基于代理的分布式应用发展的技术设计论文.docx

基于代理的分布式应用发展的技术设计论文.docx

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基于代理的分布式应用发展的技术设计论文

外文文献及译文

文献、资料题目：

Technologiesforthe

DevelopmentofAgent-basedDistributedApplications

文献、资料来源：

期刊

文献、资料发表（出版）日期：

2008.3.12

外文文献：

TechnologiesfortheDevelopmentofAgent-basedDistributedApplications

　　Mobileagentcomputingisanextensionoftheearlierideaof"processmigration."Mobileagenttechnologyoffersseveralpotentialbenefitsoverconventionalclient-servercomputingthatcouldhelpimproveclassicdistributedsystemsdesigns,whichareusuallybasedonthewell-knownremoteprocedurecall（RPC）oritsobject-orientedequivalent,remotemethodinvocation（RMI）.

　　Mobilesoftwareagentsenableashiftinthecommunicationsparadigmofdistributedsystemsfromwhatiscalled"datashipping"to"functionshipping."KeytothisshiftisthehigherlevelofabstractionthatamobilesoftwareagentcanprovidecomparedtoaRPCoraRMIcallbecauseitscodeisautonomous.Thisautonomyshouldtheoreticallyreducenetworkloadandcommunicationoverheadindistributedapplicationsaswellasfacilitatethedevelopmentofapplicationsforpotentiallyunreliable,networkedcomputingenvironments.

　　Furthermore,andperhapsevenmoreimportantly,awidespreadagent-enabledinfrastructureprovidesacommonplatformfordistributedapplicationsofarbitrarypurpose.ConsiderthePostScriptexamplegivenbyChess,Harrison,andKershenbaumtoillustratetheadvantagesastandardizedmobileagentexecutionenvironmentoffers.PostScriptdocumentsareprogramsexecutedbyaPostScriptinterpreterandsenttoprintserversthatonlyacceptpassiveinputtext,e.g.,MicrosoftWordfiles.Thedrawbackofthelattermethodisobvious:

foreverynewdocumentformat,allprintserversintheworldwouldhavetobeupdated.Theexampleiseasilytransferredtomobileagentcomputing:

mobileagentsasageneral-purposecomputingplatformdonotrequireanyspecificfunctionstobeimplementedintheservers,suchasfull-textretrievalmethodsordatabaseinterfaces.

　　DistributedapplicationsbasedonRPC-liketechniquesmightnotbeeasytoadapttolarge-scale,heterogeneous,andpotentiallyunreliableenvironments,liketheInternet,becauseimplementationofRPC-stylesemanticsreliesonrelativelystaticandreliablesystemstructures.Combiningmobileagenttechnologywithwell-establishedmechanismsmightpotentiallyleadtosystemsmuchbettersuitedtotheneedsofInternet-likecomputingenvironments.

　　Tobeabletoclearlydistinguishwhatinthiscontextisreferredtoas"conventional"client-servertechnologyfrommobileagenttechnology,sometermsneedfirstbedefined.

　　Agent

　　AsdefinedbytheObjectManagementGroup（OMG）,anorganizationthatdevelopsstandardsforcomponent-basedsoftwaresystems,anagent"isacomputerprogramthatactsautonomouslyonbehalfofapersonororganization."

　　MobileAgent

　　Amobileagentisanobjectthatis"notboundtothesystemwhereitbeginsexecution.Ithastheuniqueabilitytotransportitselffromonesystemwithinanetworktoanother."

　　Place

　　Aplaceis"acontextwhereanagentcanexecute.".Furthermore,thetermsagentplatformandagentsystemwillbeusedsynonymouslyaccordingtotheOMGdefinition:

　　"Anagentsystemisaplatformthatcancreate,interpret,execute,transfer,andterminateagents.Likeanagent,anagentsystemisassociatedwithanauthoritythatidentifiesthepersonororganizationforwhomtheagentsystemacts."

　　Becausemobileagenttechnologyisageneral-purposecomputingplatform,ithasbeencalleda"solutioninsearchofaproblem."Inthefollowing,wewillexaminetheadvantagesthatsuchaplatformoffersfordistributedandparallelcomputingandtheproblemsthatpreventitswidespreadsuccess.Subsequently,abriefoverviewofstate-of-the-artagentsystemsisprovided,togetherwithanexaminationofhowthesesystemsaddressthecontinuingchallengesofmobileagentcomputing.

　　Mobileagentsasadesignparadigmfordistributedsystems

　　Theconceptoftransmittingexecutableprogramsbetweenpotentiallyheterogeneousexecutionenvironmentsintroducesanewparadigmfortheimplementationofdistributedapplicationsforlarge-scale,potentiallyunreliable,networkedenvironments.Intraditionalclient-serversystemsthatuseonlyRPC,thedatatobemanipulated,suchastext,relationaldata,andimages,ismovedbetweenproceduresalreadyresidingontheclientandtheserver.Agent-basedcomputing,however,facilitatesthetransportationoftheprocedureitself（thatis,thealgorithmtobeapplied）totheclientbymeansofamobileagent,whichcanleadtolesstraffic.

　　Alsointeresting,isthepossibilityofimplementingdynamicapplicationprogramminginterfaces（API）overagentsthroughtheuseofanagentcommunicationlanguage;suchalanguagehasbeenproposedbytheFoundationforIntelligentPhysicalAgents（FIPA）.Suchcapabilitywouldallowpossibleintroductionandremovalofservicesthatneednotbefullyspecifiedaprioriineithertheclientortheserver.

　　Characteristicsofmobileagentsystems

　　Whatattributesuniquelydistinguishthemobileagentideafromotherdistributedcomputingconcepts?

Thereisnoagreedupondefinitiontodate;however,therearekeycharacteristicsofmobileagentsystemsthatfacilitateadvantagesinthedevelopmentofdistributedsystems.

Mobility

　　Mobility—themostimportantattributeofmobileagents—describestheabilitytomovefromoneexecutionenvironmenttoanother.Forexample,themobilityconceptenablescodetomovetothelocationwherethedataithastoworkwitharelocated,thustakingadvantageoflocalinteraction.Inthefollowing,theterm"mobility"willbeusedtorefertoamobileagent'sabilitytorelocateitselftoanotherplace.

　　Autonomy

　　Autonomymeansthatmobileagentscaninawayact"autonomously,"i.e.,performactionsbasedontheirownsituativeassessmentofcircumstances.Forexample,amobileagentdecidesonitsownwhenandwheretomove,whereasintraditionalsystemssuchdecisionsaretakenbyacentral（server）component.Hence,theautonomypropertypotentiallyenablesmobileagentstodealindependentlywithsituationslikeunavailableserversandotherresources.

　　Persistence

　　Theconceptofpersistenceistightlycoupledtomobility.Whenanagentmovesfromoneplacetoanother,itpreservesitsinternalstateanddata.Inparticular,itretainsanycomputingresultsobtainedduringexecutionintheexecutionenvironmentitistransportedfrom.

　　Advantagesofmobileagentsovertraditionalclient-serverdesigns

　　Thissectiondiscussesseveralrelativeadvantagesofmobileagentsystemsoverconventionalclient-serverdesignprinciples.Noneoftheseadvantagesareuniquelyavailablethroughmobileagenttechnology;instead,thestrengthofmobileagentsystemsemergesfromtheaggregateoftheadvantages.Thiscombinationfacilitatesseveralimprovements,suchasthefollowing:

　　Mobileagentsprovideageneral-purpose,openframeworkforthedevelopmentofdistributed,networkedsystems,i.e.,auniversalcomputingplatform.

　　New,derivativenetworkservicescouldbebasedonmobileagenttechnologythatextendorevenreplaceexistingservices.

　　Becausemobileagentsarepowerfultoolsforend-users,theymayappealstronglytothem.TheInternetcommunityisaverylargepossibletargetaudience.

　　Theaforementionedadvantagesincludehighbandwidthremoteinteraction,supportfordisconnectedoperation,supportforweakclients,semanticrouting,scalability,reducedoverheadforsecuretransactions,andcomparativelyrobustremoteinteractions.

　　Highbandwidthremoteinteraction

　　Ifthereisalargeamountofremotelystoreddatatobeprocessed,itcanbeconsiderablycheapertobringtheprocessingmethodtothelocationwherethedataisstored（"functionshipping"）,thantotransferthedatatotheclient（"datashipping"）.Functionshippingcanbeaccomplishedbymeansofamobileagent.Onemightarguethatthesamecanbeachievedthroughstoredproceduresorsimilarmechanisms;whilethisiscertainlycorrect,thepowerofthemobileagentapproachishighlightedwhenproperprocessingmethodsarenotknowninadvancebutaredata-dependent.Rothermelmentionsanalysisofhugeamountsofmeteorologicaldataandreal-timeanalysisofstockpricesasexamples.

　　Supportfordisconnectedoperationandweakclients

　　Mobileagentsimprovethesupportformobilecomputing,i.e.,mobiledevicesthatareonlyintermittentlyconnected.Mobiledevices,suchaspersonaldigitalassistants（PDAs）,webpads,andmobilephones,arenotgeneral-purposecomputingdevicesinthesenseofpersonalcomputers,butexhibittwokeycharacteristicsrelevantinthisdiscussion.

　　Non-persistentand/orlow-bandwidthconnectiontothenetwork.

　　Limitedstorageandprocessingcapacity.

　　Ausermightchoosetoputtogetheraquery（a"task"fortheagent）,connecttothenetwork,launchtheagentwiththerespectivetask,disconnectfromthenetwork,andatalatertimeretrievetheagentalongwiththeresultsithascollected.Thisisbeneficialnotonlywhenconsideringdevicesthatareoccasionallydisconnectedbutalsowhenlookingatdevices,likecellphones,thatmightbeconnectedpersistently,butonlywithaverylowbandwidth.

　　Semanticrouting

　　Interactionsinclient-serverapplicationstypicallyrequiredetailedknowledgeaboutlocations,functions,andprotocolsonboththeserverandtheclientside.Oneofthepromisesofmobileagentcomputingisthattheclient（andtheuser）canberelievedofagreatportionofthatburdenthroughtheconceptofsemanticrouting.Theideaisthatthe"intention,"i.e.,thetasktobeperformed,bytheagentshouldbeformulatedinanabstrac