RMI / clustering / remote references

Question

We are currently in the process of designing the architecture for an application. We are considering using RMI as a mechanism of communication. The architecture is as such
          / App Server 1 \   / External Server 1
Web     /                \ /
Server                     X
          \ App Server 2 /   \ External Server 2
   DatabaseWhere App Server 1 and App Server 2 are stateless servers that provide exactly the same functionality. In fact the idea is that you can add more App Servers to increase performance as you wish. To keep the servers stateless, any state information is stored on the Database.
App Server 1 and App Server 2 can talk to either External Server 1 or External Server 2 to access remote functions.
So both External Server 1 and External Server 2 can provide the same Objects but calls relating to the same data may have to go back to the same External Server. (I.E. these external Servers are statefull)
This has raised several problems.
1/ If App Server 1 and App Server 2 both register it's remote Objects with the same Registry, how does the registry deal with this. Does it maintain two references or just the latest? Would using JNDI as the registry service help? Would the registry round-robin these references?
2/ Is there are way of storing stubs in a database. I.e App Server 1 requests an External Server remote object, External Server 1 is used, App Server 1 Stores this remote object to allow any App Server to continue the process. App Server 2 picks up the Remote object from the Database and carries on talking to External Server 1 through it.
Originally I was hoping to just bypass the whole Registry section and create my own lookup process. However I cannot see how this can be done. I tried looking to see how Sun actually do the Remote Binding in the Java Foundation Classes but it all seems to be done in the underlying sun.rmi classes...
any ideas?
Cheers
Ben

Answer 1

I've just found a solution to my last question. From this I should be able to create my own registry app that is contacted initially through this mechanism, and then any required exposed objects are passed to it using .exportObject() and serialisation. My registry app can then pick up objects of the same type and allow a round robin mechanism, etc...
import java.rmi.*;
import java.rmi.server.*;
import sun.rmi.server.UnicastServerRef;
import sun.rmi.transport.LiveRef;
public class MyServerImpl extends RemoteServer implements SimpleRMIInterface
    public static int SERVER_PORT = 1234; // the client should know it
    public static int SERVER_ID = 666; // the client should also know it
    public SimpleRMIImpl(String name) throws RemoteException
        LiveRef liveref = new LiveRef(new ObjID(SERVER_ID), SERVER_PORT);
        UnicastServerRef unicastserverref = new UnicastServerRef(liveref);
        ref = unicastserverref;
        unicastserverref.exportObject(this, null);
        // No need to do Naming.rebind(name, this);
... Interface methods implementation skipped
}Client implementation:
import java.rmi.server.*;
import sun.rmi.transport.tcp.TCPEndpoint;
import sun.rmi.transport.LiveRef;
import sun.rmi.server.UnicastRef;
public class SimpleRMIClient
    public static void main(String[] argv)
        try
            TCPEndpoint tcpendpoint = new TCPEndpoint("servername",1234); // I use port=1234 here
            LiveRef liveref = new LiveRef(new ObjID(666),    tcpendpoint,false);
            MyServerInterface myServerObject = (MyServerInterface) new MyServerImpl_Stub((RemoteRef) (new UnicastRef(liveref)));
            // No need to do Naming.lookup(serverName + "/" +
objectName);
        catch(Exception e)
            e.printStackTrace();
            System.exit(0);
            System.out.println("RMI connection successful");
}nice...
Ben