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Lecture 14 Multi-Agent Systems

• Topics– Basics– MAS Architectures– Coordination

• KQML

– Collaboration• CNP

– Development of MAS– Applications

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Basics

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Basics

• MAS as seen from Distributed AI– a loosely coupled network of entities that

work together to find answers to problems that are beyond the individual capabilities or knowledge of each entity.

• A more general meaning– systems composed of autonomous

components that exhibit the following characteristics:

• each agent has incomplete capabilities to solve a problem

• there is no global system control• data is decentralized• computation is asynchronous

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Basics• Traditional

– Client-server– Low-level messages– Synchronous– Can not do the job!

• Agent breakthroughs– Peer-to-peer

topology– Blackboard

coordination model– Encapsulated

messaging– High-level message

protocols

Client Server

IntelligentAgents

IntelligentAgents

IntelligentAgents

Function(Parameters)

Return(Parameters)

Blackboard

MessageReply

Traditional Software

Agents IntelligentAgentsIntelligent

Agents

IntelligentAgentsIntelligent

Agents

IntelligentAgentsIntelligent

Agents

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Basics• MAS researchers develop communications

languages, interaction protocols, and agent architectures that facilitate the development of multi-agent systems.

• MAS researcher can tell you how to program each ant in a colony in order to get them all to bring food to the nest in the most efficient manner, or how to set up rules so that a group of selfish agents will work together to accomplish a given task.

• MAS researchers draw on ideas from many disciplines outside of AI, including biology, sociology, economics, organization and management science, complex systems, and philosophy.

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Basics

• Key elements to achieve multi-agent interaction– a coordination mechanism supported by a

common agent communication language and protocol

– a collaboration mechanism supported by agent community architecture (including agent and interaction architecture) to support the organization goal

– a shared ontology

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MAS Architectures (Standards?)

• Object Manager Group (OMG)• Foundation for Intelligent Physical

Agents (FIPA)• Knowledgeable Agent-oriented System

(KAoS)• Open Agent Architecture (OAA)• General Magic group

– seems to be out of date...

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MAS Architectures

• OMG’s Model– Composed of agents and agencies that

collaborate using general patterns and policies

– Agents are characterized by:• capabilities, type of interaction and mobility

– Agencies support:• concurrent execution of agents• security• agent mobility• ...

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MAS Architectures

• FIPA’s Model – Agents– Agent Platform (AP)– Directory Facilitator (DF)– Agent Management System (AMS)– Agent Communication Channel (ACC)– Agent Communication Language (ACL)

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MAS Architectures

• KAoS’s Model– An Open Distributed Architecture for

Software agents– Defines various agent implementations– Uses conversation policies to elaborate on

agent-to-agent communication

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MAS Architectures

• OAA Model

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MAS Architectures

• General Magic’s Model – A commercial agent technology for

electronic commerce– Views MAS as an electronic marketplace– The marketplace is modeled as a network

of computers supporting a collection of places that offer services to mobile agents.

– The mobile agents:• can travel, meet other agents, create

connections to other places• they have authority.

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• Zeus: a MAS development toolkit

MAS Architectures

A

B

C

D

Agent Facilitator

Abilit ies Database

Agent Name Server

Address Book

request

reply

Transport Protocol

MESSAGE

Common Message Format (Language)

Shared mesage content representation and ontology

Agent

Perform Task A

Agent

Perform Task C

Agent

Perform Task D

External program

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MAS Architectures•Zeus: a MAS development toolkit

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Geo-Agents

Domain (Service) Agent

Domain (Service) AgentTask Agent

Facilitator

Administrator UI Agent

Task Agent

Other Agent Systems User

Query agent

Exchange registry

Query agent

Query agent

Query agent Pass taskReply

Coordinate

Coordinate

Collaborate

Control/Reply

Task(GeoScri

pt)Reply

Collaborate

Data sources

Retrieve

• Geo-Agents (GIS agents) Architecture as a MAS example

MAS Architectures

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Coordination

• Coordination: a process to manage dependencies among activities

• Common dependencies among activities – State

• Shared resource• Producer/Consumer

– Prerequisite– Transfer– Usability

• Simultaneity constraint

– Task/Sub-Task– Worth

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Coordination

• Three aspects on coordination– Activity aspect

• What activity to execute?• When an activity should be

executed?• Model to coordinate distributed

tasks: Statecharts, Flowcharts, Process algebra, Lotos, SDL, Estelle …

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Coordination

– Conversation (state) aspect• What is the structure of the

conversation among the coordinating entities?

• FSM, Petri-Nets, State Transition Diagrams

– Implementation aspect• How to implement distributed

software systems where software components coordinate their actions.

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Coordination - KQML

• Knowledge Query and Manipulation

Language (KQML) is both a message format

and a message-handling protocol to support

run-time knowledge sharing among agents.

• KQML comprise a substrate on which to

develop higher-level models of inter-agent

interaction such as contract nets.

• KQML is a coordination mechanism from the

conversation aspect

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Coordination - KQML

• KQML contains an extensible set of

performatives, which defines the

permissible speech acts agents may

use• Example performative:

(ask-all /* message layer */ :content "price(IBM, [?price, ?time])“

/* content layer */ :receiver stock-server

/* communication layer */ :language standard_prolog :ontology NYSE-TICKS

:sender me)

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Coordination - KQML

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Coordination - KQML

• Types of performatives– Basic informative performatives: tell,

deny, … – Database performatives: insert, delete,

… – Basic responses: error, sorry, … – Basic query performatives: ask-one,

ask-all, evaluate,… – Multi-response query performatives:

stream-all, …– Basic effector performatives: achieve, …

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Coordination - KQML

• Types of performatives (Cont.)

– Generator performatives: standby, ready, next, …

– Capability-definition performatives: advertise

– Notification performatives: subscribe – Networking performatives: register,

forward, pipe, broadcast, … – Facilitation performatives: broker-one

(all), recommend-one (all), recruit-one (all)

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Collaboration• Collaboration refers to cooperative

effort among agents to reach a single goal by exchanging knowledge built upon the underlying coordination mechanism

• Example mechanism: Contract Net Protocol (CNP)– Negotiation as a collaboration mechanism – Negotiation on how tasks should be shared

• A task (plan) may be decomposed in a hierarchy of subtasks (hierarchical planning)

• An agent may subcontract another agent to perform a (sub)task.

Contract

Bid

agent agent

Collaboration - CNP

Task announceme

nt

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Collaboration - CNP

Contractor

Potential candidate agents

Task announcement ("broadcast")

Contractor

Candidate Candidate

Bid

Bid

Phase 1: Task Announcement

- The contractor agent publicly announces a task.

- Potential candidates evaluate the task according to their won skills and availability.

Phase 2: Submission of Bids / Proposals

- Agents that satisfy the requiremenst, i.e., are able to perform the task, send their bid / proposal to the contractor.

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Collaboration - CNP

Contractor

Selected candidate

Contractor

Contracted agent

Contract

Phase 3: Selection

- The selection of the best candidate is made by the contractor based on received bids and on the CVs of the candidates.

Phase 4: Contract awarding

- A contract is established between the contractor and the selected candidate.

- A privileged bilateral communication channel is established between the two agents.

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Development of MAS• Define the organization of the MAS according to

the problem specification (or solution structure)• Decide the coordination mechanism• Select a MAS implementation framework, e.g.,

Zeus, that supports the coordination mechanism

• Implement the collaborative mechanism which support the MAS organization

• Implement shared ontology • Implement each task agent (including

customizing associated communication module)

• Customize middle agents– Facilitators– Mediators

−Brokers−Matchmakers and yellow pages−Blackboards

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Applications● Advanced Manufacturing Management Systems

−Agents as representatives of machines, users, business processes, etc.

● Intelligent Information Search on Internet−Some agents may show learning capabilities

(learn the preferences of their users, ..)

● Intelligent security enforcement on Internet−Agents are representative of sensors or IDSs

● Shopping Agents in Electronic Commerce−With search, price comparison, and

bargaining capabilities

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Applications● Multi-agent auction in E-commerce

● Distributed Surveillance−For information search or to look for

special events informing their users of relevant news

● Distributed Signal Processing−For problem diagnosis, situation

assessment, etc. in the network

● Distributed Problem Solving−Collaborative design, scheduling, and

planning