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14<MAS>-1
Lecture 14 Multi-Agent Systems
• Topics– Basics– MAS Architectures– Coordination
• KQML
– Collaboration• CNP
– Development of MAS– Applications
14<MAS>-2
Basics
14<MAS>-3
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
14<MAS>-4
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
14<MAS>-5
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.
14<MAS>-6
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
14<MAS>-7
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– seems to be out of date...
14<MAS>-8
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• ...
14<MAS>-9
MAS Architectures
• FIPA’s Model – Agents– Agent Platform (AP)– Directory Facilitator (DF)– Agent Management System (AMS)– Agent Communication Channel (ACC)– Agent Communication Language (ACL)
14<MAS>-10
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
14<MAS>-11
MAS Architectures
• OAA Model
14<MAS>-12
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.
14<MAS>-13
• 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
14<MAS>-14
MAS Architectures•Zeus: a MAS development toolkit
14<MAS>-15
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
14<MAS>-16
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
14<MAS>-17
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 …
14<MAS>-18
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.
14<MAS>-19
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
14<MAS>-20
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)
14<MAS>-21
Coordination - KQML
14<MAS>-22
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, …
14<MAS>-23
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)
14<MAS>-24
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
14<MAS>-26
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.
14<MAS>-27
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.
14<MAS>-28
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
14<MAS>-29
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
14<MAS>-30
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