Multi-Agent Multi-Agent SystemsSystemsLecture 3Lecture 3
University “Politehnica” of University “Politehnica” of BucarestBucarest2004-20052004-2005
Adina Magda FloreaAdina Magda [email protected]@cs.pub.ro
http://http://turingturing..cscs.pub..pub.roro//blia_2005blia_2005
Agent communicationAgent communicationLecture outlineLecture outline
The nature of communicationThe nature of communication Indirect communicationIndirect communication Direct communicationDirect communication
Agent Communication LanguagesAgent Communication Languages• Content languagesContent languages• OntologiesOntologies• Theory of speech actsTheory of speech acts• KQMLKQML• FIPA and FIPA-ACLFIPA and FIPA-ACL
Interaction protocolsInteraction protocols
1. The nature of 1. The nature of communicationcommunication
1.1 Human communication1.1 Human communication Communication is the intentional exchange of information brought about
by the production and perception of signs drawn from a shared system of conventional signs (AIMA, Russell&Norvig) language
Communication seen as an action (communicative act) and as an intentional stance
1.2 Component steps of communication1.2 Component steps of communicationSpeakerSpeaker HearerHearer Intention Perception Generation Analysis Synthesis Disambiguation
Incorporation
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SyntaxSemanticsPragmatics
Communication modelCommunication model
1.3 1.3 Artificial CommunicationArtificial Communication low-level language vs high-level languages direct communication vs. indirect communication
Computer communicationComputer communication shared memory message passing
Agent communication/ MAS communicationAgent communication/ MAS communication low-level communication: simple signals, traces, low-level languages high-level communication - cognitive agents, mostly seen as intentional
systems Communication in MAS = more than simple communication, implies
interaction The environment provides a computational infrastructure where
interactions among agents take place. The infrastructure includes protocols for agents to communicate and protocols for agents to interact
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Communication protocolsCommunication protocols = enables agents to exchange and understand messages
Interaction protocolsInteraction protocols = enable agents to have conversations, i.e., structured exchanges of messages
AimAim Communication enables agents to: coordinate their actions and behavior, a property of
a MAS performing some activity in a shared environment
attempt to change state of the other agents attempt to make the other agents perform some
actions
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2. Indirect communication2. Indirect communication2.1 Signal propagation2.1 Signal propagation - Manta, A. Drogoul 1993 An agent sends a signal, which is broadcast into the environment, and
whose intensity decreases as the distance decreases At a point x, the signal may have one of the following intensities
V(x)=V(x0)/dist(x,x0) or V(x)=V(x0)/dist(x,x0)2
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x0
SS - stimulus
Agent A(stimulus triggersbehavior P)
Agent B(stimulus triggersbehavior P)
2.2 Trails2.2 Trails - L. Steels, 1995 agents drop "radioactive crumbs" making trails an agent following a trail makes the trail faint until it disappears
Reactive agentsReactive agents
2.32.3 Blackboard systemsBlackboard systems, Barbara Hayes-Roth, 1985
Blackboard = a common area (shared memory) in which agents can exchange information, data, knowledge
Agents initiates communication by writing info on the blackboard Agents are looking for new info, they may filter it Agents must register with a central site to receive an access
authorization to the blackboard Blackboard = a distributed knowledge computation paradigm Agents = Knowledge sources (KS)
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Cognitive agentsCognitive agents
KS
KS
KS
KS
KSAR
Control
Blackboard
3. Direct communication3. Direct communicationSending messages Sending messages method invocation – Actors exchange of partial plans – coordination of cooperative
agents
ACL = Agent Communication LanguagesACL = Agent Communication Languages Need to communicate knowledge knowledge
representation Need to understand the message in a context ontologies Communication is seen as an action - communicative acts
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Concepts (distinguish ACLs from RPC, RMI or Concepts (distinguish ACLs from RPC, RMI or CORBA, ORB):CORBA, ORB): An ACL message describes a desired state in a declarative
language, rather than a procedure or method invocation ACLs handle propositions, rules, and actions instead of
objects with no associated semantics - KR ACLs are mainly based on BDI theories: BDI agents
attempt to communicate their BDI states or attempt to alter interlocutor's BDI state – Cognitive Agents
ACLs are based on Speech Act Theory – Communicative Acts
ACLs refer to shared Ontologies Agent behavior and strategy drive communication
and lead to conversations - Protocols
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3.1 Agent Communication 3.1 Agent Communication LanguagesLanguages
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Origins of ACLsOrigins of ACLsKnowledge Sharing Effort - DARPA, 1990 External Interface Group - interaction between KBS - KQML Interlingua - common language of KB - KIF Shared, Reusable Knowledge Bases - Ontolingua
ACL
Content language
Ontology
Communication primitivesand protocols
Content languages• KIF• Prolog• Clips• SQL• FIPA-SL, FIPA-KIF
Ontologies DAML OWL
DARPA Agent Markup Language (August 2000).The goal of the DAML effort is to develop a languageand tools to facilitate the concept of the Semantic Web.
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3.2 Content languages for ACL3.2 Content languages for ACL(Knowledge representation)
Description Logics (DL) - a formalism for expressing concepts and their interrelationships.
In DL, concepts are organized into IS-A hierarchies.Concepts are specifications such that given an individual
(object instance) a DL system can recognize the individual and determine which concepts it belongs to.
DL systems also perform subsumption checking.
Knowledge Interchange Format (KIF) - is based on first order predicate logic and has a LISP-like prefix syntax.
KIF is capable of expressing facts and complex relationships (rules).
KIF provides constructs for describing procedures, i.e. programs to (possibly) be executed by an agent.
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Knowledge Interchange Format (KIF)
Facts(salary 015-46-3946 john 72000)(salary 026-40-9152 michael 36000)(salary 415-32-4707 sam 42000)
Asserted relation (> (* (width chip1) (length chip1)) (* (width chip2) (length chip2)))
Rule(=> (and (real-number ?x) (even-number ?n)) (> (expt ?x ?n) 0))
Procedure(progn (fresh-line t)
(print "Hello!")(fresh-line t))
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3.3 Ontologies3.3 OntologiesOntology = a specification of objects,
concepts, and reationships in a particular domain; it comprises a vocabulary, a domain theory and a conceptual schemata to describe organization and interpretation
An ontology is more than a taxonomy of classes, it must also describe relationships
Lexicalized ontologies (WordNet, EuroWordNet, BalkanNet, FrameNet, MikroKosmos).
Ontologies for knowledge representation Implicit ontologies (class libraries of
OOP)
Person
PupilStudEmpl
Sun_EIBM_E
Person
Empl
Woman
Stud
Man
Joe Alice
Joe Alice
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x (Block x) (PhysicalObject x)
Instead of saying (Block A)
(InstanceOf A Block)
Define the hierarchy
(Class Block)
(Class PhysicalObjects)
(SubclassOf Block PhysicalObjects)
x,y,z (InstanceOf x y) (SubclassOf y z) (InstanceOf x z)
Ontology editors = frame-based KR systems that allow the user to define an ontology and its components: classes, instances, relationships, and functions
OWL - Web Ontology Language
W3C OWL - designed for use by applications that need to process
the content of information instead of just presenting information to humans.
OWL facilitates greater machine interpretability of Web content than that supported by XML, RDF, and RDF Schema (RDF-S) by providing additional vocabulary along with a formal semantics.
OWL has three increasingly-expressive sublanguages: OWL Lite supports a classification hierarchy and simple constraints. OWL DL supports maximum expressiveness while retaining
computational completeness (all conclusions are guaranteed to be computed) and decidability (all computations will finish in finite time).
OWL Full supports maximum expressiveness and the syntactic freedom of RDF with no computational guarantees.
OWL - Web Ontology Language
Ontology header<owl:Ontology rdf:about="">
<rdfs:comment>An example OWL ontology</rdfs:comment>
<owl:priorVersion rdf:resource="http://www.w3.org/TR/2003/CR-owl-guide-20030818/wine"/>
<owl:imports rdf:resource="http://www.w3.org/TR/2003/PR-owl-guide-20031215/food"/>
<rdfs:label>Wine Ontology</rdfs:label>
…
Simple Named Classes
Class, rdfs:subClassOf<owl:Class rdf:ID="Winery"/>
<owl:Class rdf:ID="Region"/>
<owl:Class rdf:ID="ConsumableThing"/>
3.4 Theory of Speech Acts3.4 Theory of Speech ActsJ. Austin - How to do things with words, 1962, J. Searle - Speech acts,
1969
A speech act has 3 aspects: locution = physical utterance by the speaker illocution = the intended meaning of the utterance by the speaker (performative) prelocution = the action that results from the locution
Alice told Tom: "Would you please close the door"locution illocution content
prelocution: door closed (hopefully!)
Illocutionary aspect - several categories Assertives, which inform: the door is shut Directives, which request: shut the door, can pelicans fly? Commissives, which promise something: I will shut the door Permissive, which gives permission for an act: you may shut the door Prohibitives, which ban some act: do not shut the door Declaratives, which causes events: I name you king of Ruritania Expressives, which express emotions and evaluations: I am happy 18
3.5 KQML - 3.5 KQML - Knowledge Query and Knowledge Query and Manipulation Manipulation
LanguageLanguageA high-level, message-oriented communication language and protocol for information exchange, independent of content syntax (KIF, SQL, Prolog,…) and application ontology
KQML separates: semantics of the communication protocol (domain independent) semantics of the message (domain dependent)
3 (conceptual) layers3 (conceptual) layers
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Content
Communication
Message
Describes low levelcommunication parameters:- identity of sender and receiver- an unique id associated with the communication
Core of KQML- identity of the network protocol with which to deliver the message- speech act or performative Optional- content language- ontology
SyntaxSyntax S-expressions used in LISPKQML performatives are classified:
Queries - These performatives are used to send questions for evaluation somewhere. Generative - Used for controlling and initiating the exchange of messages. Response - Used by a agent in order reply to queries. Informational - Informational performatives are used to transfer information. Capability definition - Allows an agent to learn about the capabilities of other agents and to announce its
own to the agent community. Networking - Networking performatives make it possible to pass directives to underlying communication
layers.
ExampleExample(ask-one :sender joe
:receiver ibm-stock:reply-with ibm-stock:language PROLOG:ontology NYSE-TICKS:content (price ibm ?price) )
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(tell :sender willie:receiver joe:reply-with block1:language KIF:ontology BlockWorld:content (AND (Block A)(Block
B)(On A B)) )
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1. Query performatives:ask-one, ask-all, ask-if, stream-all,...
(stream-all :sender willie:receiver ibm-stock
:content (price ?VL ?price ) )
ask-one(P)
tell(P)
tell(P1)
stream-all(P)
tell(P2)eos
tell(P1,P2,...)
ask-all(P)
B
B
B
A
A
A
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4. Generic informational performatives:tell, untell, insert, delete, ...
In fact, KQML contains only 2 types of illocutionary acts: assertives and directives
+ facilitator and network-related performatives (no necessarily speech acts)
tell(P)
untell(P)
delete(P)
insert(P)
2. Generative performatives:standby, ready, next, rest, discard, generate,...
6. Network performatives:register, unregister, forward, route, ...
3. Response performatives:reply, sorry ...
5. Capability performatives:advartise, subscribe, recommend...
Facilitator
A
A
B
B
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Facilitator agentFacilitator agent= an agent that performs various useful communication services:
maintaining a registry of service names (Agent Name Server) forwarding messages to named services routing messages based on content matchmaking between information providers and clients providing mediation and translation services
tell(P)
ask(P)
subscribe(ask(P)) tell(P)
tell(P)
advertise(ask(P))
tell(P)
recruit(ask(P))
reply(A)
recommend(ask(P)) advertise(ask(P))
tell(P)
ask(P)
reply(B)
A
A
A
A
B
B
B
B
point-to-point
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Semantics of KQML Semantics of KQML (Labrou & Finin)(Labrou & Finin) Use preconditions and postconditions that govern the use of a performative + the
final state for the successful performance of the performative Uses propositional attitudes: belief, knowledge, desire, intentions
Preconditions: the necessary states for an agent to send a performative and for the receiver to accept it and successfully process it; if the precondition does not hold, the most likely response is error or sorry
Postconditions - describe the state of the sender after successful utterance of a performative and of the receiver after the receipt and processing of a message
Completion condition - the final state after a conversation has taken place and that the intention associated with the performative that started the conversation has been fulfilled
Propositional attitudes
Bel(A,P) Know(A,S) Want(A,S) Int(A,S)
Instances of action
Proc(A,M) SendMsg(A,B,M)
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tell(A,B,X)tell(A,B,X)A states to B that A believes the content X to be true, Bel(A,X)
Pre(A): Bel(A,X) Know(A, Want(B, Know(B, Bel(A,X))))
Pre(B): Int(B, Know(B, Bel(A,X)))
or Bel(A,X)
Post(A): Know(A, Know(B, Bel(A,X))) no unsolicited information
Post(B): Know(B, Bel(A,X))
Completion: Know(B, Bel(A,X))
advertise(A,B,M)advertise(A,B,M)A states to B that A can and will process the message M from B, if it receives one
Int(A, Proc(A,M)) commisive act
Pre(A): Int(Proc(A,M))
Pre(B): NONE
Post(A): Know(A, Know(B, Int(A, Proc(A,M)))
Post(B): Know(B, Int(A, Proc(A,M)))
Completion: Know(B, Int(A, Proc(A,M)))
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KQML Software architectureKQML Software architecture KQML – implemented in different languages
Design – a router, a facilitator, a library of interface routines (KRIL)
Eack KML agent is associated with its separate router process; all routers are identical
AgentKRILRouterNetwork
KQML objects
function callsNetwork connections
KQML strings
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KRILKRILAn API – KQML Router Interface Library
While the router is a separate process, with no undestanding of the content field of a KQML message, the KRIL API is embedded in the application and has access to the application tools for analyzing the content.
Variuos KRILs Tightly embedded or lightly embedded Simple KRIL
- send-kqml-message – accepts a message- declare-message-handler – which function to invoke
when a message arrives- different types of declarations which allow to register the
application with local facilitators and contact remote agents KRIL: Common Lisp, C, Prolog, SQL, etc.
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3.6 FIPA3.6 FIPA andand FIPA - ACL FIPA - ACL Foundation for Intelligent Physical Agents, 1996
Goal of FIPA = make available specifications that maximize interoperability across agent-based systems
FIPA Committees: ACL, agent specification, agent-software interaction As KQML, FIPA ACL is based on speech act theory; it sees messages as
communication acts (CA); syntax similar to KQML Differs in: the names of CAs, set of CAs, and semantics
FIPA communicative actsInformatives - query_if, subscribe, inform, inform_if, confirm, disconfirm, not_understoodTask distribution - request, request_whenever, cancel, agree, refuse, failureNegotiation - cfp, propose, accept_proposal, reject_proposal
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FIPA's specifications are grouped into 5 categories: 1. Applications 2. Abstract Architecture 3. Agent Communication 4. Agent Management 5. Agent Message Transport
3. Agent Communication: FIPA ACL Message Structure Communicative Acts: FIPA Communicative Acts Library Content Languages:
FIPA Content Languages FIPA SL Content Language FIPA CCL Content Language FIPA KIF Content Language FIPA RDF Content Language
Interaction Protocols
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FIPA-SL (Semantic Language)(inform :sender Agent1
:receiver Agent2:content (price good2 150):in-reply-to round-1: reply-with bid03: language S1:ontology hp-auction:reply-by 10:protocol offer:conversation-id conv-1 )
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FIPA-SL(request :sender (agent-identifier :name i) :receiver (set (agent-identifer :name j)
:content ((action (agent-identifier :name j)(deliver box7 (loc 10 15))))
:protocol fipa-request :language fipa-sl :reply-with order56 )
(agree :sender (agent-identifier :name j) :receiver (set (agent-identifer :name i)
:content ((action (agent-identifier :name j)(deliver box7 (loc 10 15))) (priority order56 low))
:protocol fipa-request :language fipa-sl :in-reply-to order56 )
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FIPA - SemanticsFIPA - SemanticsSL (Semantic Language) - a quantified, multi-modal logic, with
modal operators
Allows to represent:beliefsuncertain beliefsdesiresintentions
B - belief D - desire U - uncertain belief
PG - intention
Bif - express whether an agent has a definite opinion one way or another about the truth or falsity of
Uif - the agent is uncertain about
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FIPAFIPA - - SemanticsSemanticsThe semantics of a CA is specified as a set of SL's formulae that
describe: Feasibility preconditions - the necessary conditions for the sender -
the sender is not obliged to perform the CA Rational effect - the effect that an agent can expect to occur as a
result of performing the action; it also typically specifies conditions that should hold true of the recipientThe receiving agent is not required to ensure that the expected effect comes aboutThe sender can not assume that the rational effect will necessary follow
<A, inform(B, )>
Pre: BA BA (BifB UifB )
Post: BB
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Using ACLs in MAS
Any MAS that is to use an ACL must provide: a finite set of APIs for composition, sending, and receiving ACL
messages an infrastructure of services that assist agents in naming,
registration, and basic facilitation services (finding other agents that can do things for your agent)
code for every reserved message type that takes the action prescribed by the semantics for the particular application;
the code depends on the application language, the domain, and the details of the agent system using the ACL
4. Interaction protocols4. Interaction protocolsInteraction protocolsInteraction protocols = enable agents to have conversations, i.e., structured exchanges of messages
Finite automata Conversations in KQML Petri nets FIPA IP standards:
• FIPA-query, FIPA-request, FIPA-contract-net, ...
4.1 Finite state automata4.1 Finite state automata
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A:B<<ask(do P)
B:A<<accept(do P)
B:A<<refuse(do P)
B:A<<result(do P) B:A<<fail(do P)
proposeS(P)
acceptR(P) rejectR(P)
counterR(P)counterS(P)
acceptS(P) rejectS(P)Winograd, Flores, 1986
COOL, Barbuceanu,95
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4.2 Conversations in KQML4.2 Conversations in KQMLUse Definite Clause Grammars (DCG) formalism for the specification of conversation policies for KQML performatives
DCGs extend Context Free Grammars in the following way: non-terminals may be compound terms the body of the rule may contain procedural attachments, written as "{" and "}"
that express extra conditions that must be satisfied for the rule to be valid
Ex: noun(N) [W], {RootForm(W,N), is_noun(N)}
S s(Conv, P, S, R, inR, Rw, IO, Content), {member(P, [advertise, ask-if]}
s(Conv, ask-if, S, R, inR, Rw, IO, Content) [ask-if, S, R, inR, Rw, IO, Content] |
[ask-if, S, R, inR, Rw, IO, Content], {OI is inv(IO)},
r(Conv, ask-if, S, R, _, Rw, OI, Content)
r(Conv, ask-if, R, S, _, inR, IO, Content) [tell, S, R, inR, Rw, IO, Content] |
problem(Conv, R, S, inR, _, IO) Labrou, Finin, 1998
DAA wants to do P,A cannot do P
Request do(P)
Refuse do(P)
Accept/request do(P)
Fail to do(P)
Notification of end(P)
DB
AR1
AR2
FA2FA1
BR
FBFailure Satisfaction
Impossibleto do(P)
B does not wantto do(P)
B is willingto do(P)
Completed(P)
4.3 Petri nets4.3 Petri nets
Petri net = oriented graph with 2 type of nodes:places and transitions;there are moving tokens through the net - representation of dynamic aspect of processes.Tokens are moved from place to place, following firing rules.A transition T is enabled if all the input places P of T posses a token (several other rules may be defined).A marking is a distribution of tokens over places. Colored Petri-nets
Ferber, 1997
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Success
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4.4 FIPA Interaction Protocols4.4 FIPA Interaction Protocols
FIPA Interaction Protocol Library FIPA Request Interaction Protocol FIPA Query Interaction Protocol FIPA Request When Interaction Protocol FIPA Contract Net Interaction Protocol FIPA Iterated Contract Net Interaction Protocol FIPA English Auction Interaction Protocol FIPA Dutch Auction Interaction Protocol FIPA Brokering Interaction Protocol FIPA Recruiting Interaction Protocol FIPA Subscribe Interaction Protocol FIPA Propose Interaction Protocol
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ReferencesReferences M. Huhns, L. Stephens. Multiagent systems and societies of agents. In
Multiagent Systems - A Modern Approach to Distributed Artificial Intelligence, G. Weiss (Ed.), The MIT Press, 2001, p.79-120.
M. Wooldrige. Reasoning about Rational Agents. The MIT Press, 2000, Chapter 7
Y. Labrou, T. Finin. Semantics and conversations for an agent communication language. In Readings in Agents, M. Huhns & M. Singh (Eds.), Morgan Kaufmann, 1998, p.235-242.
J. Ferber - Multi-Agent Systems. Addison-Wesley, 1999, Chapter 6 T. Finnin, R. Fritzson - KQML as an agent communication language. In Proc. of
the Third International Conference on Information and Knowledge Management (CIKM'94), ACM Press, 1994.
M. Singh. Agent communication languages: Rethinking the principles. IEEE Computer, Dec. 1998, p.40-47.
Y. Labrou, T. Finnin, Y. Peng. Agent communication languages: The current Landscape. IEEE Computer, March/April 1999, p. 45-52.
FIPA97. "Agent Communication Language" Specification FIPA, 11/28/97
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Web ReferencesWeb References
DARPA KSE http://www-ksl.stanford.edu/knowledge-sharing/
KQML http://www.cs.umbc.edu/kqml/
KIF http://logic.stanford.edu/kif/
Ontolingua http://www-ksl-svc.stanford.edu:5915/&service=frame-editor
FIPA http://www.fipa.org/
DAML http://www.daml.org/
OWL http://www.w3.org/TR/owl-guide/
References for OntologiesReferences for Ontologies (due to prof. Stefan Trausan) Constandache, G.G., Ştefan Trăuşan-Matu, Ontologia şi hermeneutica
calculatoarelor, Ed. Tehnică, 2001 Gruber, T., What is an Ontology, http://www.kr.org/top/definitions.html J. Sowa, Ontologia şi reprezentarea cunoştinţelor, în (Constandache şi Trăuşan-
Matu, 2001) http://www.w3.org/2001/sw/WebOnt/ http://www.cs.man.ac.uk/~horrocks/Slides/index.html
