Lectures
TDDD10AIProgramming
Multi-AgentsandCommunication
CyrilleBerger
1AIProgramming:Introduction
2IntroductiontoRoboRescue
3AgentsandAgentsArchitecture
4Multi-AgentsandCommunication
5MultiagentDecisionMaking
6CooperationAndCoordination1
7CooperationAndCoordination2
8MachineLearning
9KnowledgeRepresentation
10PuttingItAllTogether
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Q/AFromthelabs
Lecturegoals
Thereisnorightwaytodothelabs,
don'thesitatetobecreative
Agentshavearadiusof500mm
URNsforChangeSet/Propertyare
definedinStandardEntityURNand
StandardPropertyURN
Ifyouhavefinishedlab2,helptheothers
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Acquireknowledgeonmutliagent
systems.
Acquireknowledgeonhowagents
communicate
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Lecturecontent
Multi-AgentSystems
CMUCaseStudy
Communication
Multi-AgentSystems
KQML
FoundationforIntelligentPhysicalAgents(FIPA)
ContractNet
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Multi-AgentSystems
CharacteristicsofMulti-AgentSystems
“Amulti-agentsystem(MAS)canbe
definedasalooselycouplednetworkof
problemsolversthatinteracttosolve
problemsthatarebeyondtheindividual
capabilitiesorknowledgeofeach
problemsolver.”DurfeeandLesser,1989
Theparticipantsareself-interested.
Theparticipantsandtheircapabilitieschanges
overtime,i.e.opensystems.
Eachparticipanthasincompleteinformationor
capabilitiesforsolvingtheproblemand,thus,
hasalimitedviewpoint.
Thereisnosystemglobalcontrol.
Dataisdecentralized.
Computationisasynchronous.
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ViewofacanonicalMulti-AgentSystem
Motivation
CooperativedistributedProblemSolving
NoAgentcansolvetheproblembyitself
Differentcapabilities,resourcesandknowledge
Cooperation:Systemsdointeracttofulfill
theirtasks
Task&ResultSharing
TaskDecomposition&Allocation
Coordination:Systemsdynamicallyadapt
theirbehaviortootheragents
MultiagentplanningandTaskDeconfliction
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Challenges
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Solutions
Organizations
Taskallocationandmultiagentplanning
Recognizingandresolvingconflicts
Modelingotheragents
Communication
Managingresources
Adaptationandlearning
Howtoformulate,describe,decompose,andallocateproblems
andsynthesizeresultsamongagroupofagents.
Howtoenableagentstocommunicateandinteract.
Howtomakesurethatagentsactcoherentlyinmakingdecision
ortakingaction.
Howtoenableindividualagentstorepresentandreasonabout
theactions,plans,andknowledgeofotheragentstocoordinate
withthem.
Howtorecognizeandreconciledisparateviewpointsand
conflictingintentionsamongacollectionofagentstryingto
coordinatetheiractions.
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Footballsimulator
CMUCaseStudy
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CMUCaseStudy
CMUAgentArchitecture
Abehavior-basedarchitecture.
Flexibleagentroleswithprotocolsforswitching
amongthem.
Collectionofrolesbuiltintoteamformations.
Locker-roomagreement.
Set-plays:Multi-step,multi-agentplansfor
executioninspecificsituations.
http://www.cs.cmu.edu/afs/cs/usr/pstone/public/
papers/98springer/final-champ/final-champ.html
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CMUBehaviors
CMURoles
Abehaviorisasetofcondition/actionpairs,where
conditionsarelogicalexpressionsovertheinputsand
actionsarebehaviors
Leavesaretheoutputs:internalstatechangesforinternal
behaviorsandactionprimitivesforexternalbehaviors.
Specificationofanagent’sinternaland
externalbehaviours.
Eitherrigid,i.e.completelyspecifyan
agent’sbehaviour,orflexible,i.e.leave
acertaindegreeofautonomytothe
agentfillingtherole.
Dynamicswitchingofroles.
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CMURolesandFormations-Example
CMUFormations
Decomposethetaskspace.
Consistofasmanyrolesasagents.
Canincludesub-formations,units,for
localproblemsolving,consistingof:
Asubsetoftheformationroles.
Aleader,thecaptain.
Intra-unitinteractionsamongtheroles.
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CMULocker-RoomAgreement
CMUCommunication
Aninitialformation.
Aninitialmappingfromagentstoroles.
Orderofprecedencebetweenroles.
Run-timetriggersfordynamicchanging
offormations.
Set-plays.
Syntaxandsemanticsofcommunication.
Request/respondballlocation.
Request/respondteammatelocation.
Informpassdestination.
Informgoingtoball.
Informtaking/leavingposition.
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ApplyCMUtotheRescueSimulator
CMUChallenges
Howtorepresentandfollowlocker-room
agreements?
Howtodeterminetheappropriatetimesfor
agentstochangerolesand/orformations?
Howtoensurethatallagentsareusingthe
sameformation?
Howtoensurethatallrolesinaformation
arefilled?
Roles
Teamleader
Formation
Strategies
...
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Communication
“Communicationistheintentional
exchangeofinformationbroughtabout
bytheproductionandperceptionofsigns
drawnfromasharedsystemof
conventionalsigns.”
RusselandNorvig,1995
Communication
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AgentCommunication(1/3)
AgentCommunication(2/3)
Communicationinconcurrentsystems:
Exchangeinformationandknowledge.
High-levelcommunication–agent
conversations,suchasnegotiationsand
auctions.
Makeheterogeneousagents
interoperate.
Synchronizationofmultipleprocesses
CommunicationinObject-OrientedProgramming
Methodinvocationbetweendifferentmodules
CommunicationinMulti-Agent
Autonomousagentshavecontroloverbothstateandbehavior
Methodsareexecutedaccordingtotheagent’sself-interest
However,agentscanperformcommunicativeactions,i.e.attemptto
influenceotheragents
Agentcommunicationimpliesinteraction,i.e.agentsperform
communicationacts
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AgentCommunication(3/3)
SpeechActs(1/2)
Tocommunicateagentsneed:
acommonlanguage;
acommonunderstandingoftheknowledge
exchanged;and
theabilitytoexchangetheabove
information.
Awayofdiscoveringotheragents,network
addressesandtheircapabilities.
MosttreatmentofcommunicationinMASisinspired
fromspeechacttheory
Thetheoryofspeechactsisgenerallyrecognizedto
havebegunwiththeworkofthephilosopherJohn
Austin:“HowtoDoThingswithWords”(Austin,
Henoticedthatsomeutterancesareratherlike
1962)
physicalactionsthatappeartochangethestateof
theworld.
declaringwar
christening
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SpeechActs(2/2)
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TypesofSpeechAct
Searle(1969)extendedAustin’sworkandidentifiedthe
followingfivekeyclassesofpossibletypesofspeechacts:
Representatives:suchasinforming,e.g.,“Itisraining”
Directives:attemptstogetthehearertodosomething
e.g.,“pleasemakethetea”
Commisives:whichcommitthespeakertodoing
something,e.g.,“Ipromiseto…”
Expressives:wherebyaspeakerexpressesamentalstate,
e.g.,“thankyou!”
Declarations:suchasdeclaringwarorchristening
Atheoryofhowutterancesareusedto
achieveintentionsisaspeechacttheory.
Speechacttheoriesarepragmatictheoriesof
language,i.e.,theoriesoflanguageuse.Theyattempt
toaccountforhowlanguageisusedbypeopleevery
daytoachievetheirgoalsandintentions.
Austinidentifiedanumberofperformativeverbs,
whichcorrespondtovariousdifferenttypesof
speechacts
Examplesofperformativeverbsarerequest,inform,andpromise
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PlanBasedSemantics(1/2)
TypesofSpeechActs
Howdoesonedefinethesemanticsof
speechacts?
performative=request
content=Thebuildingisonfire
speechact=pleasehelpmeextinguishthebuilding
Whencanoneuttersomething,e.g.,arequestoraninform?
Whencanonesaysomeonehasuttered,e.g.,arequestoran
inform?
performative=inform
content=Thebuildingisonfire
speechact=Thebuildingisonfire!
Cohen&Perrault(1979)definedsemantics
ofspeechactsusingthepreconditiondelete-addlistformalismof(STRIPS)
planningresearch.
performative=inquire
content=Thebuildingisonfire
speechact=Isthebuildingonfire?
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SemanticsforRequest(A,B,⚔)
PlanBasedSemantics(2/2)
pre-conditions:
STRIPS(StanfordResearchInstitute
ProblemSolver)
AbelievesBcando⚔
(youdon’tasksomeonetodosomethingunlessyouthink
theycandoit)
AbelievesBbelievesBcando⚔
(youdon’tasksomeoneunlesstheybelievetheycandoit)
AbelievesAwants⚔
(youdon’tasksomeoneunlessyouwantit)
Pisasetofconditions
Oisasetofoperators,eachoperatorisquadruplet
1conditionsthatneedstobetrue
2conditionsthatneedstobefalse
3conditionsthataresettotrue
4conditionsthataresettofalse
Iistheinitialstate
Gisthegoalstate
effect:
BbelievesAbelievesAwantsBtodo⚔
(theeffectistomakethemawareofyourdesire)
STRIPScangenerateaplanfromItoG
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SpeechActonlyupdatebelief
Notethataspeakercannot(generally)forceahearerto
acceptsomedesiredmentalstatenortodoanaction.
Ontology(1/2)
Inordertobeabletocommunicate,agents
musthaveagreedonacommonsetofterms.
Aformalrepresentationofasetofterms
andconceptsisknownasanontology.
Theknowledgesharingefforthasassociated
withitalargeeffortatdefiningcommon
ontologies–softwaretoolslikeOntolingua
isdevelopedforthispurpose.
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Ontology(2/2)
AgentCommunicationLanguages
Sharingcommonunderstandingofthe
structureofinformationamongpeople
orsoftwareagents
Anagentcommunicationlanguage(ACL)is
astandardformatforexchangingmessages:
KnowledgeQueryandManipulationLanguage(KQML)
FIPAAgentCommunicationLanguage(FIPAACL)
Example:usingthesametechnicaltermonawebsite
(softwareisasynonymofprogram)
Enablingreuseofdomainknowledge
Example:usethesamerepresentationoftime
Analysedomainknowledge
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KnowledgeSharingEffort
KSE(KnowledgeSharingEffort)inearly
1990sdesignedtwoACLswith
differentpurpose
KQML
TheKnowledgeQueryandManipulationLanguage
(KQML),whichisan'outer'languageforagent
communication
TheKnowledgeInterchangeFormat(KIF),alanguage
forexpressingcontent,closelybasedonFirstOrder
Logic
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KnowledgeQueryandManipulationLanguage-KQML(1/2)
KnowledgeQueryandManipulationLanguage-KQML(2/2)
Itisan“outer”language,thatdefinesvarious
acceptable“communicativeverbs”,or
performatives.Exampleperformatives:
High-level,message-oriented
communicationlanguageandprotocol
forinformationexchangeindependentof
contentsyntaxandapplicableontology.
KQMLisdividedintothreelayers:
ask-if
(isittruethat…)
perform
(pleaseperformthisaction…)
tell(itistruethat…)
reply(theansweris…)
contentlayer
communicationlayer
messagelayer
KnowledgeInterchangeFormatisalanguagefor
expressingmessagecontent.
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KnowledgeInterchangeFormat-KIF(1/2)
KnowledgeInterchangeFormat-KIF(2/2)
Examples:
Aprefixversionoffirstorderpredicate
calculuswithextensionstosupportnonmonotonicreasoninganddefinitions.
Itallowsagentstoexpress
“Thetemperatureofm1is83Celsius”:
(=(temperaturem1)(scalar83Celsius))
“Anobjectisabacheloriftheobjectisamanandisnot
(defrelationbachelor(?x):=
(and(man?x)(not(married?x))))
propertiesofthingsinadomain,e.g.,“Michaelisa
vegetarian”
relationshipsbetweenthingsinadomain,e.g.,“Michaeland
Janinearemarried”
generalpropertiesofadomain,e.g.,“Allstudentsare
registeredforatleastonecourse”(quantification∀)
“Chip1hasalargersurfacethanchip2”:
(>(*(widthchip1)(lengthchip1))
(*(widthchip2)(lengthchip2)))
“Anyindividualwiththepropertyofbeingapersonalsohasthepropertyofbeing
amammal”:
(defrelationperson(?x):=>(mammal?x))
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KQML-Messagestructure
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KQML-Example(1/3)
(ask-one
:senderjoe
:receiverstock-server
:reply-withibm-stock
:content“(PRICEIBM?price)”
:languageLPROLOG
:ontologyNYSE-TICKS)
:contentcontentofthemessage
:languageformallanguageofthemessage
:ontologyterminologyofthemessage
:forcewillsendereverdenycontentofmessage?
:reply-withreplyexpected?identifierofreply.
:in-reply-toifitisaresponse?idofreply
:sendersenderID
:receiverreceiverID
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KQML-Example(2/3)
KQML-Example(3/3)
(tell
:senderstock-server
:receiverjoe
:in-reply-toibm-stock
:content“(PRICEIBM14)”
:languageLPROLOG
:ontologyNYSE-TICKS)
(ask-all
:receiverstock-server
:content“price(IBM,[?price,?time])“
:languagestandard_prolog
:ontologyNYSE-TICKS)
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KQMLPerformatives
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KQMLCriticism
Basicquery:ask-if,ask-one,ask-all
Multi-response(query):stream-all,eos
Response:error,sorry
Genericinformational:tell,deny,untell,achieve,
unachieve,insert,uninsert
Generator:standby,ready,next,rest,discard
Capability-definition:advertise,unadvertise,subscribe,
recommend-one,recommend-all,broker-one,broker-all
Networking:register,unregister,forward,broadcast
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ThebasicKQMLperformativesetwasoverlylarge
andnotwellstandardized.
NorigoroussemanticsforKQML.
differentimplementationsofKQMLwheredevelopedthatcouldnot,
infact,interoperate
Thelanguagewasmissingtheperformative
commissives
Commissivesarecrucialforagentscoordinatingtheiractions.
Noprecisedefinitionofthetransportmechanism
forKQMLmessages.
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FIPA
FoundationforIntelligentPhysicalAgents(FIPA)
1997,theFoundationforIntelligent
PhysicalAgents(FIPA)startedworkona
programofagentstandards-the
centrepieceisanACL.
ReleasedspecificationsforFIPAACLin1999and2002.
Basicstructureisquitesimilarto
performative;20performativeinFIPA.
housekeeping;e.g.,senderetc.
content,theactualcontentofthemessage.
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Informperformative
FIPAACL
InformandRequestarethetwobasic
performativesinFIPA.Allothersaremacro
definitions,definedintermsofthese.
Themeaningofinformandrequestis
definedintwoparts:
Inform(j,ɸ)
Thecontentɸisastatement.
Feasibilitycondition:
Biɸ˄¬Bi(Bifjɸ˅Ui
(ibelievesɸanddoesnotalreadybelievethat
therecipientjisawareofwhetherɸistrueornot)
feasibilitycondition:whichthesendermustsatisfyto
conformtotheFIPAACLstandard.
rationaleffect:whatthesenderofthemessageattempts
toachieve.
Rationaleffect:
Bjɸ
(jbelieveɸ)
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Requestperformative
FIPAperformatives
requestinformation:subscribe,query-if,
query-ref
passinginformation:inform,inform-ref,
confirm,disconfirm
negotiation:cfp,propose,accept-proposal,
reject-proposal
performingactions:request,request-when,
request-whenever,agree,cancel,refuse
Request(j,α)
Thecontentαisanaction.
Feasibilitycondition:
BiAgent(α,j)˄¬BiIjDone(α)
(ibelievesjwilldoαandthatjdoesnot
alreadyintendtodoα)
Rationaleffect:
Done(α)
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FIPAInteractionProtocols
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FIPARequest
InteractionProtocols(IPs)arestandardizedexchanges
ofperformativesaccordingtowellknownsituations
FIPARequest
FIPAQuery
FIPARequestWhen
FIPAContractNet
FIPAIteratedContractNet
FIPAAuctionEnglish
FIPAAuctionDutch
FIPABrokering
FIPARecruiting
FIPASubscribe
FIPAPropose
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ContractNetProtocol(1/3)
Smith(1980)
Simpleandeasytoimplement
ManagerandSupplierscommunicatevia
offers
SmithdefinedaContractSpecification
Language
ContractNet
Requirements
Formats
DeadlineforPresentation
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ContractNetProtocol(2/3)
ContractNetProtocol(3/3)
Announcement:Managessendsataskdescriptiontoall
possiblesuppliers(RequestForBid(T,M))
Bidding:Suppliersevaluatetheofferandsendaproposal
totheManager(Propose(T,Off,AG)or
NotInterested(T,Ag))
Awarding:ManagerEvaluatestheProposalandallocates
thecontracttothebestsupplier(Award(T,Ag,M))
Expediting:Thechosensupplierrepliespositivelyor
negativelytotheManager(Accept(T,Ag),Refuse(T,Ag))
Rolesarenotspecifiedinadvance,butaredynamic
TheManagercandecomposeataskandissue
severalCallforProposals(CFPs)
Alsoacontractormightfurtherdecomposeatask
andgivesomepartsawaytoothercontractors!
HierarchicalControlStructureforTaskSharing
Typically,anodewilltakeonbothroles,often
simultaneouslyfordifferentcontracts
FullyDistributedControl
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CNPFireBrigadeexample(1/2)
CNPFireBrigadeexample(2/2)
FirebrigadeAneedshelp
toextinguishabuilding
Themanagerawardsa
contracttothemost
appropriateagent
Forexample,agentB,
whichisclosertothefire
Taskspecification:needed
amountofwater,thelocationof
thefire,andadeadline
AgentBandDsubmit
theirbits
Thecontractorsendsbacka
reportafterfinishingthetaskor
furthersubdividesthetask...
Thebitcontainsestimatedcosts
fortravelingtothelocationand
forrefillingthetank
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LimitationsofContractNets
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FIPAContractNet
Limitations:
Taskdecompositionandproblemsynthesescanbenontrivial
Communicationoverhead
Theawardedcontractormightnotbethebestchoice,a
bettercandidatecouldbetemporarilybusyduringaward
time
Efficiencymodifications:
Focusedaddressing/directcontracts(e.g.teamstructure)
Agentsendstatusmessage,e.g.eligiblebutbusy,ineligible,
uninterested,…
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CommunicationinRoboRescue
Summary
ACLsprovidestandardsforcommunicationamong
selfishagents,e.g.withinanopensystems
Motivatedfromthetheoryofspeechacts,
communicationisimplementedintermsof
TheFIPAACLcanbeconsideredasthedefacto
standardforagentcommunication
Agentscommunication
Shareobservations
Distributetasks
Coordinatedecisions
Lowbandwidthunreliablechannels
IsitsuitabletouseanACLhere?
TheJADEframeworkimplementsitinJAVA
Notastandardone:ittakestoomuch
Butsimilarconceptscanbeapplied:informand
requestareusefull
Thereisstilllongwaytogountilwehavescalable
systems!
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