Multi-Agent Systems Mehdi Dastani
What is JADE? JADE (Java Agent DEvelopment framework) is a FIPA compliant agent platform and a Java framework for the development of MAS. The Jade platform is based on a middleware that facilitates the development of distributed multi-agent applications based on a peerto-peer communication architecture. The environment can evolve dynamically with agents that appear and disappear in the system according to the needs and the requirements of the context. JADE Project started in July 1998 as joint development of Telecom Italia Lab and Parma University. JADE is the Leading Open Source FIPA compliant agent platform. Currently JADE development is driven by a board composed of five industrial partners:tilab, Motorola, Whitestein Technologies AG, Profactor GmbH, and France Telecom R&D.
JADE An agent platform that implements the basic services and infrastructure of a distributed multi-agent application: agent life-cycle, agent mobility, and agent security white & yellow-page services peer-to-peer message transport & parsing; also multi-party communication scheduling of multiple agent tasks set of graphical tools to support monitoring, logging, and debugging Some relevant features: Distributed Agent Platform seen as a whole from the outside world spanning multiple machines enables interoperability through FIPA compliance Two levels concurrency model Inter-agent (pre-emptive, Java threads) Intra-agent (co-operative, Behaviour classes)
The architectural model A JADE-based application is composed of a collection of active components called Agents. Each agent has a unique name. Each agent is a peer since he can communicate in a bidirectional way with all other agents Each agent lives in a container (that provides its run time) and can migrate within the platform One container plays the role of main (where AMS, DF live)
Agent class and names A type of agent is created by extending the jade.core.agent class and redefining the setup() method. Each Agent instance is identified by an AID (jade.core.aid), composed of a unique name plus some addresses; the getaid() method of the Agent class Agent names are of the form <local-name>@<platform-name>. The complete name of an agent must be globally unique. The default platform name is <main-host>:<main-port>/jade. The platform name can be set using the name option Within a single JADE platform agents are referred through their names only. Given the name of an agent its AID can be created as AID id = new AID(localname, AID.ISLOCALNAME); AID id = new AID(name, AID.ISGUID);
Hello World: A Simple Example import jade.core.agent; public class HelloAgent extends Agent { protected void setup() { System.out.println("Hello World. "); System.out.println("My name is "+ getlocalname()); } } $> javac HelloAgent.java $> java jade.boot fred:helloagent This is JADE 3.0b1. Hello World. My name is fred
Starting Agent Execution Birth of a new agent The agent is given an identifier It is registered with the AMS It is put in the AP_ACTIVE state Its setup() method is executed The setup() method is therefore the point where any application-defined agent activity starts Initialise the agent Add tasks using the method addbehaviour() Scheduled as soon as the setup() method ends
The Behaviour class The actual job that an agent does is typically carried out within behaviours Behaviours are created by extending jade.core.behaviours.behaviour class To make an agent execute a task it is sufficient to create an instance of the corresponding Behaviour subclass and call the addbehaviour() method of the Agent class. Each Behaviour subclass must implement public void action(): what the behaviour actually does public boolean done(): Whether the behaviour is finished An agent can execute several behaviours in parallel, however, behaviour scheduling is not preemptive, but cooperative and everything occurs within a single Java Thread Behaviour switch occurs only when the action() method of the currently scheduled behaviour returns.
Hello World: A Simple Example import jade.core.agent; import jade.core.behaviours.*; public class myagent extends Agent { protected void setup() { addbehaviour( new mybehaviour( this ) ); } class mybehaviour extends SimpleBehaviour { public void action() { //... the real programming!! } private boolean finished = false; public boolean done() { return finished; } } // ----------- End mybehaviour } //end class myagent Hello World! My name is fred Hello World! My name is fred Hello World! My name is fred Hello World! My name is fred... loops until stopped with CTL-C...!
Hello World: A Simple Example import jade.core.agent; import jade.core.behaviours.*; public class myagent extends Agent { protected void setup() { addbehaviour( new mybehaviour( this ) ); } class mybehaviour extends SimpleBehaviour { int n=0; public void action() { ; n++; } public boolean done() { return n>=3; } } // ----------- End mybehaviour } //end class myagent Hello World! My name is fred Hello World! My name is fred Hello World! My name is fred
Behaviour types One shot behaviours. Cyclic behaviours. Complex behaviours. WakerBehaviour The action() and done() method are already implemented so that the handleelapsedtimeout() method (to be implemented by subclasses) is executed after a given timeout After that execution the behaviour completes. TickerBehaviour The action() and done() method are already implemented so that the ontick() (to be implemented by subclasses) method is executed periodically with a given period The behaviour runs forever unless its stop() method is executed
WakerBehaviour: A Simple Example public class MyAgent extends Agent { protected void setup() { System.out.println( Adding waker behaviour ); addbehaviour(new WakerBehaviour(this, 10000) { protected void handleelapsedtimeout() { // perform operation X } } ); } } Operation X is performed 10 seconds after the Adding waker behaviour printout appears.
TickerBehaviour: A Simple Example public class tickeragent extends Agent { Behaviour loop; protected void setup() { } } loop = new TickerBehaviour( this, 10000 ) { protected void ontick() { // perform operation X } }; addbehaviour( loop ); Operation X is performed periodically every 10 seconds.
Stopping Agent Execution Any behaviour can call the Agent.doDelete() method in order to stop agent execution. The Agent.takeDown() method is executed when the agent is going to be destroyed The agent is still registered with the AMS and can therefore send messages to other agents The takedown() method can be overridden to implement any necessary cleanup. Just after the takedown() method is completed, the agent will be de-registered and its thread destroyed
More about behaviours The onstart() method of the Behaviour class is invoked only once before the first execution of the action() method. The onend() method of the Behaviour class is invoked only once after the done() method returns true. The removebehaviour() method of the Agent class can be used to remove a behaviour from the agent pool of behaviours. The onend() method is not called. When the pool of active behaviours of an agent is empty, the agent enters the IDLE state and its thread goes to sleep
ACLMessage Based on asynchronous message passing Message format defined by the ACL language (FIPA) Messages exchanged by agents are instances of the jade.lang.acl.aclmessage Sending a message = creating an ACLMessage object and calling the send() method of the Agent class Reading messages from the private message queue is accomplished through the receive() method Provide accessor methods to get and set all the fields defined by the ACL language : get/setperformative(), get/setsender(), add/getallreceiver(), get/setlanguage(), get/setontology(), get/setcontent()
Blocking or selecting The block() method of the Behaviour class removes a behaviour from the agent pool of behaviours and puts it in a blocked state. Each time a message is received all blocked behaviours are inserted back in the agent pool and have a chance to read and process the message. It is possible to read only messages with certain characteristics specifying a jade.lang.acl.messagetemplate parameter in the receive() method. The Agent class also provides the blockingreceive() method; there are overloaded versions that accept a MessageTemplate and/or a timeout.
Blocking or selecting public void action() { ACLMessage msg = myagent.receive(); if (msg!= null) { // Message received. Process it... } else { block(); } }
FIPA ACL Message Elements p e r f o r m a t i v e What action the message performs s e n d e r Initiator of the message r e c e i v e r Recipient of the message r e p l y - t o Recipient of the message reply c o n t e n t Content of the message l a n g u a g e Language used to express content e n c o d i n g Encoding used for content o n t o l o g y Ontology context for content p r o t o c o l Protocol message belongs to c o n v e r s a t i o n - i d Conversation message belongs to r e p l y - w i t h Reply with this expression i n - r e p l y - t o Action to which this is a reply r e p l y - b y Time to receive reply by
ACL Message Example (request :sender (:name dominicagent@whitestein.com:8080) :receiver (:name rexhotel@tcp://hotelrex.com:6600) :ontology personal-travel-assistant :language FIPA-SL :protocol fipa-request :content (action movenpickhotel@tcp://movenpick.com:6600 (book-hotel (:arrival 25/11/2000) (:departure 05/12/2000)... ))) Any language can be used as a Content Language, e.g.: KIF, Prolog, SQL, Serialized Objects, Binary Large Objects FIPA-SL, FIPA-CCL, FIPA-RDF, FIPA-KIF
Sending/receiving Message ACLMessage msg = receive(); ACLMessage reply = new ACLMessage( ACLMessage.INFORM ); reply.setcontent( "Pong" ); reply.addreceiver( msg.getsender() ); send(reply); public void action() { ACLMessage msg = receive(); if (msg!=null) {...; ACLMessage reply = msg.createreply(); reply.setperformative(aclmessage.inform ); reply.setcontent(" Pong" ); reply.send(); } block(); }
DF (Directory Facilitator)
Interacting with the DF Agent The DF is an agent, it communicates using ACL The ontology and language that the DF understands are specified by FIPA The jade.domain.dfservice class provides static utility methods that facilitate the interactions with the DF register(); modify(); deregister(); search(); The JADE DF also supports a subscription mechanism
DFDescription format When an agent registers with the DF it must provide a description The agent AID A collection of service descriptions (class ServiceDescription): The service type (e.g. Weather forecast ) The service name (e.g. Meteo-1 ) The languages, ontologies and interaction protocols that must be known to exploit the service A collection of service-specific properties in the form key-value pair When an agent searches/subscribes to the DF it must specify another DFAgentDescription that is used as a template
Interacting with the DF Agent protected void setup() {... DFAgentDescription dfd = new DFAgentDescription(); dfd.setname(getaid()); ServiceDescription sd = new ServiceDescription(); sd.settype( book-selling ); sd.setname( JADE-book-trading ); dfd.addservices(sd); DFService.register(this, dfd);... } protected void takedown() { DFService.deregister(this); }
Agent Management System The authority in a JADE platform; all platform management actions (creating/killing agents, killing containers...) Other agents can request the AMS to perform these actions by using The fipa-request interaction protocol The SL language The JADE-Management ontology and related actions getams() => the AID of the AMS
The main graphical tools of JADE Management, control, monitoring, and debugging of a multi-agent platform RMA (Remote Monitoring Agent) Dummy Agent Sniffer Agent Introspector Agent Log Manager Agent DF (Directory Facilitator) GUI
Advantages No need to implement the Agent Platform AMS, DF executed at start-up No need to implement agent-management ontology and functionalities An agent is registered with the Agent Platform within its constructor, it is given a name and an address The DFService class provides a simplified interface to access the services of the DF (registration, searching, lease-renewal, ) No need to implement Message Transport and Parsing Automatically (and possibly efficiently) done by the framework when sending/receiving messages Interaction Protocols must only be extended via handle methods Standard FIPA!