itispossibletoimplement applicationsdevelopedwithanyparadigmbyusinganykindoftechnology,given thatsuchtechnologiesallowforthecommunicationbetweeneus.however,we havefoundthatsometechnologiesaremoresuitabletoimplementapplications inecient,counter-intuitiveuseoftheexistingones. opertoprogram,attheapplicationlevel,somemobilitymechanismsorforcean designedusingparticularparadigms.unsuitabletechnologiesforcethedevel-

9 mentingarchitecturesbasedonthecsparadigm.iftheyareusedtoimplement REV-basedarchitectures,theyforcetheimplementortouse(unnaturally)code asdataandtoprogramtheevaluationofsuchcodeexplicitly.evenworse,if message-basedtechnologiesareusedtoimplementma-basedarchitectures,the AsshowninTable2,message-basedtechnologiesarewellsuitedforimple- programmerhasalsotocodestatemanagement,i.e.,auxiliaryvariablesmustbe usedtokeepthestateofthecomputationandunnaturalcodestructuresmust arenaturallyorientedtowardstheimplementationofmcasdesignedaccording beusedtorestorethestateofacomponentaftermigratingtoadierentsite. chitecturessincetheyforcetheremoteexecutionofsegmentsofcodecomposed totherevparadigm.thesetechnologiesareinecienttoimplementcsar- ofasingleinstruction.thereforeaneweuiscreatedinordertoexecutethis Weaklymobiletechnologiesthatallowtoexecutesegmentsofcoderemotely packing/unpackingofthevariablesrepresentingthestateandtherestoringof themaparadigm,theprogrammerhastomanage,attheprogramlevel,the \degenerate"code.onthecontrary,inordertoimplementmcasbasedon theeuexecutionow. whiletheyarenotsuitedforimplementingapplicationsbasedonthecsand REVparadigms.Intheformercase,theprogrammerhasto\overcode"anagent inordertohaveitmovedtotheserversite,executeasingleoperationandjump backwiththeresults.suchimplementationscouldberatherinecientsince StronglymobiletechnologiesareorientedtowardsMA-basedapplications thewholeeustateistransmittedbackandforthacrossthenetwork.inthe lattercase,inadditiontothecodetobeexecutedremotely,theimplementor hastoaddthemigrationprocedures.furthermore,thestateoftheeuistobe transmittedoverthenetwork. plementaparticulararchitecture.intherstcaseresourcesarewasted,resulting andpoliciesthatthetechnologydoesnotprovide. inineciency.inthesecondcasetheprogrammerhastocodeallmechanisms Summingup,technologiesmayrevealtobetoopowerfulortoolimitedtoim- andtechnologiesthatconstitutethecomputationalinfrastructureofthisnew Theever-increasinggrowthofMCAshasstimulatedresearchonmethodologies 10Conclusions classofapplications.severaltechnologies,withdierentcharacteristicshave beenproposed.yet,aclearmethodologicalframeworkisstillmissing;many andtechnologies conceptsarenotclearlydenedandthereisoftenconfusionaboutparadigms tectureofanapplicationcanbedesignedintermsofinteractingcomponents. distributedapplications. Technologiesprovidemechanismsthatcanbeusedtoimplementnetwork-centric Mobilecodeparadigmsarethestylesaccordingtowhichthesoftwarearchi- thedevelopmenttheseconceptscomeinplayandwhicharetherelationships Wehavedevelopedacasestudyinordertounderstandinwhichphaseof

10 Technologies Message-based CS Paradigms Weaklymobile Codeisasingle Wellsuited PrograminterpretationProgramstaterestoring Codeasdata Wellsuited REV Codeandstateasdata Stateasdata MA StronglymobileCodeisasingle Createsunnecessary executionunits instruction ManagemigrationProgramstaterestoring Createsunnecessary Movestateback executionunits instruction Movestateback andforth Wellsuited Table2.Relationshipsamongparadigmsandtechnologies. andforth betweenparadigmsandtechnologies. applicationsdesignedusingaparticularparadigmandwhicharethediculties ortheinecienciesthatstemoutfromtheusetheinappropriatetechnology. Wehaveidentiedwhichclassoftechnologiesarewellsuitedtoimplement couldconsiderthestudyofareal-lifemobilecodeapplication.inaddition,more technologiesandparadigmsmightbeconsidered,otherthanthosediscussedhere. Atpresent,weareextendingourworkbydevelopingacasestudythatincludes thecodparadigm,whichwasnotcoveredhere,andusesothertechnologieslike Ourcasestudyisa\toy"applicationoflimitedcomplexity.Futurework JavaandTACOMA. implementation.asanexample,testinganddebuggingphasesarestronglyin- thantestingof\traditional"applicationssincethecongurationofcomponents andthebindingamongcomponentsandcodeisdynamic.inaddition,debugging uencedbytheintroductionofmobility.testingofmcasismuchmoredicult Wedidnotcoverphasesofthedevelopmentprocessotherthandesignand codethatisexecutedremotelyisachallengingtask.furtherworkisneededto fullyunderstandtheeectofthenewcomputationalinfrastructureprovidedby thenetworkontheentirelifecycleofmcas. Apreliminarytreatmentoftheissuesdiscussedinthispapercanbefoundin[6]. Acknowledgments WewishtothankTullioDoveraandRobertoNespoliforimplementingmostof bilecodelanguagesandparadigmswascarriedoutwithg.cugola,a.carzaniga, theexperimentsillustratedhere.thepreviousworkaboutclassicationofmo- G.P.Picco,andA.Fuggetta.

11 References 1.J.Baumann,C.Tschudin,andJ.Vitek,editors.Proceedingsofthe2ndECOOP 2.A.D.BirrellandB.J.Nelson.Implementingremoteprocedurecalls.Technical 3.L.Cardelli.Obliq:Alanguagewithdistributedscope.Technicalreport,Digital ReportCSL-83-7,XEROX,October1983. EquipmentCorporation,SystemsResearchCenter,May1995. WorkshoponMobileObjects("AgentsontheMove"), A.Carzaniga,G.P.Picco,andG.Vigna.DesigningDistributedApplicationsusing 5.G.Cugola,C.Ghezzi,G.P.Picco,andG.Vigna.AnalyzingMobileCodeLanguages.InSpecialIssueonMobileObjectSystems,LNCS.Springer-Verlag,1997. SoftwareEngineering,May1997. MobileCodeParadigms.InProceedingsofthe1997InternationalConferenceon 6.T.DoveraandR.Nespoli.Paradigmietecnologieperlosviluppodiapplicazioni 7.C.Ghezzi,G.Cugola,G.P.Picco,andG.Vigna.ACharacterizationofMobilityandStateDistributioninMobileCodeLanguages.InProceedingsofthe2nd basatesucodicemobile.master'sthesis,politecnicodimilano,1996. Toappear. 8.R.S.Gray.AgentTcl:ATransportableAgentSystem.InProceedingsofthe 9.D.Johansen,R.vanRenesse,andF.B.Schneider.AnIntroductiontothe CIKM'95WorkshoponIntelligentInformationAgents,1995. ECOOPWorkshoponMobileObjectSystems,July SunMicrosystems.TheJavaLanguage:AWhitePaper.Technicalreport,Sun sityoftromsandcornelluniversity",june1995. TACOMADistributedSystem-Version1.0.TechnicalReport95-23,"Univer- 13.M.Straer,J.Baumann,andF.Hohl.MOLE:AJavaBasedMobileAgentSystem. 12.B.SmithandL.Rowe.Tcl-DP.Documentation, J.K.Ousterhout.TclandtheTkToolkit.Addison-Wesley,1994. Microsystems, C.F.Tschudin.AnIntroductiontotheM0MessengerLanguage.Universityof 15.ChristianF.Tschudin.OO-AgentsandMessengers.InECOOP'95Workshopon Geneva,Switzerland,1994. InProceedingsofthe2ndECOOPWorkshoponMobileObjects("Agentsonthe Move"), JamesE.White.TelescriptTechnology:TheFoundationfortheElectronicMarketplace.Technicalreport,GeneralMagic,Inc.,1994.WhitePaper. ObjectsandAgents,August1995.