The MPJ slave

The implementation class associated with the MPJSlave interface normally behaves as follows (schematically):

  public class MPJSlaveImpl extends UnicastRemoteObject {
    public static void main(String args []) {
      int slaveID       = Integer.parseInt(args [0]) ;
      String masterPort = args [1] ;

      MPJService master =
              (MPJService) Naming.lookup("rmi://localhost:" + args [1] +
                                         "/MPJService") ;

      master.addSlave(int slaveID, int new MPJSlaveImpl()) ;
    }

    public runTask(MPJApplication task, String [] args, ...)
                                  throws RemoteException {

      ... create default communicator, `world'

      task.main(world, args) ;
    }
  }

The main method creates a remote object and ``registers'' it with its daemon by calling a remote method addSlave on the master. Later the client calls back with the runTask method, passing an instance of the actual user class. Because this is a serializable object it is passed by value to the remote runTask method. Importantly, the byte code for the user class will be loaded by RMIClassLoader from the code-base specified in the serialized object. As discussed below, this will be the URL of a process serving a (typically very much stripped-down) subset of the HTTP protocol.

Hence, using the dynamic class-loading mechanisms provided as standard in RMI, we ensure that all user code is automatically available to the remote host.