1. Developing Mobile Agent Systems
with Java
Prof. Dr. Jürgen Dunkel
Computer Science Department
Hannover University of Applied Science and Arts
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2. Overview
Overview 1. Mobile Agents Concept of Agent Mobility Benefits and Problems of Mobile Agents
2. Architecture of Mobile Agent Systems
3. The Jade-Framework
3.1. Overview
3.2. Creating JADE Agents
3.3. Agent Behaviours
3.4. Agent Communication
3.5. Yellow Pages Service
3.6. JADE Review
4. Conclusion
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3. how can agents use services in distributed systems ?
1. Mobile Agents
Mobile Agents
agents must live in distributed systems
because services and information are provided in computer networks
how can agents use services in distributed systems ?
A. Agents can use Interprocess Communication Mechanisms
Remote Procedure Calls (RPC), RMI, Corba, Web Services,MOM,…
B. Agents can be mobile
move autonomously across a network
transport program + state
continue execution at remote site
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4. Migration steps of a Mobile Agent:
1. Mobile Agents
Migration steps of a Mobile Agent:
interrupt processing of the agent
save state agent and provides authentication information
transfer agent code and state to server 2
Agent Environment
Server 1
Server 2
agent 1
recover agent state
resume execution of agent on server 2
agent cooperates with agents on server 2 and completes work
agent returns home to server 1
agent communicates the results of his work
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5. Benefits of Mobile Agents over RPC (1)
Mobile Agents replace expensive RPC with local calls
reducing number of remote interactions and transmitted data
Server 1
Client
Service
Server 2
Remote Procedure Calls:
Agent Environment
agent 1
Server 1
Server 2
Service
Mobile Agents:
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6. Benefits of Mobile Agents over RPC (2)
Mobile Agents: can work in spontaneous networks
clients must be connected to the network only long enough to
sending the agent on its way
receiving the agent
clients must not be connected when the agent carries out its task
asynchronous communication
Mobile Agents: are well suited for small devices
lack of computational power: agent moves to another host
Mobile Agents: can extend server functionality
Agent contains its own functionality
not completely dependent on the installed services
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7. Problems of Mobile Agents
Host Protection
host must trust in agents from another host
Agent platform realizes Java Sandbox Model
agents have only restricted access on system resources
authenticated agents can have more access rights
Agent Protection
private data, agent knowledge base
secure transport protocols (SSL)
encryption of private agent information
Interoperability of heterogenous agents
different server platforms
agents might be implemented with different agent frameworks
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8. 2. Architecture of Mobile Agents Systems
Standard Architectures of Mobile Agent Systems
FIPA (Foundation For Intelligent Physical Agents)
focus on interoperation of heterogenous agents
defines ACL (agent communication language)
ontologies for agents
no emphasis on mobility
JADE /LEAP, ZEUS, Grasshopper (
OMG /MASIF (Mobile Agent System Interoperability Facility)
focus on interoperability between agent systems
defines standard interfaces for
Agent Management: creation, suspension, resumption, termination
Mobility Infrastructure: communicate with and migrate to other agent systems
Naming Services: to find and locate agents (CORBA based)
Grashopper, Aglets
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9. 2. Architecture of Mobile Agents Systems
Architectural Components of a Multi Agent System
User-Agent-Interface
Agent Life Cycle Management
Execution Layer
Mobility
Communication
Persistence Services
Agent Security
Name Service
Host Security
Network
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10. 2. Architecture of Mobile Agents Systems
User-Agent-Interface
user interface to interact with the agent
user manages local and remote agents
creating, destroying agents
bidirectional communicating
sending data to the agent
presentation of agent results
Agent Life Cycle Managment
management of agent lifecycle
creation, suspension, resumption, termination
different kind of agents
system agents: manager agents, monitor agents, repository agents, interface agents..
participant agents: support cooperative processes (negotiation, mediation,..)
user agents: act on behalf of the user
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11. 2. Architecture of Mobile Agents Systems
Execution Layer
execution environment for agents
environment where agents live (and meet other agents)
contains the virtual machine for the agent execution
granting access to services
Mobility Layer
provides agent mobility
physical transport of agent code
classes are transported as part of the agent transfer protocol
classes are loaded on demand from a designated code base
weak mobility: agent restarts on the new host with its current data
strong mobility: agent continues execution according to its program counter
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12. 2. Architecture of Mobile Agents Systems
Communication Layer
responsible for agent collaboration
defines Agent Communication Language (ACL)
message content based on common vocabulary (ontology) necessary
Standards: FIPA/ACL, Knowledge Interchange Format (KIF), KQML
Message Delivery: 1. locate agent with naming service
2. transfer message to agent
asynchronous communication
Persistence Services Layer
provides agent persistence
Serialization/Deserialization of an agent state:
used for agent mobility
Checkpoint-Restore-Mechanism
avoids loss of agents in case of host or network errors
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13. 2. Architecture of Mobile Agents Systems
Agent Security
protect agent against malicious hosts
Cryptographic techniques
Trust-base computing: defining a trusted network
Name Service
provides name service
requires globally unique agent name
seeking Mobile Agents
Brute Force Method: searching in every agent system in the region
Logging: follows an agent is by its trail, (migrating agent leaves its next destination at each server)
Agent Registration: every agent registers its current location in a database
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14. 2. Architecture of Mobile Agents Systems
Host Security
protect host against malicious agents
limits resource consumption
verification (check agent code),
access control policies
use of authentication and authorisation mechanisms
Network Layer
responsible for agent transport
Agent Transport Protocol:
platform-independent
based on standard protocols (HTTP, RMI/IIOP,..
includes security mechanism (SSL)
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15. 3. The JADE Framework 3. The Jade Framework 3.1. Overview
3.2. Creating JADE Agents
3.3. Agent Behaviours
3.4. Agent Communication
3.5. Yellow Pages Service
3.6. JADE Review
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16. JADE JADE Components 3.1. JADE-Overview
Java Agent DEvelopment Framework
developed by TILAB (formerly CSELT)
FIPA compliant
includes LEAP framework for PDA, mobile phones,..
Middleware that facilitates development of multi-agent systems
fully implemented in Java
Open source: distributed under LPGL
JADE Components
runtime environment where JADE agents can live
class libraries for agent development
graphical tools for administrating and monitoring agents
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17. JADE Agent Platform 3.1. JADE-Overview
JADE Agent Platform: is a set of active containers
Container: Instance of the JADE Runtime Environment
Main Container: one in every JADE platform
all other containers must register in main container
containers might be distributed over different servers
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18. Main Container JADE Agents 3.1. JADE-Overview
each main container constitutes a different platform
contains two special agents
AMS (Agent Management System)
provides Naming Service
Life Cycle Management (e.g. can start or kill agents)
DF (Directory Facility)
Yellow Pages: seeking agents with a special service
JADE Agents
identified by a unique name
agents can communicate transparently regardless of their actual location
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19. Book Trading Example 3.1. JADE-Overview two different types of agents
BookSellerAgent
wants to sell books
books with different prices in his stock (can be inserted by a simple GUI)
BookBuyerAgent
wants to buy a book:
LookUp: BookBuyerAgent looks for all BookSellerAgents, who sell books
Call For Proposal: asks all BookSellers for the price of a book title
BookSellers send their offers
BookBuyerAgent sends an order to the BookSellerAgent with the lowest price
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20. HelloWorldAgent 3.2. Creating JADE Agents
import jade.core.Agent;
public class HelloAgent extends Agent {
protected void setup() {
System.out.println("Hallo! "+getAID().getName()+" is ready."); }
setup() is intended for agent initialization
is a hook-method, i.e. automatically invoked by the agent platform
each agent is identified by a unique identifier
instance of class jade.core.AID
is of the form e.g.
String nickname = “pepe”;
AID id = new AID(nickname, AID.ISLOCALNAME);
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21. BookBuyerAgent - Initialization
3.2. Creating JADE Agents
BookBuyerAgent - Initialization
public class BookBuyerAgent extends Agent {
private String targetBookTitle;
private AID[] sellerAgents = {new AID(“seller1”, AID.ISLOCALNAME), new AID(“seller2”, AID.ISLOCALNAME)};
protected void setup() {
System.out.println("Hallo! Buyer-agent "+getAID().getName()+" is ready.");
Object[] args = getArguments();
if (args != null && args.length > 0) {
targetBookTitle = (String) args[0];
System.out.println("Trying to buy "+targetBookTitle);
} else
System.out.println("No book title specified"); }
protected void takeDown() {
System.out.println("Buyer-agent "+getAID().getName()+" terminating.");
getArguments() reads arguments from the command line
takeDown(): invoked before an agent terminates and used for clean-up
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22. Agent Behaviour 3.3. Agent Behaviours 4 3 1 2 QUEUE
real job of an agent is carried out within behaviours
there are different Behaviour-classes in JADE
each Behaviour-class must implement action()-method
each Behaviour-class must implement done()-method:
if it yields true, the behaviour is removed from the agent’s behaviour pool
same agent can have different Behaviours:
an Behaviour is added to an agent by addBehaviour() –method
behaviours are scheduled co-operatively
agent sleeps if there is no behaviour available 4 3 1 2
QUEUE
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23. Behaviour Types 3.3. Agent Behaviours Generic Behaviour
One-shot Behaviour
action()-method is executed only once
WakerBehavior-class: executes a method after a given timeout
addBehaviour(new WakerBehaviour(this, 10000) {
protected void handleElapsedTimeout() { // perform operation X } }
Cyclic Behaviour
its action()-method is executed periodically in an endless loop
TickerBehavior-class: executes a method repetitively waiting a period
addBehaviour(new TickerBehaviour(this, 10000) {
protected void onTickt() { // perform operation Y }
Generic Behaviour
embeds a state
executes different operations depending on the state
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24. Behaviour for the Book Trading Example
3.3. Agent Behaviours
Behaviour for the Book Trading Example
Behaviours of BookBuyerAgent
TicketBehavior: only for scheduling periodically requests seller agents
protected void setup() { …
addBehaviour(new TickerBehaviour(this, 60000) {
protected void onTick() {
myAgent.addBehaviour(new RequestPerformer()); }
} ); …
RequestPerformer implements the real and complex behaviour
sends Call for Proposal to the Sellers
receives the proposals
sends the purchase order to the seller with best offer
receives the purchase order reply
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25. Behaviours of BookSellerAgent
3.3. Agent Behaviours
Behaviours of BookSellerAgent
two cyclic behaviours
OfferRequestsServer: serves a request for a offer
PurchaseOrdersServer: serves a purchase order
protected void setup() { …
// Create and show the GUI
myGui = new BookSellerGui(this);
myGui.show();
addBehaviour(new OfferRequestsServer() );
addBehaviour(new PurchaseOrdersServer() ); ....
OneShotBehaviour: updating the catalogue
/** is invoked by the GUI when the user adds a new book for sale*/
public void updateCatalogue(final String title, final int price) {
addBehaviour(new OneShotBehaviour() {
public void action() { catalogue.put(title, new Integer(price)); }
} ); }
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26. JADE Communication Mechanism
3.4. Agent Communication
JADE Communication Mechanism
asynchronous message passing
each agent has a private queue of ACL messages
other agents can post messages into the queue
agent is notified whenever a message arrives
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27. ACL Messages 3.4. Agent Communication FIPA message format:
sender of the messages
list of receivers
performative: communicative intention, e.g.
REQUEST: sender wants the receiver perform an action
INFORM: sender wants to inform the receiver
CFP: sender calls the receiver for a proposal
content of the message
content language: syntax of the message
ontology: vocabulary of the message
additional control fields (timeout,
ACLMessage msg = new ACLMessage(ACLMessage.INFORM);
msg.addReceiver(new AID(“Pepe”, AID.ISLOCALNAME));
msg.setLanguage(“English”); msg.setOntology(“Weather-forecast-ontology”);
msg.setContent(“Today it’s raining”);
send(msg);
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28. ACL Messages for the Book Trading Example
3.4. Agent Communication
ACL Messages for the Book Trading Example
BuyerAgent  SellerAgents
request of a book offer: CFP-message containing book title
ACLMessage cfp = new ACLMessage(ACLMessage.CFP);
for (int i = 0; i < sellerAgents.lenght; ++i) { cfp.addReceiver(sellerAgents[i]); } cfp.setContent(targetBookTitle);
myAgent.send(cfp);
SellerAgents  BuyerAgent
send offer to buyer: PROPOSE-message containing price
REFUSE-messsage, if requested book not in stock
BuyerAgent  SellerAgents
send accept of the offer to the cheapest seller: ACCEPT_PROPOSAL-message
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29. 3.4. Agent Communication ACL Message Processing – in Behaviours
OfferRequestsServer – BookSellerAgent sends offer to Buyer
private class OfferRequestsServer extends CyclicBehaviour {
public void action() {
ACLMessage msg = myAgent.receive();
if (msg != null) {
String title = msg.getContent();
ACLMessage reply = msg.createReply();
Integer price = (Integer) catalogue.get(title);
if (price != null) {
reply.setPerformative(ACLMessage.PROPOSE);
reply.setContent(String.valueOf(price.intValue())); }
else {
reply.setPerformative(ACLMessage.REFUSE);
reply.setContent(“not-available”);
myAgent.send(reply);
} block(); // agent thread is going asleep
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30. 3.4. Agent Communication Message Filtering
a behaviour is only interested certain message
MessageTemplates filter messages in the queue
public void action() {
MessageTemplate mt = MessageTemplate.MatchPerformative(ACLMessage.CFP);
ACLMessage msg = myAgent.receive(mt);
if (msg != null) {
// CFP Message received. Process it
... }
else {
block();
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31. Yellow Pages Service 3.5. Yellow Page Services
an agent wants to find other agents offering a certain service
E.g. BuyerAgent wants to discover dynamically all SellerAgents
Yellow Page Service
provided by DF (Directory Facilitator) agent, according to FIPA
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32. 3.5. Yellow Page Services Publishing a Service
providing a description of the service
AID, languages and ontologies an agent needs to know
service name, service type
protected void setup() { ...
// Register the book-selling service in the yellow pages
DFAgentDescription dfd = new DFAgentDescription();
dfd.setName(getAID());
ServiceDescription sd = new ServiceDescription();
sd.setType(“book-selling”);
sd.setName(“JADE-book-trading”);
dfd.addServices(sd);
try {
DFService.register(this, dfd); }
...
in the example no ontology, language,.. is specified
in takeDown()-method services should be deregistered
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33. 3.5. Agent Name Service Searching for Services
agent seeks all other agents offering a certain service
necessary information: AID, required languages and ontologies
public class BookBuyerAgent extends Agent {
private AID[] sellerAgents;
protected void setup() {…
// Update the list of seller agents
DFAgentDescription template = new DFAgentDescription();
ServiceDescription sd = new ServiceDescription();
sd.setType(“book-selling”);
template.addServices(sd);
try {
DFAgentDescription[] result = DFService.search(myAgent, template);
sellerAgents = new AID[result.length];
for (int i = 0; i < result.length; ++i) {
sellerAgents[i] = result.getName(); }
}….
AID[ ] contains the names of all agent, offering the “book-selling”-service
also publish-subscribe service available
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34. How much of FIPA is hidden by the JADE?
3.6. JADE Review
How much of FIPA is hidden by the JADE?
no need to implement the Agent Platform
AMS, and DF executed at start-up
no need to implement Message Transport and Parsing
automatically done by the framework when sending/receiving messages
no need to implement agent-management
an agent is registered with the AP within its constructor
simplified interface to access the services of the DF (registration, searching, …)
Interaction Protocols must only be extended via handle methods
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35. Conclusion 4. Conclusion Concept of Agent Mobility
necessary for multi-agent systems in a distributed world
benefits over interprocess communication mechanisms
problems of mobile agents
Software Architecture of a Mobile Multi Agent Systems
short description of the essential layers
Developing a multi agent system with JADE
behaviours
ACL messages
many open fields for further research
Software architecture of Mobile Multi Agent Systems: Integrating of new standards (Web Services, Semantic Web, Security Issues,..)
Agents on Small devices (LEAP)
Agents in adhoc networks
Cooperation and negotiation models,..
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36. Software Architecture of Multi Agent Systems
5. References
Agent Mobility
J.E. White: Mobile Agents; in: M. Bradshaw: Software Agents, MIT Press, 1997, pp
D. Miojicic: Trend Wars: Mobile Agent Applications, IEEE Concurrency, 1999.
Software Architecture of Multi Agent Systems
M. Schoeman, E. Cloete: Architectural Components of the Efficient Design of Mobile Agent Systems, Proceedings of SACISIT, 2003.
FIPA (Foundation of Intelligent Agents)
OMG: The Mobile Agent Facility Specification
JADE
JADE homepage:
JADE documentation
D. Grisham: JADE Administrative Tutorial, 5/2003
C. Giovanni: JADE Tutorial, JADE Programming for Beginners, 12/2003
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37. Example of complex behavior (1)
3.6. Agent Communication
Example of complex behavior (1)
private class RequestPerformer extends Behaviour {
private AID bestSeller; private int bestPrice; private int repliesCnt = 0; private MessageTemplate mt; private int step = 0;
public void action() {
switch (step) {
case 0: // Send the cfp to all sellers
ACLMessage cfp = new ACLMessage(ACLMessage.CFP);
for (int i = 0; i < sellerAgents.length; ++i) {
cfp.addReceiver(sellerAgents[i]); }
cfp.setContent(targetBookTitle);
cfp.setConversationId(“book-trade”);
cfp.setReplyWith(“cfp”+System.currentTimeMillis()); // Unique value
myAgent.send(cfp);
// Prepare the template to get proposals
mt = MessageTemplate.and(MessageTemplate.MatchConversationId(“book-trade”),
MessageTemplate.MatchInReplyTo(cfp.getReplyWith()));
step = 1;
break;
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38. Example of complex behavior (2)
3.6. Agent Communication
Example of complex behavior (2)
case 1:
// Receive all proposals/refusals from seller agents: conversation-ID = “book-trade”
ACLMessage reply = myAgent.receive(mt);
if (reply != null) {
if (reply.getPerformative() == ACLMessage.PROPOSE) { // offer received
int price = Integer.parseInt(reply.getContent());
if (bestSeller == null || price < bestPrice) {
bestPrice = price;
bestSeller = reply.getSender(); }
repliesCnt++;
if (repliesCnt >= sellerAgents.length) step = 2; // We received all replies
else {
block();
break;
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39. Example of complex behavior (3)
3.6. Agent Communication
Example of complex behavior (3)
case 2: // Send the purchase order to the seller that provided the best offer
ACLMessage order = new ACLMessage(ACLMessage.ACCEPT_PROPOSAL);
order.addReceiver(bestSeller);
order.setContent(targetBookTitle);
order.setConversationId(“book-trade”);
order.setReplyWith(“order”+System.currentTimeMillis());
myAgent.send(order);
mt = MessageTemplate.and(MessageTemplate.MatchConversationId(“book-trade”),
MessageTemplate.MatchInReplyTo(order.getReplyWith())); step = 3; break;
case 3: // Receive the purchase order reply
reply = myAgent.receive(mt);
if (reply != null) {
if (reply.getPerformative() == ACLMessage.INFORM) {
System.out.println(targetBookTitle+“ successfully purchased. Price = ”+bestPrice);
myAgent.doDelete(); }
step = 4; } else { block(); }
break; }
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