CS 696 Emerging Technologies: Distributed Objects

Design Patterns: Elements of Reusable Object-Oriented Software, Gamma, Helm, Johnson, Vlissides, 1995, pp 207-218

Doc 18, Some RMI Patterns Slide # 2

A client should be able to use a service independent of the location of the service provider (server)

The code implementing the functional core of a client should be separate from the code use to establish a connection with the server

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When a client needs to connection to a server on a remote machine:

The client connections to its local dispatcher

The local dispatcher connects to the remote dispatcher

The remote dispatcher returns the server communication channel to the local dispatcher

The local dispatcher returns to server communication channel to the client

The client now uses the communication channel to interact with the remote server

The client manages all the communication with the server including opening the communication channel

Doc 18, Some RMI Patterns Slide # 6

Sockets

Pipes

The dispatcher looks up all servers that provide that service and opens a communication channel to one of those servers

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• Exchangeability of servers

Servers can be added or modified without modifications to the dispatcher or clients

• Location and migration transparency

Servers can be dynamically migrated to other machines as long as a client is not connected to the server

• Re-configuration

Decisions about where and which servers are part of the system can be decided have start-up time or later

• Fault Tolerance

A new server can be activated on a different machine when there is a network or server failure

• Dispatcher overhead

• Sensitivity to change in the interfaces of the dispatcher

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  Doc 18, Some RMI Patterns Slide # 8

  Know Uses

Java's RMI

OMG Corba

Doc 18, Some RMI Patterns Slide # 9

Use common interface (or abstract class) for both the proxy and original object

Proxy contains a reference to original object, so proxy can forward requests to the original object

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The actual object is on a remote machine (remote address space)

Hide real details of accessing the object

Used in CORBA, Java RMI

public class HelloClient  {
     public static void main(String args[]) {
          try {
               String server = getHelloHostAddress( args);
               Hello proxy = (Hello) Naming.lookup( server );
               String message = proxy.sayHello();
               System.out.println( message );
          } 
          catch ( Exception error) 
               {  error.printStackTrace(); }
     }

Doc 18, Some RMI Patterns Slide # 12

Creates/accesses expensive objects on demand

You may wish to delay creating an expensive object until it is really accessed

It may be too expensive to keep entire state of the object in memory at one time

Provides different objects different level of access to original object

Multiple local clients can share results from expensive operations: remote accesses or long computations

Protect local clients from outside world

Doc 18, Some RMI Patterns Slide # 13

Synchronize multiple accesses to real subject

public class Table {     
     public Object elementAt( int row, int column )
          { blah     }

     public void setElementAt(Object element, int row, int column )
          { blah}
}
     
public class RowLockTable {
     Table realTable;
     Integer[] locks;
     
     public RowLockTable( Table toLock) {
          realTable = toLock;
          locks = new String[ toLock.numberOfRows() ];
          for (int row = 0; row< toLock.numberOfRows(); row++ )
               locks[row] = new Integer(row);
          }
     
     public Object elementAt( int row, int column ) {
          synchronized ( locks[row] ) {
               return realTable.elementAt( row, column);
          }
     }

     public void setElementAt(Object element, int row,
                                              int column ){
          synchronized ( locks[row] )     {
               return realTable.setElementAt(element, row,
                                                   column);
          }
     }
}

Doc 18, Some RMI Patterns Slide # 14

Delete original object when there are no references to it

Prevent accidental deletion of real subject

Collect usage statistics

Sample use is making C++ pointer safe

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