CS 580 Client-Server Programming

Spring Semester, 2005

Doc 20 RPC

RPC    

XML-RPC    

Issues    

XmlRpc ServersJava Example    

RMI    

The Remote Interface    

The Server Implementation    

The Client Code    

Running The Example    

Server Side    

Client Side    

Proxies    

 

References

 

http://www.xmlrpc.com/ Main XML_RPC web site

 

http://davenet.userland.com/1998/07/14/xmlRpcForNewbies XML_RPC For Newbies

 

http://xml.apache.org/xmlrpc/ Home page for Java XML-RPC implementation

 

 

Copyright ©, All rights reserved. 2005 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA. OpenContent ( http://www.opencontent.org/opl.shtml ) license defines the copyright on this document.

 

 

 

RPC

Remote Procedure Call

 

A client can "directly" call a function or procedure on the server

 

 

Issues

 

1. Cross platform

Primitive data types may be different on client & server

 

2. Marshalling/unmarshalling of parameters and results

Procedure parameters must be sent from client to server

How can one handle pointers as parameters?

Result of procedure call must be sent back to client

 

3. Different contexts of client and server

 

4. Registering and finding servers

 

Sample Uses

 

Unix NFS (Network File System)

Unix license managers

 

RPC implementations

SUN RPC

Distributed Computing Environment (DCE)

 

 

 

XML-RPC

 

RPC using

5. HTTP as transport layer and

6. XML to encode request/response

7. Language and platform independent

 

Started by Userland ( http://frontier.userland.com/ ) in 1998

 

Languages/Systems with XML-RPC implementations

 

8. Java, Perl, Python, Tcl, C, C++, Smalltalk

9. ASP, PHP, AppleScript, COM

10. Zope, WebCrossing

 

 

Led to the development of SOAP

 

 

Java Example

import java.util.*;

import org.apache.xmlrpc.*;

 

public class XmlRpcExample

    {

     public static void main (String args[])

        {

         try

            {

            XmlRpcClient xmlrpc =   new XmlRpcClientLite

                        ( "http://xmlrpc.usefulinc.com/demo/server.php" );

            Vector parameters = new Vector ();

            parameters.addElement ( new Integer( 5 ) );

            parameters.addElement ( new Integer( 3 ) );

            Integer sum =

                    (Integer) xmlrpc.execute ( "examples.addtwo" ,

                                                    parameters);

            System.out.println( sum.intValue() );

            }

         catch (java.net.MalformedURLException badAddress)

            {

            badAddress.printStackTrace( System.out);

            }

         catch (java.io.IOException connectionProblem)

            {

            connectionProblem.printStackTrace( System.out);

            }

         catch (Exception serverProblem)

            {

            serverProblem.printStackTrace( System.out);

            }

        }

    }

 

 

Issues

 

Client program has to know

 

11. Server machine name or IP ( xmlrpc.usefulinc.com)

12. Path to server program (/demo/server.php)

13. Name of remote method (examples.addtwo)

14. Number, Type and Order of arguments

 

Supported Data Types

 

 

 

 

How do you know about methods on the Server?

 

The server:

15. url

16. method name

17. method arguments

 

Need to be documented some place

 

 

 

 

How does this work?

 

Client marshals (serialize) the rpc request

 

Converts the requests in to a format that can be sent on the network

 

Client

 

18. Sends the marshaled version to the server

19. Waits for server response

 

 

Server

 

20. Unmarshals the request,

21. Runs the requested method

22. Marshals the result

23. Send the marshaled result back to the client

 

 

Client unmarshals the result

 

 

Complete Request sent to Server

 

POST /demo/server.php HTTP/1.1

Host: xmlrpc.usefulinc.com

Content-length: 190

Content-type: text/xml;charset=iso-8859-1

User-Agent: Smalltalk XMLRPC version 0.5 (VisualWorksÆ NonCommercial, Release 7 of June 14, 2002)

Connection: keep-alive

 

<?xml version="1.0"?>

<methodCall>

    <methodName>examples.addtwo</methodName>

    <params>

        <param>

            <value><int>5</int></value>

        </param>

        <param>

            <value><int>3</int></value>

        </param>

    </params>

</methodCall>

 

 

XmlRpc ServersJava Example

 

The following starts an addtwo server on port 8080

Server URL is serverMachinename:8080

Method name is: examples.addtwo

 

How come the server is still running after the last println?

 

import org.apache.xmlrpc.*;

 

public class JavaServer

    {

     public Integer addtwo( int x, int y)

        {

         return new Integer( x + y);

        }

    

     public static void main( String[] args)

        {

         try

            {

            System.out.println( "Starting server on port 8080" );

            WebServer addTwoServer = new WebServer( 8080 );

            addTwoServer.addHandler( "examples" , new JavaServer());

            System.out.println( "server running" );

          }

         catch (Exception webServerStartError)

            {

            System.err.println( "JavaServer " +

                            webServerStartError.toString());

            }

        }

    }

 

 

Notice

 

We have not explicitly handled sockets in any example

 

 

 

RMI

 

Java's Remote Method Invocation

 

Allows easy communication between remote Java VMs

 

 

Hello World Example

 

Modified from "Getting Started Using RMI"

 

The Remote Interface

 

public interface Hello extends java.rmi.Remote

    {

    String sayHello() throws java.rmi.RemoteException;

    }

 

 

 

The Server Implementation

 

// Required for Remote Implementation

import java.rmi.*;

import java.rmi.server.UnicastRemoteObject;

 

// Used in method getUnixHostName

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStreamReader;

 

public class HelloServer

        extends UnicastRemoteObject

        implements Hello

    {

 

    public HelloServer() throws RemoteException

        {

        }

 

    // The actual remote sayHello

    public String sayHello() throws RemoteException

        {

        return  "Hello World from " + getUnixHostName();

        }

 

 

 

// Works only on UNIX machines

 

    protected String getUnixHostName()

        {

        try

            {

            Process  hostName;

            BufferedReader  answer;

 

            hostName = Runtime.getRuntime().exec( "hostname" );

            answer = new BufferedReader(

                                new InputStreamReader(

                                    hostName.getInputStream()) );

 

            hostName.waitFor();

            return answer.readLine().trim();

            }

        catch (Exception noName)

            {

            return "Nameless";

            }

        }

        

 

 

// Main that registers with Server with Registry

 

    public static void main(String args[])

        {

        // Create and install a security manager

        System.setSecurityManager(new RMISecurityManager());

 

        try

            {

            HelloServer serverObject = new HelloServer ();

 

            Naming.rebind("//roswell.sdsu.edu/HelloServer",

                                    serverObject );

 

            System.out.println("HelloServer bound in registry");

 

            }

        catch (Exception error)

            {

            System.out.println("HelloServer err: ");

            error.printStackTrace();

            }

        }

    }

 

 

The Client Code

 

import java.rmi.*;

import java.net.MalformedURLException;

 

public class HelloClient

    {

    public static void main(String args[])

        {

        try {

            Hello remote = (Hello) Naming.lookup(

                                        "//roswell.sdsu.edu/HelloServer");

 

            String message = remote.sayHello();

            System.out.println( message );

            }

        catch ( Exception error)

            {

            error.printStackTrace();

            }

        }

    }

 

Note the multiple catches are to illustrate which exceptions are thrown

 

 

Running The Example

Server Side

 

Step 1 . Compile the source code

 

Server side needs interface Hello and class HelloServer

 

javac Hello.java  HelloServer.java

 

 

Step 2 . Generate Stubs and Skeletons (to be explained later)

 

The rmi compiler generates the stubs and skeletons

 

rmic   HelloServer

 

This produces the files:

 

HelloServer_Skel.class

HelloServer_Stub.class

 

The Stub is used by the client and server

The Skel is used by the server

 

 

The normal command is:

 

rmic   fullClassname

 

 

 

 

Step 3 . Insure that the RMI Registry is running

 

For the default port number

 

rmiregistry &

 

For a specific port number

 

rmiregistry portNumber &

 

On a UNIX machine the rmiregistry will run in the background and will continue to run after you log out

 

This means you manually kill the rmiregistry

 

 

Step 4. Register the server object with the rmiregistry by running HelloServer.main()

 

 

java HelloServer &

 

 

 

 

Client Side

 

The client can be run on the same machine or a different machine than the server

 

Step 1 . Compile the source code

 

Client side needs interface Hello and class HelloClient

 

javac Hello.java HelloClient.java

 

Step 2. Make the HelloServer_Stub.class is available

 

Either copy the file from the server machine

 

or

 

Compile HelloServer.java on client machine and rum rmic

 

Step 3. Run the client code

 

java HelloClient

 

 

 

Proxies

How do HelloClient and HelloSever communicate?

 

 

 

 

Client talks to a Stub that relays the request to the server over a network

 

Server responds via a skeleton that relays the response to the Client