CS 696 Emerging Technologies: Java Distributed Computing Spring Semester, 1999 Voyager Agent Basics
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References
Starting Voyager
Naming Service
RMI & Voyager
Modile Code

moveTo() Explained
Move Notification

Mobile Agents

Agent Example

HelloAgent

Voyager and Rohan

References

Doc 34, Voyager Agent Basics Slide # 2

Starting Voyager

addSystemListener(SystemListener listener) 
getSystemThreadGroup() 
getVersion() 
removeSystemListener(SystemListener listener) 
shutdown() 
startup() 
startup(java.lang.Object object, java.lang.String url) 
startup(java.lang.String url) 

shutdown(SystemEvent event) 
startup(SystemEvent event) 
Doc 34, Voyager Agent Basics Slide # 3

import com.objectspace.voyager.Voyager;
import com.objectspace.voyager.SystemEvent;
import com.objectspace.voyager.SystemListener;
public class StartupTest implements SystemListener
   {
   public static void main(String[] arguments )
      {
      try
         {
         System.out.println( "Start program" );
         Voyager.addSystemListener( new StartupTest() );
         System.out.println( "Call start up" );
         Voyager.startup( "tcp://rohan.sdsu.edu:6000" );
         System.out.println( "Call shut down" );
         Voyager.shutdown(  );
         System.out.println( "Good by" ); 
         }
      catch (Exception voyagerProblem )
         {
         voyagerProblem.printStackTrace();
         }
      }
   
   public void shutdown( SystemEvent goingDown )
      {
      System.out.println( "Going down" );
      }
   public void startup( SystemEvent starting )
      {
      System.out.println( "Starting up" );
      }
   }
Doc 34, Voyager Agent Basics Slide # 4

Doc 34, Voyager Agent Basics Slide # 5

Naming Service

bind(java.lang.String name, java.lang.Object object) 
lookup(java.lang.String name) 
rebind(java.lang.String name, java.lang.Object object) 
setServerURL(java.lang.Object obj)

Used only in lightweight clients (applets) to be able to use the applet server machine as a proxy for a naming service on a different machine.

unbind(java.lang.String name) 

RMI & Voyager

Ifoo aFooStub = 
   (Ifoo) Namespace.lookup( "rmi://eli.sdsu.edu:5645/myFoo" );

Namespace.bind( "rmi://eli.sdsu.edu:5645/myBar", aBarObject );
Doc 34, Voyager Agent Basics Slide # 6

Modile Code

Move the receiver to the JVM containing the specified object

Move the receiver to the JVM specified by the URL.

   public static void mobile() throws Exception
      {
      Voyager.startup();
      String serverClass =  "whitney.voyager.examples.hello.HelloImpl";
      Proxy aProxy =  Factory.create( serverClass);
      Hello helloProxy = (Hello) aProxy;
      System.out.println( helloProxy.sayHello());
      IMobility mover = Mobility.of( helloProxy );
      mover.moveTo( "//eli.sdsu.edu:8000" );
      System.out.println( helloProxy.sayHello());
      mover.moveTo( "//fargo.sdsu.edu:8000" );
      System.out.println( helloProxy.sayHello());
      Voyager.shutdown();
      }
Doc 34, Voyager Agent Basics Slide # 7

moveTo() Explained

Current messages to the object are allowed to complete

New messages to the object are suspended

The code can only detect method calls that are synchronized, so do not attempt to move an object that might be executing non-synchronized methods.

The object is serialized and sent to the new location

This means that if the object is not serializable an exception will be thrown. You may wish to consider using proxies in the objects fields to reduce serializable problems. It also means that all the fields of the object are serialized and sent also.

java.rmi.Remote and com.objectspace.voyager.IRemote are ignored

This means that a Voyager object that is also an RMI server can be moved.

The new addresses of the object and all of its non-transient parts are cached at the old location.

When a message sent via a proxy arrives at the old address of a moved object, a special exception containing the object’s new address is thrown back to the stale proxy. The proxy traps this exception, rebinds to the new address, and then resends the message to the updated address. If the program at the old location crashes before a stale proxy is updated, the stale proxy is unable to successfully rebind and a message sent via the proxy generates an ObjectNotFoundException.

If an exception occurs in this process, the old object is restored to its original condition, suspended messages are resumed, and the exception is rethrown wrapped in a MobilityException.

Doc 34, Voyager Agent Basics Slide # 8

Move Notification

At this point, the copy of the object has become the real object, the object at the source has become the stale object, and the move is deemed successful and cannot be aborted.

This method is executed on the original stale object at the source, and is typically defined to perform activities such as removing the stale object from persistence. Messages sent to the stale object via a proxy will be redirected to the new object, so postDeparture() should not utilize proxies to the original object or any of its facets. Note that because the user-supplied callback on the new object is executed using a fresh thread, it is possible for this postDeparture() to be executing concurrently with the user-supplied callback.

This method is executed on the copy of the object at the destination. If the method throws a MobilityException, the move is aborted and no more IMobile callbacks occur

preDeparture(String source, String destination) throws MobilityException

This method is executed on the original object at the source. If the method throws a MobilityException, the move is aborted and no more IMobile callbacks occur.

Doc 34, Voyager Agent Basics Slide # 9

Mobile Agents

static IAgent get(java.lang.Object object) 

If the specified object implements IAgent, return the object.

static IAgent of(java.lang.Object object) 

If the specified object implements IAgent, return the object.

getHome() 
getResourceLoader() 
isAutonomous() 
moveTo(Object destination) 
moveTo(Object destination,  String callback) 
moveTo(Object destination, String callback, Object[] args) 
moveTo(String destination) 
moveTo(String destination, String callback) 
moveTo(String destination, String callback, Object[] args) 
setAutonomous(boolean flag) 
setResourceLoader(IResourceLoader resourceLoader)
Doc 34, Voyager Agent Basics Slide # 10

Agent Example

package whitney.voyager.examples.agent;
public interface IHello
   {
   String sayHello();
   }
Doc 34, Voyager Agent Basics Slide # 11

HelloAgent

package whitney.voyager.examples.agent;
import java.net.InetAddress;
import com.objectspace.voyager.agent.*;
import com.objectspace.voyager.mobility.*;
import com.objectspace.voyager.*;
public class HelloAgent implements IHello, java.io.Serializable
   {
   private int count = 0;
   
   public  String sayHello() 
      {
      return  "Hello World from " + getHostName() + " count " +  count++;
      }
   public void move( String newAddress ) throws MobilityException
      {
      System.out.println( "Start move" );
      IAgent agentProxy = Agent.of( this );
      agentProxy.moveTo( newAddress, "movedDone" );
      }
   
   public void movedDone()
      {
      System.out.println( "I have moved" + sayHello() );
      IAgent agentProxy = Agent.of( this );
      
      //rohan host name problem forced this odd check to see if we are home
      if ( getHostName().startsWith( "local" );
         System.out.println( "I am home" );
      else
         try
            {
            System.out.println( "Going home" );
            move( agentProxy.getHome() );
            }
         catch (MobilityException moveProblem )
            {
            System.out.println( "Trouble moving" );
            moveProblem.printStackTrace();
            }
      }
Doc 34, Voyager Agent Basics Slide # 12

   public static void main( String[] arguments)
      {
      try 
         {
         Voyager.startup( "8000" );
         String serverClass =  "whitney.voyager.examples.agent.HelloAgent";
         HelloAgent bond = new HelloAgent();
         System.out.println( bond.sayHello());
         bond.move( "tcp://fargo.sdsu.edu:8000" );
         
         // No shutdown to make sure the orb will get the return agent
         } 
      catch (Exception e) 
         {
         System.out.println("HelloServer err: ");
         e.printStackTrace();
         }
      }
   }

   protected static String getHostName()
      {
      try
         {
         return InetAddress.getLocalHost().getHostName();
         }
      catch (java.net.UnknownHostException who)
         {
         return "Unknown";
         }
      }
Doc 34, Voyager Agent Basics Slide # 13

Hello World from localhost count 0
Start move
I have movedHello World from localhost count 2
I am home

I have movedHello World from fargo.sdsu.edu count 1
Going home
Start move

Doc 34, Voyager Agent Basics Slide # 14

Voyager and Rohan

voyager tcp://rohan.sdsu.edu

voyager tcp://rohan.sdsu.edu:portNumber

Voyager.startup( "tcp://rohan.sdsu.edu:6000" );

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