4005-730 Distributed Systems
Lecture Notes -- Module 7. Agent Systems

Agents and Agent Systems

• An agent is autonomous software object
  • Executes on its own (in its own thread)
  • Other agents cannot call it directly

• Agents interact by sending messages to each other
  • Each agent decides for itself when to receive messages
  • Each agent decides for itself in which order (priority) to receive messages
  • Each agent decides for itself how to respond to messages

• Agents are proactive
  • They don't have to merely react to external stimuli
  • They can initiate actions on their own

• Agents can say no
  • An agent can refuse a request sent by another agent
  • Agents tend to interact by negotiation rather than obeying commands

• Multiple agents constitute a peer-to-peer system
  • Different agents can run on different compute nodes
  • Some agents may get inputs from the external world (sensors)
  • Some agents may send outputs to the external world (actuators)
  • Some agents may take action to achieve some objective
  • There is no central server that controls the whole system

• Agents can even move from one node to another -- agent mobility

Agent Middleware

• Foundation for Intelligent Physical Agents (FIPA): http://www.fipa.org/
  • Formerly a primarily-European industrial consortium
  • Now an IEEE standards committee
  • Sets standards for agents and agent systems

• Java Agent Development (JADE) Framework: http://jade.tilab.com/
  • A widely-used open-source Java implementation of the FIPA standard
  • Includes a complete agent middleware system
  • Includes classes for writing agents in Java

• F. Bellifemine, G. Caire, and D. Greenwood. Developing Multi-Agent Systems with JADE. John Wiley & Sons, 2007.

• JADE architecture

  
  Bellifemine et al., op. cit.

  • Containers
    • Main container
    • Other containers
  • Agents
    • Agent Management System (AMS)
    • Directory Facilitator (DF) -- agent registry/yellow pages
    • Application agents
    • Each has a globally unique agent identifier (AID)
  • Container Table (CT)
  • Global Agent Descriptor Table (GADT)
    • AIDs, names, etc. of all agents in entire system
    • Cached in containers other than main container
    • Tells where to send messages for each agent
  • Local Agent Descriptor Table (LADT)
    • Just agents in the local container
  • Message Transport Protocol (MTP)
    • FIPA standard
    • For interoperating with non-JADE FIPA-compliant systems
  • Internal Message Transport Protocol (IMTP)
    • Used between JADE containers
    • More efficient than MTP
  • A hybrid architecture
    • Agents themselves are P2P
    • AMS and DF are on central server

Agent Messages

• In the FIPA standard, every message has:
  • Communicative act (CA)
  • Content
  • Ontology

• 22 possible CAs:

  Accept Proposal Agree Cancel Call for Proposal Confirm Disconfirm Failure Inform

  Inform If Inform Ref Not Understood Propagate Propose Proxy Query If Query Ref

  Refuse Reject Proposal Request Request When Request Whenever Subscribe

• The ontology attaches application-specific meaning to the content

• The FIPA standard also specifies message protocols

• FIPA Request Interaction Protocol:

  
  Bellifemine et al., op. cit.

• FIPA Contract Net Interaction Protocol:

  
  Bellifemine et al., op. cit.

JADE Ping Agent Example

• Class PingAgent

• Agent behaviors
  • A piece of code that gets executed by the agent's thread
  • An agent can be executing multiple behaviors
  • The behaviors provide cooperative (single-threaded, non-preemptive) multitasking within an agent

• Demo

• JADE includes many useful graphical distributed agent debugging tools
  • Remote Monitoring Agent (RMA)
  • Dummy Agent
  • Sniffer Agent
  • Introspector Agent

JADE Book Trading Example

• Book buyer agents
  • Looking for a certain title
  • Will buy from whichever seller has the lowest price

• Book seller agents
  • Has a catalog of books for sale and their prices
  • Books added by the user
  • Books removed when sold to a buyer

• Class BookBuyerAgent

• Class BookSellerAgent

• Agent behaviors
  • Behaviour -- performed repeatedly until done
    • Often coded as a state machine for complex conversations/protocols
    • block() -- suspends execution of the behavior until a message is received
  • CyclicBehaviour -- performed repeatedly forever
  • OneShotBehaviour -- performed just once
  • TickerBehaviour -- performed at regular time intervals
  • WakerBehaviour -- performed once after a time interval

• Demo

JADE Agent Mobility

• A JADE agent can move from one container to another, e.g. on a different host
  • An agent can move itself
  • One agent can cause another agent to move

• A JADE agent can be cloned in another container, e.g. on a different host
  • An agent can clone itself
  • One agent can cause another agent to be cloned

• Moving/cloning process
  • A message containing the following is sent to the destination container:
    • Agent's class name
    • Agent's data (field values)
    • Agent's codebase (Java class files)
  • The destination reconstructs the agent and starts executing it from where it left off
  • If the agent moved, the original is terminated in the source container

• Mobile agent example
  • Class MobileAgent
  • Class CounterBehaviour
  • Class GetAvailableLocationsBehaviour
  • Class ServeIncomingMessagesBehaviour
  • Class MobileAgentGui
  • Class LocationTableModel

• Demo

• Use cases for mobile agents:

• Computation on a large data set
  • Impractical to move the data set to the user's host
  • So, move the computation (agent) to the data set's host

• Computation on a widely distributed data set
  • Move the computation (agent) from host to host, accumulating information from each host's data set
  • Eventually, the agent returns to the user's host with the answer
  • Or, reports the answer back to the user's agent

• Heavy computation on a limited device
  • Move the computation (agent) to a high-end compute server
  • "Limited device" = smartphone, e.g.
  • Depending on the computation, even a desktop PC might be a "limited device"

Other JADE Capabilities

• Lightweight Extensible Agent Platform (LEAP)
  • Run agents on Java 2 Micro Edition (J2ME) devices

• Fault-tolerant JADE platform
  • The main container is a single point of failure
  • Can run multiple main containers, one master, the others backup
  • The backup replicas are kept in sync with the master replica
  • If the master crashes, a new master is chosen from among the backups (leader election)
  • The Directory Facilitator (DF) can be persisted in a database and can be restored from there after a crash

• JADE Web Services Integration Gateway
  • Any JADE agent, when registered in the DF, can also be published as a web service
  • Any external web service can be called by a JADE agent

Distributed Systems 4005-730-01 Spring Quarter 2013

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Alan Kaminsky • Department of Computer Science • Rochester Institute of Technology • 4486 + 2220 = 6706

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Copyright © 2011 Alan Kaminsky. All rights reserved. Last updated 15-Oct-2011. Please send comments to ark­@­cs.rit.edu.