面向Agent软件工程 Agent-Oriented Software Engineering

1

Agent-OrientedSoftwareEngineering

NicholasR.JenningsandMichaelWooldridge

DepartmentofElectronicEngineering

QueenMary&WestfieldCollege

UniversityofLondon

LondonE14NS,UnitedKingdom

{N.R.Jennings,M.J.Wooldridge}@qmw.ac.uk

Abstract

Agent-orientedtechniquesrepresentanexcitingnewmeansofanalysing,designingandbuildingcom-plexsoftwaresystems.Theyhavethepotentialtosignificantlyimprovecurrentpracticeinsoftwareengineeringandtoextendtherangeofapplicationsthatcanfeasiblybetackled.Yet,todate,therehavebeenfewseriousattemptstocastagentsystemsasasoftwareengineeringparadigm.Thispaperseekstorectifythisomission.Specifically,itwillbearguedthat:(i)theconceptualapparatusofagent-orientedsystemsiswell-suitedtobuildingsoftwaresolutionsforcomplexsystemsand(ii)agent-orientedapproachesrepresentagenuineadvanceoverthecurrentstateoftheartforengineeringcomplexsys-tems.Followingonfromthisview,themajorissuesraisedbyadoptinganagent-orientedapproachtosoftwareengineeringarehighlightedanddiscussed.

1.Introduction

Designingandbuildinghighqualityindustrial-strengthsoftwareisdifficult.Indeed,ithasbeenclaimedthatsuchdevelopmentprojectsareamongthemostcomplexconstructiontasksunder-takenbyhumans.Againstthisbackground,awiderangeofsoftwareengineeringparadigmshavebeendevised(e.g.,proceduralprogramming,structuredprogramming,declarativeprogramming,object-orientedprogramming,designpatterns,applicationframeworksandcomponent-ware).Eachsuccessivedevelopmenteitherclaimstomaketheengineeringprocesseasierortoextendthecomplexityofapplicationsthatcanfeasiblybebuilt.Althoughthereissomeevidencetosup-porttheseclaims,researcherscontinuallystriveformoreefficientandpowerfulsoftwareengi-neeringtechniques,especiallyassolutionsforevermoredemandingapplicationsarerequired.

Thispaperwillarguethatanalysing,designingandimplementingsoftwareasacollectionofinteracting,autonomousagents(i.e.,asamulti–agentsystem

[26][39]

)representsapromisingpointofdepartureforsoftwareengineering.Whilethereissomedebateaboutexactlywhatcon-stitutesanautonomousagentandwhatconstitutesinteraction,thisworkseekstoabstractawayfromparticulardogmaticstandpoints.Instead,wefocusonthosecharacteristicsforwhichthereissomeconsensus.Fromthisstandpoint,thepaper’scentralhypothesiswillbeadvanced:forcertainclassesofproblem(thatwillbedefined),adoptingamulti-agentapproachtosystemdevelopmentaffordssoftwareengineersanumberofsignificantadvantagesovercontemporarymethods.Notethatwearenotsuggestingthatmulti-agentsystemsareasilverbullet

[4]

—thereisnoevidencetosuggesttheywillrepresentanorderofmagnitudeimprovementinsoftwareengineeringproduc-

2

tivity.However,webelievethatforcertainclassesofapplication,anagent-orientedapproachcansignificantlyimprovethesoftwaredevelopmentprocess.

Inseekingtodemonstratetheefficacyoftheagent-orientedapproach,themostcompellingformofanalysiswouldbetoquantitativelyshowhowadoptingsuchtechniqueshadimproved,accord-ingtosomestandardsetofsoftwaremetrics,thedevelopmentprocessinarangeofprojects.However,suchdataissimplynotavailable(asitisstillnotformoreestablishedmethodssuchasobject-orientation).However,therearecompellingargumentsforbelievingthatanagent-orientedapproachwillbeofbenefitforengineeringcertaincomplexsoftwaresystems.Theseargumentshaveevolvedfromadecadeofexperienceinusingagenttechnologytoconstructlarge-scale,real-worldapplicationsinawidevarietyofindustrialandcommercialdomains

[20

].

Thecontributionofthispaperistwofold.Firstly,despitemulti-agentsystemsbeingtoutedasatechnologythatwillhaveamajorimpactonfuturegenerationsoftware(“pervasiveineverymar-ketbytheyear2000”

[17]

and“thenewrevolutioninsoftware”

[14]

),therehasbeennosystem-aticevaluationofwhythismaybethecase.Thus,althoughthereareanincreasingnumberofdeployedagentapplications(see

[5]

,

[19]

,

[20]

,

[27]

forareview),nobodyhasanalysedpreciselywhatmakestheparadigmsoeffective.Thisisclearlyamajorgapinknowledge,whichthispaperseekstoaddress.Secondly,therehasbeencomparativelylittleworkonviewingmulti-agentsys-temsasasoftwareengineering.Thisshortcomingisrectifiedbyrecastingtheessentialcompo-nentsofagentsystemsintomoretraditionalsoftwareengineeringconcepts,andbyexaminingtheimpactonthesoftwareengineeringlife-cycleofadoptinganagent-orientedapproach.

Theremainderofthepaperisstructuredasfollows.Sectiontwomakesthecaseforanagent-ori-entedapproachtosoftwareengineering.Itanalysesthetypeofcomplexitypresentinindustrial-strengthsoftware,characterisesthekeyconceptualmechanismsoftheagent-orientedapproach,andexplainshowthesemechanismsarewellsuitedtotacklingthecomplexitypresentinsoftwaredevelopment.Sectionthreeexaminestheimpactofadoptinganagent-orientedapproachonthesoftwareengineeringlifecycle—focusinginparticularonthespecification,implementationandverificationphases.Sectionfourdealswiththepragmaticsofagent-orientedsoftwareengineeringbypresentingsomecommonpitfallsthatfrequentlybedevilagent-orienteddevelopments.Sectionfiveconcludesbyidentifyingthemajoropenissuesthatneedtobeaddressedifagent-orientedtechniquesaretoreachthesoftwareengineeringmainstream.

2.TheCaseforanAgent-OrientedApproachtoSoftwareEngineering

Thissectioncharacterisestheessentialnatureofreal-worldsoftwaresystems(section2.1)andthengoesontopresentexactlywhatwemeanbythenotionofagent-orientedsoftware(section2.2).Usingthesecharacterisations,argumentsareadvancedastowhyagent-orientedtechniquesarewellsuitedtodevelopingcomplexsoftwaresystems(section2.3).

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2.1TheNatureofComplexSoftwareSystems

Industrial-strengthsoftwareiscomplexinnature:itistypicallycharacterisedbyalargenumberofpartsthathavemanyinteractions

[37]

.Moreoverthiscomplexityisnotaccidental

[4]

:itisaninnatepropertyofthetypesoftasksforwhichsoftwareisused.Theroleofsoftwareengineeringisthereforetoprovidestructuresandtechniquesthatmakeiteasiertohandlethiscomplexity.For-tunately,thiscomplexityexhibitsanumberofimportantregularities

[37

]:

•Complexityfrequentlytakestheformofahierarchy1.Thatis,thesystemiscomposedofinter-relatedsub-systems,eachofwhichisitselfahierarchy.Theprecisenatureoftheorganisationalrelationshipsvariesbetweensub-systems,althoughsomegenericforms(suchasclient-server,peer,team)canbeidentified.Organisationalrelationshipsarenotstatic:theycan,andfrequentlydo,varyovertime.

•Thechoiceofwhichcomponentsinthesystemareprimitiveisrelativelyarbitraryandisdefinedverymuchbytheobserver’saimsandobjectives.

•Hierarchicsystemsevolvemorequicklythannon-hierarchiconesofcomparablesize.Inotherwords,complexsystemswillevolvefromsimplesystemsmuchmorerapidlyiftherearestableintermediateforms,thaniftherearenot.

•Itispossibletodistinguishbetweentheinteractionsamongsub-systemsandtheinterac-tionswithinsub-systems.Thelatterarebothmorefrequent(typicallyatleastanorderofmagnitudemore)andmorepredictablethantheformer.Thisgivesrisetotheviewthatcomplexsystemsarenearlydecomposable.Thus,sub-systemscanbetreatedalmostasiftheyareindependentofoneanother,butnotquitesincetherearesomeinteractionsbetweenthem.Moreover,althoughmanyoftheseinteractionscanbepredictedatdesigntime,somecannot.

Drawingtheseinsightstogether,itispossibletodefineacanonicalviewofacomplexsystem(figure1).Thesystem’shierarchicalnatureisexpressedthroughthe“composedof”links,compo-nentswithinasub-systemareconnectedthrough“frequentinteraction”links,andinteractionsbetweencomponentsareexpressedthrough“infrequentinteraction”links.Thevariablenotionofprimitivecomponentscanbeseeninthewaythatatomiccomponentsatonelevelareexpandedouttoentiresub-systemsatsubsequentlevels.

1.Heretheterm“hierarchy”isnotusedtomeanthateachsub-systemissubordinatedbyanauthorityrelationtothesystemtowhichitbelongs.Rather,itshouldbeinterpretedinabroadsensetomeanasystemthatisanalysableintosuccessivesetsofsub-systems.

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sub-system

sub-systemcomponent

>composedof

frequentinteractioninfrequentinteraction

Figure1:ViewofaCanonicalComplexSystem

Giventheseobservations,softwareengineershavedevisedanumberofpowerfultoolsinordertomanagethiscomplex

ity.Theprincipalmechanismsinclude[3]:

•Decomposition:Themostbasictechniquefortacklinganylargeproblemistodivideitintosmaller,moremanageablechunkseachofwhichcanthenbedealtwithinrelativeiso-lation(notethenearlydecomposablesub-systemsinfigure1).Decompositionhelpstacklecomplexitybecauseitlimitsthedesigner’sscope:atanygiveninstantonlyaportionoftheproblemneedstobeconsidered.

•Abstraction:Theprocessofdefiningasimplifiedmodelofthesystemthatemphasisessomeofthedetailsorproperties,whilesuppressingothers.Again,thistechniqueworksbecauseitlimitsthedesigner’sscopeofinterestatagiventime.Attentioncanbefocusedonthesalientaspectsoftheproblem,attheexpenseofthelessrelevantdetails.

•Organisation2:Theprocessofidentifyingandmanagingtheinter-relationshipsbetweenthevariousproblemsolvingcomponents(notethesub-systemandinteractionlinksoffig-ure1).Theabilitytospecifyandenactorganisationalrelationshipshelpsdesignerstacklecomplexityintwoways.Firstly,byenablinganumberofbasiccomponentstobegroupedtogetherandtreatedasahigher-levelunitofanalysis.Forexample,theindividualcompo-nentsofasub-systemcanbetreatedasasingleunitbytheparentsystem.Secondly,byprovidingameansofdescribingthehigh-levelrelationshipsbetweenvariousunits.Forexample,anumberofcomponentsmayneedtoworktogetherinordertoprovideapartic-ularfunctionality.

2.Boochusestheterm“hierarchy”forthisfinalpoint

[3]

.However,hierarchyinvariablygivestheconnotationofcontrol,hencethemoreneutralterm“organisation”isusedhere.Organisationscanbearrangedsuchthattheycorre-spondtocontrolhierarchies,howevertheycanalsocorrespondtogroupsofpeers,andanythingthatfallsin-between.

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Theprecisenatureandwayinwhichthesetoolsareusedvariesenormouslybetweensoftwareparadigms.Hencewhencharacterisinganewparadigm,suchasagent-orientedsoftware,clearpositionsneedtobedeterminedoneachoftheseissues(section2.2).Moreover,whenassessingthepowerofaparadigm,argumentsneedtobeadvancedastowhythechosenwayofdealingwiththeseissueshelpssoftwareengineersbuildsystemsmoreeffectively(section2.3).

2.2WhatisAgent-OrientedSoftware?

Atpresent,thereisagreatdealofongoingdebateaboutexactlywhatconstitutesanagent,yetthereisnothingapproachingauniversalconsensus.However,anincreasingnumberofresearch-ersfindthefollowingcharacterisationuseful

[41

]:

anagentisanencapsulatedcomputersystemthatissituatedinsomeenvironment,

andthatiscapableofflexible,autonomousactioninthatenvironmentinorderto

meetitsdesignobjectives

Thereareanumberofpointsaboutthisdefinitionthatrequirefurtherexplanation.Agentsare:(i)clearlyidentifiableproblemsolvingentitieswithwell-definedboundariesandinterfaces;(ii)situ-ated(embedded)inaparticularenvironment—theyreceiveinputsrelatedtothestateofthatenvi-ronmentthroughtheirsensorsandtheyactontheenvironmentthroughtheireffectors3;(iii)designedtofulfilaspecificrole—theyhaveparticularobjectivestoachieve,thatcaneitherbeexplicitlyorimplicitlyrepresentedwithintheagents;(iv)autonomous—theyhavecontrolbothovertheirinternalstateandovertheirownbehaviour;(v)capableofexhibitingflexible(context-dependent)problemsolvingbehaviour—theyneedtobereactive(abletorespondinatimelyfashiontochangesthatoccurintheirenvironmentinordertosatisfytheirdesignobjectives)andproactive(abletoopportunisticallyadoptnewgoalsandtaketheinitiativeinordertosatisfytheirdesignobjectives)

[42]

.

Whenadoptinganagent-orientedviewoftheworld,itsoonbecomesapparentthatasingleagentisinsufficient4.Mostproblemsrequireorinvolvemultipleagents:torepresentthedecentralisednatureoftheproblem,themultiplelociofcontrol,themultipleperspectives,orthecompetinginterests.Moreover,theagentswillneedtointeractwithoneanother,eithertoachievetheirindi-vidualobjectivesorelsetomanagethedependenciesthatensuefrombeingsituatedinacommonenvironment.Theseinteractionsrangefromsimplesemanticinteroperation(theabilitytoexchangecomprehensiblecommunications),throughtraditionalclient-servertypeinteractions(theabilitytorequestthataparticularactionisperformed),torichsocialinteractions(theabilitytocooperate,coordinateandnegotiateaboutacourseofaction).Whateverthenatureofthesocialprocess,however,therearetwopointsthatqualitativelydifferentiateagentinteractionsfrom

3.Typicallyeachagenthasapartialviewoftheenvironment(thatmayormaynotoverlapwiththatofothers)andalimitedsphereofinfluencethroughwhichitcanaltertheenvironment.

4.Itcanbearguedthatthereisnosuchthingasasingleagentsystem;everythinginvolvesmultipleagents.

6

thosethatoccurinothersoftwareengineeringparadigms.Firstly,agent-orientedinteractionsgen-erallyoccurthroughahigh-level(declarative)agentcommunicationlanguage(typicallybasedonspeechacttheory

[1]

).Consequently,interactionsareusuallyconductedattheknowledgelevel

[25]

:intermsofwhichgoalsshouldbefollowed,atwhattime,andbywhom(cf.methodinvoca-tionorfunctioncallsthatoperateatapurelysyntacticlevel).Secondly,asagentsareflexibleproblemsolvers,operatinginanenvironmentoverwhichtheyhaveonlypartialcontrolandobservability,interactionsneedtobehandledinasimilarlyflexiblemanner.Thus,agentsneedthecomputationalapparatustomakecontext-dependentdecisionsaboutthenatureandscopeoftheirinteractionsandtoinitiate(andrespondto)interactionsthatwerenotnecessarilyforeseenatdesigntime.

Inmostcases,agentsacttoachieveobjectivesonbehalfofindividualsorcompanies.Thus,whenagentsinteractthereistypicallysomeunderlyingorganisationalcontext.Thiscontexthelpsdefinethenatureoftherelationshipbetweentheagents.Forexample,theymaybepeersworkingtogetherinateam,onemaybethebossoftheothers,ortheymaybeinvolvedinaseriesofemployer-subcontractorrelationships.Tocapturesuchlinks,agentsystemsoftenhaveexplicitconstructsformodelingorganisationalrelationships(e.g.,peer,boss,etc.)andorganisationalstructures(e.g.,teams,groups,coalitions,etc.).Itshouldbenotedthatinmanycases,theserela-tionshipsmaychangewhilethesystemisoperating.Socialinteractionmeansexistingrelation-shipsevolve(e.g.,anagentawardsanewcontract)andnewrelationsarecreated(e.g.,anumberofagentsmayformateamtodeliveraparticularservicethatnooneindividualcanoffer).Thetemporalextentoftheserelationshipscanvaryenormously:fromjustlongenoughtodeliveraparticularserviceoncetoapermanentbond.Tocopewiththisvarietyanddynamicity,agentresearchershaveexpendedconsiderableeffort:devisingprotocolsthatenableorganisationalgroupingstobeformedanddisbanded,specifyingmechanismstoensuregroupingsacttogetherinacoherentfashion,anddevelopingstructurestocharacterisethemacrobehaviourofcollectives.

Drawingthesepointstogether(figure2),itcanbeseenthatadoptinganagent-orientedapproachtosoftwareengineeringmeansdecomposingtheproblemintomultiple,interacting,autonomouscomponents(agents)thathaveparticularobjectivestoachieve.Thekeyabstractionmodelsthatdefinethe“agent-orientedmindset”areagents,interactionsandorganisations.Finally,explicitstructuresandmechanismsareoftenavailablefordescribingandmanagingthecomplexandchangingweboforganisationalrelationshipsthatexistbetweentheagents.

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Organisationalrelationship

.A3

A4

A2

Interaction

AiAgenti

A1

viewofenvironment/sphereofinfluence

A5

Environment

Figure2:Canonicalviewofamulti-agentsystem

2.3TheCaseforanAgent-OrientedApproach

Havingcharacterisedcomplexsystemsanddescribedagentsoftware,wenowconsiderwhysuchagent-orientedtechniquesarewellsuitedtodevelopingsuchsoftwaresystems.Thisargumenthasthreeparts:

•showthatagent-orienteddecompositionsareaneffectivewayofpartititioningtheprob-lemspaceofacomplexsystem(section2.3.1);

•showthatthekeyabstractionsoftheagent-orientedmindsetareanaturalmeansofmodel-ingcomplexsystems(section2.3.2);

•showthattheagent-orientedphilosophyforidentifyingandmanagingorganisationalrela-tionshipsisappropriatefordealingwiththedependenciesandinteractionsthatexistinacomplexsystem(section2.3.3).

Tomakethecaseforagent-orientedsoftwareengineeringevenmorecompelling,thefinalstepistoarguethatagent-orientedtechniquesrepresentagenuineadvanceoverthecurrentstateoftheart.Tothisend,theagent-orientedapproachwillbecomparedwithleading-edgetechniquesfrommainstreamsoftwareengineering(section2.3.4).Inparticular,thisinvolvescomparisonswithobject-orientedanalysisanddesign(systemisbuiltoutofinteractingobjectsthatencapsulatebothdataandtheproceduresthatoperateonthatdata

[3],[23])

andwithcomponent-ware(systemisbuiltbyassemblingpre-existingcomponentsintosomeoverallstructure

[38

]).

2.3.1Agent-OrientedDecompositions

Complexsystemsconsistofanumberofrelatedsub-systemsorganisedinahierarchicalfashion(figure1).Atanygivenlevel,thesub-systemsworktogethertoachievethefunctionalityoftheirparentsystem.Moreover,withinasub-system,theconstituentcomponentsworktogethertodelivertheoverallfunctionality.Thus,thesamebasicmodelofinteractingcomponents,working

8

togethertoachieveparticularobjectivesoccursthroughoutthesystem.

Giventheabovesituation,itisentirelynaturaltomodularisethecomponentsintermsoftheobjectivestheyachieve5.Inotherwords,eachcomponentcanbethoughtofasachievingoneormoreobjectives.Asecondimportantobservationisthatcurrentsoftwareengineeringtrendsaretowardsincreasingthedegreesoflocalisationandencapsulationinproblemdecompositions

[27]

.Applyingthisphilosophytoobjective-achievingdecompositionsmeansthattheindividualcom-ponentsshouldhavetheirownthreadofcontrol(i.e.,componentsshouldbeactive)andthattheyshouldencapsulatetheinformationandproblemsolvingabilityneededtomeettheseobjectives.Sincethecomponentstypicallyhavetooperateinanenvironmentinwhichtheyhaveonlypartialinformation,theymustbeabletodetermine,atrun-time,whichactionstheyshouldperforminpursuitoftheirobjectives.Inshort,componentsneedautonomyovertheirchoiceofaction.

Inorderfortheactiveandautonomouscomponentstofulfilboththeirindividualandcollectiveobjectives,theyneedtointeractwithoneanother(recallcomplexsystemsareonlynearlydecom-posable).Howeverthesystem’sinherentcomplexitymeansthatitisimpossibletoaprioriknowaboutallthepotentiallinks:interactionswilloccuratunpredictabletimes,forunpredictablerea-sons,betweenunpredictablecomponents.Forthisreason,itisfutiletotryandpredictoranalyseallthepossibilitiesatdesign-time.Itismorerealistictoendowthecomponentswiththeabilitytomakedecisionsaboutthenatureandscopeoftheirinteractionsatrun-time.Fromthisitfollowsthatcomponentsneedtheabilitytoinitiate(andrespondto)interactionsinaflexiblemanner.

Thepolicyofdeferringtorun-timedecisionsaboutcomponentinteractionsfacilitatestheengi-neeringofcomplexsystemsintwoways.Firstly,problemsassociatedwiththecouplingofcom-ponentsaresignificantlyreduced(bydealingwiththeminaflexibleanddeclarativemanner).Componentsarespecificallydesignedtodealwithunanticipatedrequestsandcangeneraterequestsforassistanceiftheyfindthemselvesindifficulty.Moreoverbecausetheseinteractionsareenactedthroughahigh-levelagentcommunicationlanguage,couplingbecomesaknowledge-levelissue.This,inturn,removessyntacticlevelconcernsfromthetypesoferrorscausedbyunexpectedinteractions.Secondly,theproblemofmanagingcontrolrelationshipsbetweenthesoftwarecomponents(ataskthatbedevilsmoretraditionalfunctionaldecompositions)issignifi-cantlyreduced.Allagentsarecontinuouslyactiveandanycoordinationorsynchronisationthatisrequiredishandledthroughinter-agentinteraction.Thus,theorderingofthesystem’stop-levelgoalsisnolongersomethingthathastoberigidlyprescribedatdesigntime.Rather,itbecomessomethingthatcanbehandledinacontext-sensitivemanneratrun-time.

Fromthisdiscussion,itisapparentthatanaturalwaytomodulariseacomplexsystemisintermsofmultiple,interacting,autonomouscomponentsthathaveparticularobjectivestoachieve.Inshort,agent-orienteddecompositionsmakeiteasiertodevelopcomplexsystems.

5.Theviewthatdecompositionsbaseduponfunctions/actions/processesaremoreintuitiveandeasiertoproducethanthosebasedupondata/objectsisevenacknowledgedwithintheobject-orientedcommunity(see,forexample,

[23]

pg44).

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2.3.2TheAppropriatenessoftheAgent-OrientedAbstractions

Asignificantpartofalldesignendeavoursistofindtherightmodelsforviewingtheproblem.Ingeneral,therewillbemultiplecandidatesandthedifficulttaskispickingthemostappropriateone.Turningtothespecificcaseofdesigningsoftware,themostpowerfulabstractionsarethosethatminimisethesemanticgapbetweentheunitsofanalysisthatareintuitivelyusedtoconceptu-alisetheproblemandtheconstructspresentinthesolutionparadigm.Inourcase,theproblemtobecharacterisedconsistsofsub-systems,sub-systemcomponents,interactionsandorganisationalrelationships.Takingeachoftheseinturn:

•Thecaseforviewingsub-systemcomponentsasagentshasalreadybeenmadeabove(sec-tion2.3.1).

•Theinterplaybetweenthesub-systemsandbetweentheirconstituentcomponentsismostnaturallyviewedintermsofhigh-levelsocialinteractions:“atanygivenlevelofabstrac-tion,wefindmeaningfulcollectionsofobjectsthatcollaboratetoachievesomehigherlevelview”

[3]

pg34.Thisviewaccordspreciselywiththeknowledgelevel(orevensociallevel

[18]

)treatmentofinteractionaffordedbytheagent-orientedapproach.Agentsystemsareinvariablydescribedintermsof“cooperatingtoachievecommonobjectives”,“coordinatingtheiractions”or“negotiatingtoresolveconflicts”.Thus,theagent-orientedmindsetisentirelyappropriateforcapturingthetypesofinteractionthatoccurincomplexsystems.

•Complexsystemsinvolvechangingwebsofrelationshipsbetweentheirvariouscompo-nents.Theyalsorequirecollectionsofcomponentstobetreatedasasingleconceptualunitwhenviewedfromadifferentlevelofabstraction.Again,thisviewmatchescloselywiththeabstractionsprovidedbytheagent-orientedmindset.Thus,facilitiesaretypicallypro-videdforexplicitlyrepresentingorganisationalrelationships.Interactionprotocolshavebeendevelopedforformingnewgroupingsanddisbandingunwantedones.Finally,struc-turesareavailableformodelingcollectives.Thelatterpointisespeciallyusefulinrelationtorepresentingsub-systemssincetheyarenothingmorethanateamofcomponentswork-ingtogethertoachieveacollectivegoal.

2.3.3TheNeedforFlexibleManagementofChangingOrganisationalStructures

Complexsystemsinvolveavarietyoforganisationalrelationships,rangingfrompeerstocontrolhierarchies,fromtheshort-termtotheongoing.Theserelationshipsareimportantfortwomainreasons.Firstly,theyallowanumberofseparatecomponentstobegroupedtogetherandtreatedasasingleconceptualentity.Secondly,theyenablethehigh-levellinksbetweenthedifferententi-tiestobecharacterised.Giventheinfluenceandimpactoforganisationalrelationshipsandstruc-turesonsystembehaviour,theimportanceofprovidingexplicitsupportforflexiblyspecifyingandmanagingthemisself-evident.Moreover,giventhattheserelationshipsfrequentlychange,theabilitytodynamicallyadapttoprevailingcircumstancesisalsoessential.

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Asalreadyindicated,organisationsarefirst-classentitiesinagentsystems.Thusexplicitstruc-turesandflexiblemechanismsarecentraltotheagentparadigm.Thisrepresentationalpower,whencoupledwiththesupportingcomputationalmechanisms,enablesagent-orientedsystemstoexploittwofacetsofthenatureofcomplexsystems.Firstly,thenotionofaprimitivecomponentcanbevariedaccordingtotheneedsoftheobserver.Thusatonelevel,entiresub-systemscanbeviewedasasingleton,alternativelyteamsorcollectionsofagentscanbeviewedasprimitivecomponents,andsoonuntilthesystemeventuallybottomsout.Secondly,suchstructuresprovideavarietyofstableintermediateforms,that,asalreadyindicated,areessentialfortherapiddevel-opmentofcomplexsystems.Theiravailabilitymeansthatindividualagentsororganisationalgroupingscanbedevelopedinrelativeisolationandthenaddedintothes