CS 696 Emerging Technologies: Java Distributed Computing Spring Semester, 1999 Agents & Voyager
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References
What is an Agent
Why Mobile Agents
Some Java-based Mobile Agents Toolkits
Hello World Example

Simple Example

Hello Interface
HelloImplmentation
Client Side Startup
Running the Example

Using Naming Service
Mobile Code Example

References

Doc 31, Agents & Voyager Slide # 2

What is an Agent

• Ongoing Execution

Agents run for a long time

• Autonomy

Agents do not require human supervision to function

• Character or Anthropomorphism

Agents have personality, and exhibit human-like qualities

• Adaptive

Agents adapt to their environment and customize themselves to the preferences of their user

• Agent awareness

Agents can communicate and interact with other agents

• Mobile

Agents can move around on the network

Doc 31, Agents & Voyager Slide # 3

Doc 31, Agents & Voyager Slide # 4

Why Mobile Agents

Distributed systems often require many messages to achieve a task. With agents you send to where the work needs to be done. The agent then works locally.

Since the agent is working locally on the task, it does not have to deal with network latency.

Distributed systems use protocols define how messages and data are transferred. To modify the protocol requires changing the code on all the machines in the system. With agents, the protocol is just accept an agent and let is work. So creating a new agent can create new functionality.

Mobile devices are often not continuously connected to a network. Systems that require open connections will not work on mobile devices. With agents, a mobile device can connect to the network to check for work/messages. An agent can be sent to the device and work even after the device disconnects. The agent can wait until the device is reconnected to report the result of its task.

Agents can distribute themselves around on machines on the network to best solve the task at hand.

If a host is being shut down, agent can move on to another host to continue to operate.

Doc 31, Agents & Voyager Slide # 5

Some Java-based Mobile Agents Toolkits

Doc 31, Agents & Voyager Slide # 6

No need for a class to implement java.rmi.Remote. All classes can be used remotely

Client can create a server object remotely.

Voyager orb (or daemon) has a build-in http server.

This feature allows you to add secondary objects (termed facets) to a primary object at runtime. For example, you can dynamically add hobbies to an employee, a repair history to a car, or a payment record to a customer.

There is full native support for IDL, IIOP, and bidirectional IDL<->Java translation. No stub generators or helper classes are required. Full support for RMI. Soon it will support DCOM

You can move any serializable object between programs at runtime. If a message is sent from a proxy to an object’s old location, the proxy is automatically updated with the new location and the message is resent.

The activation framework allows objects to be persisted to any kind of database and automatically re-activated in the case that the program is restarted. An object does not have to be modified in any way to be activatable.

Voyager-enabled applets and servlets are supported

Voyager orb (or daemon) has a build-in naming service

You can multicast a Java message to a distributed group of objects without requiring the sender or receiver to be modified in any way.

You can publish a Java event on a specified topic to a distributed group of subscribers. The publish-subscribe facility supports server-side filtering and wildcard matching of topics.

You can send oneway, sync, and future messages. Oneway messages return immediately and discard the return value. Future messages immediately return a placeholder to the result, which may then be polled or read in a blocking fashion.

Doc 31, Agents & Voyager Slide # 7

Hello World Example

Simple Example

Hello Interface

package whitney.voyager.examples.hello;
public interface Hello
   {
   String sayHello();
   }

HelloImplmentation

package whitney.voyager.examples.hello;
import java.net.InetAddress;
public class HelloImpl implements Hello
   {
   private int count = 0;
   
   public String sayHello() 
      {
      return  "Hello World from " + getHostName() + " count " + count++;
      }
   protected static String getHostName()
      {
      try
         {
         return InetAddress.getLocalHost().getHostName();
         }
      catch (java.net.UnknownHostException who)
         {
         return "Unknown";
         }
      }
   }
Doc 31, Agents & Voyager Slide # 8

Client Side Startup

package whitney.voyager.examples.hello;
import com.objectspace.voyager.Factory;
import com.objectspace.voyager.Voyager;
public class HelloUser
   {
   public static void main(String args[])
      {
      try
         {
         // Start the Voyager client
         Voyager.startup();
         String serverClass =  "whitney.voyager.examples.hello.HelloServer";
         // Have the remote Voyager system create the server class
         // return a proxy. Assumes Voyager server is using port 8000 on fargo
         Hello proxy = (Hello) Factory.create( serverClass , "//fargo.sdsu.edu:8000");
         // Send a remote message to the remote object. Print return message
         System.out.println( proxy.sayHello());
         //Shutdown the Voyager client
         Voyager.shutdown();
         }
      catch (Exception voyagerProblem)
         {
         voyagerProblem.printStackTrace();
         }
      }
   }
Doc 31, Agents & Voyager Slide # 9

Running the Example

• Start Voyager

Use the command:

or if you want voyager to select a port use:

voyager

then voyager will print out the port selected

Make sure that the classpath of voyager contains the classes to be run by the client

• Compile the server side classes

The server side needs the classes Hello and HelloImpl. Compile them with javac

• Compile the classes

The clients side needs the classes Hello, HelloImpl, and HelloUser. Compile them with javac

• Run HelloUser

java HelloUser runs the program

Doc 31, Agents & Voyager Slide # 10

Using Naming Service

package whitney.voyager.examples.hello;
import com.objectspace.voyager.Factory;
import com.objectspace.voyager.Voyager;
import com.objectspace.voyager.Namespace;
public class Register
   {
   public static void main(String args[])
      {
      try
         {
         Voyager.startup();
         String serverClass =  "whitney.voyager.examples.hello.HelloImpl";
         Hello proxy = (Hello) Factory.create( serverClass , "//fargo.sdsu.edu:8000");
         Namespace.bind( "//fargo.sdsu.edu:8000/Greeter", proxy );
         Voyager.shutdown();
         }
      catch (Exception voyagerProblem)
         {
         voyagerProblem.printStackTrace();
         }
      }
   }
Doc 31, Agents & Voyager Slide # 11

package whitney.voyager.examples.hello;
import com.objectspace.voyager.Voyager;
import com.objectspace.voyager.Namespace;

public class HelloUserLookup
   {
   public static void main(String args[])
      {
      try
         {
         Voyager.startup();
         Hello proxy = (Hello) 
            Namespace.lookup( "//fargo.sdsu.edu:8000/Greeter");
      
         System.out.println( proxy.sayHello());
         Voyager.shutdown();
         }
      catch (Exception voyagerProblem)
         {
         voyagerProblem.printStackTrace();
         }
      }
Doc 31, Agents & Voyager Slide # 12

Mobile Code Example

package whitney.voyager.examples.hello;
public interface Hello
   {
   String sayHello();
   }

package whitney.voyager.examples.hello;
import java.net.InetAddress;
public class HelloImpl implements Hello, java.io.Serializable
   {
   private int count = 0;
   
   public String sayHello() 
      {
      return  "Hello World from " + getHostName() + " count " + count++;
      }
   protected static String getHostName()
      {
      try
         {
         return InetAddress.getLocalHost().getHostName();
         }
      catch (java.net.UnknownHostException who)
         {
         return "Unknown";
         }
      }
   }
Doc 31, Agents & Voyager Slide # 13

package whitney.voyager.examples.hello;
import com.objectspace.voyager.Factory;
import com.objectspace.voyager.Voyager;
import com.objectspace.voyager.Proxy;
import com.objectspace.voyager.mobility.Mobility;
import com.objectspace.voyager.mobility.IMobility;
public class HelloMover
   {
   public static void main(String args[])
      {
      try 
         {
         // Start local Voyager system
         Voyager.startup();
         String serverClass =  "whitney.voyager.examples.hello.HelloImpl";
         // Create object locally
         Proxy aProxy =  Factory.create( serverClass);
         Hello helloProxy = (Hello) aProxy;
         System.out.println( helloProxy.sayHello());
         // Create the mover
         IMobility mover = Mobility.of( helloProxy );
         // Move the object and reference it
         mover.moveTo( "//fargo.sdsu.edu:8000" );
         System.out.println( helloProxy.sayHello());
         // Move the object and reference it again
         mover.moveTo( "//eli.sdsu.edu:8000" );
         System.out.println( helloProxy.sayHello());
         // Destroy the object and shutdown
         Voyager.shutdown();
         } 
      catch (Exception e) 
         {
         e.printStackTrace();
         }
      }
   }
Doc 31, Agents & Voyager Slide # 14

Hello World from cx137265-a.fed1.sdca.home.com count 0
Hello World from fargo.sdsu.edu count 1
Hello World from eli.sdsu.edu count 2

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