	CS 683 Emerging Technologies Spring Semester, 2003 IDL, RMI, CORBA, WSDL
	Previous Lecture Notes Index Next © 2003, All Rights Reserved, SDSU & Roger Whitney San Diego State University -- This page last updated 13-Mar-03

Contents of Doc 14, IDL, RMI, CORBA, WSDL

CS 683 Emerging Technologies

Spring Semester, 2003

Doc 14 IDL, RMI, CORBA, WSDLContents
References
Web Services Description Language (WSDL) 1.1 http://www.w3.org/TR/wsdl
Building Web Services with Java, Graham et all, Chapter 6

CS 696 Emerging Technologies: Distributed Objects Spring Semester, 1998, Lecture notes Doc 4 and Doc 19.

2003 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.


Doc 14, IDL, RMI, CORBA, WSDL Slide # 2

Some Basic Questions

How do I find a service?
Where is the service?
What does the service do?
How does one interact with the service?


Doc 14, IDL, RMI, CORBA, WSDL Slide # 3

The Broker Pattern


Doc 14, IDL, RMI, CORBA, WSDL Slide # 4

A broker

Handles the transmission of requests from clients to servers

Handles the transmission of responses from servers to clients

Must have some means to identify and locate server

If server is hosted by different broker, forwards the request to other broker

If server is inactive, active the server

Provides APIs for registering servers and invoking server methods

Bridge

Optional components used for hiding implementation details when two brokers interoperate


Doc 14, IDL, RMI, CORBA, WSDL Slide # 5

Dynamics

Registering Server


Doc 14, IDL, RMI, CORBA, WSDL Slide # 6

Client Server Interaction


Doc 14, IDL, RMI, CORBA, WSDL Slide # 7

Variants

Direct Communication Broker System

Broker gives the client a communication channel to the server

Client and server interact directly

Many CORBA implementation use this variant

Message Passing Broker System

Clients and servers pass messages rather than services (methods)

Trader System

Clients request a service, not a server

Broker forwards the request to a server that provides the service

Adapter Broker System

Hide the interface of the broker component to the servers using an additional layer

The adapter layer is responsible for registering servers and interacting with servers

For example if all server objects are on the same machine as application a special adapter could link the objects to the application directly


Doc 14, IDL, RMI, CORBA, WSDL Slide # 8

Callback Broker System

Eliminate the difference between clients and servers

When an event is registered with a broker, it calls the component that is registered to handle the event


Doc 14, IDL, RMI, CORBA, WSDL Slide # 9

Known Uses

CORBA

IBM's SOM/DSOM

Mircosoft OLE 2.x

RMI

Consequences

Benefits
Location Transparency
Clients (servers) do not care where servers (clients)are located

Changeability and extensibility of components
Changes to server implementations are transparent to clients if they don't change interfaces

Changes to internal broker implementation does not affect clients and servers

One can change communication mechanisms without changing client and server code

Portability of Broker System
Porting client & servers to a new system usually just requires recompiling the code


Doc 14, IDL, RMI, CORBA, WSDL Slide # 10

Benefits - Continued

Interoperability between different Broker System
Different broker systems may interoperate if they have a common protocol for the exchange of messages

DCOM and CORBA interoperate

DCOM and RMI interoperate

RMI and CORBA interoperate

Reusability
In building new clients you can reuse existing services


Doc 14, IDL, RMI, CORBA, WSDL Slide # 11

Liabilities
Restricted Efficiency

Lower fault tolerance compared to non-distributed software

Benefits and Liabilities
Testing and Debugging

A client application using tested services is easier to test than creating the software from scratch

Debugging a Broker system can be difficult


Doc 14, IDL, RMI, CORBA, WSDL Slide # 12

Some RMI

A First Program - Hello World
Modified from "Getting Started Using RMI"

The Remote Interface

public interface Hello extends java.rmi.Remote 
   {
   String sayHello() throws java.rmi.RemoteException;
   }


Doc 14, IDL, RMI, CORBA, WSDL Slide # 13

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();
      }
Doc 14, IDL, RMI, CORBA, WSDL Slide # 14

// 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";
         }
      }
      
Doc 14, IDL, RMI, CORBA, WSDL Slide # 15

// 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();
         }
      }
   }
Doc 14, IDL, RMI, CORBA, WSDL Slide # 16

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


Doc 14, IDL, RMI, CORBA, WSDL Slide # 17

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


Doc 14, IDL, RMI, CORBA, WSDL Slide # 18

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 &


Doc 14, IDL, RMI, CORBA, WSDL Slide # 19

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


Doc 14, IDL, RMI, CORBA, WSDL Slide # 20

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


Doc 14, IDL, RMI, CORBA, WSDL Slide # 21

Some Corba

A Simple CORBA Example using OrbixWeb

Step 1 Create an interface for the server using the CORBA interface definition language (IDL)

Place the following code in the file Hello.idl

interface Hello
   {
   string sayHello();
   };

Step 2 Compile the IDL interface

idl Hello.idl

This creates a directory called java_output which contains:

Hello.java	HelloPackage/	_HelloSkeleton.java
HelloHelper.java	_HelloImplBase.java	_HelloStub.java
HelloHolder.java	_HelloOperations.java	_tie_Hello.java


Doc 14, IDL, RMI, CORBA, WSDL Slide # 22

IDL output Hello

Client interface to server

_HelloStub

Client side proxy for server

_HelloSkeleton

Server side proxy

_HelloImplBase

Abstract class to use as parent class to server

HelloPackage

A Java package used to contain any IDL types nested in the Hello interface

HelloHelper

A Java class that allows IDL user-defined types to be manipulated in various ways

HelloHolder

Used for passing Hello objects as parameters


Doc 14, IDL, RMI, CORBA, WSDL Slide # 23

Step 3 Implementing the Server using Inheritance

Classes generated by OrbixWeb IDL compiler Hello

public interface Hello
    extends org.omg.CORBA.Object
{
    public String sayHello() ;
    public java.lang.Object _deref() ;
}
Doc 14, IDL, RMI, CORBA, WSDL Slide # 24

_HelloImplBase

import IE.Iona.OrbixWeb._OrbixWeb;
public abstract class _HelloImplBase 
    extends _HelloSkeleton 
    implements Hello 
{
    public _HelloImplBase() {
        org.omg.CORBA.ORB.init().connect(this);
    }
    public _HelloImplBase(String marker) {
        _OrbixWeb.ORB(org.omg.CORBA.ORB.init()).connect(this,marker);
    }
    public _HelloImplBase(IE.Iona.OrbixWeb.Features.LoaderClass loader) {
        _OrbixWeb.ORB(org.omg.CORBA.ORB.init()).connect(this,loader);
    }
    public _HelloImplBase(String marker,
                        IE.Iona.OrbixWeb.Features.LoaderClass loader) {
        _OrbixWeb.ORB(org.omg.CORBA.ORB.init()).connect(this,marker,loader);
    }
    public java.lang.Object _deref() {
        return this;
    }
}
Doc 14, IDL, RMI, CORBA, WSDL Slide # 25

Programmer Implemented ClassesHelloImplementation

public class HelloImplementation extends _HelloImplBase
   {
   public String sayHello()
      {
      return "Hello World";
      }
   }

HelloServer

import IE.Iona.OrbixWeb._CORBA;
import IE.Iona.OrbixWeb.CORBA.ORB;
public class HelloServer
   {
   public static void main (String args[])
      {
      org.omg.CORBA.ORB ord =
         org.omg.CORBA.ORB.init();
      
      try
         {
         Hello server = new HelloImplementation();
         _CORBA.Orbix.impl_is_ready( "HelloServer" );
         System.out.println("Server going Down");
         }
      catch ( org.omg.CORBA.SystemException corbaError)
         {
         System.out.println("Exception " + corbaError);
         }
      }
   }
Doc 14, IDL, RMI, CORBA, WSDL Slide # 26

Step 4 Compiling the Server

The classpath must include the following:

Java JDK classes
org.omg.CORBA package
IR.Iona.OrbixWeb package

Must compile the following classes:

Hello.java
_HelloSkeleton.java
_HelloImplBase.java
HelloImplementation.java
HelloServer.java


Doc 14, IDL, RMI, CORBA, WSDL Slide # 27

Step 5 Registering and running the Server

Make sure that the OrbixWeb daemon (orbixdj) is running on the server machine

You start the deamon by the command:

orbixdj -textConsole

Now register the server via:

putit HelloServer -java HelloServer

Now run the server via

java HelloServer

Note running the server is not normally required, however, since the server is not in a package it is hard to get the ORB to activate the server. We will address this issue later

Details of the above process will be discussed later


Doc 14, IDL, RMI, CORBA, WSDL Slide # 28

Step 6 Writing the client

HelloClient.java

import IE.Iona.OrbixWeb._CORBA;
import org.omg.CORBA.ORB;
public class HelloClient
   {
   public static void main(String args[])
      {
      ORB.init();
      
      String hostname = "eli.sdsu.edu";
      String serverLabel = ":HelloServer";
      Hello server = HelloHelper.bind( serverLabel, hostname);
      System.out.println( server.sayHello() );
      }
   }


Doc 14, IDL, RMI, CORBA, WSDL Slide # 29

Step 7 Compiling and Running the client

Compile the classes:

_HelloStub.java
HelloClient.java

Now run the client with the command:

java HelloClient

Output - Client Window

[New IIOP Connection (eli.sdsu.edu,IT_daemon, null,null,pid=0) ]
[New IIOP Connection (eli.sdsu.edu,HelloServer, null,null,pid=0) ]
Hello World

Output - Server Window

[ HelloServer: New Connection (eli.sdsu.edu:59201) ]
[ HelloServer: End of IIOP connection (eli.sdsu.edu:59201) ]

Output - Daemon Window

[ IT_daemon: New Connection (eli.sdsu.edu:59200) ]
[ IT_daemon: End of IIOP connection (eli.sdsu.edu:59200) ]


Doc 14, IDL, RMI, CORBA, WSDL Slide # 30

IDL
Interface Definition language

RMI

public interface Hello extends java.rmi.Remote 
   {
   String sayHello() throws java.rmi.RemoteException;
   }

CORBA

interface Hello
   {
   string sayHello();
   };


Doc 14, IDL, RMI, CORBA, WSDL Slide # 31

WSDL

WSDL Elements

Types

Container for data type definitions
XML schema defining types used

Message

Abstract, typed definition of the data being communicated
Defines the set of parameters used in method signatures

Operation

Abstract description of an action supported by the service
Set of messages

Port Type

Abstract set of operations supported by one or more endpoints
Each child operation defines an abstract method signature
Doc 14, IDL, RMI, CORBA, WSDL Slide # 32

Binding

A concrete protocol and data format specification for a particular port type
Details of how elements in portType are converted into concrete representation of data formats (XML) and protocols( http)

Port

A single endpoint defined as a combination of a binding and a network address

Service

A collection of related endpoints


Doc 14, IDL, RMI, CORBA, WSDL Slide # 33

Standard WSDL Namespaces

Namespace	URI	Definition
wsdl	http://schemas.xmlsoap.org/wsdl/	WSDL framework
soap	http://schemas.xmlsoap.org/wsdl/soap/	WSDL SOAP binding
http	http://schemas.xmlsoap.org/wsdl/http/	WSDL HTTP GET & POST binding
mime	http://schemas.xmlsoap.org/wsdl/mime/	WSDL MIME binding
soapenc	http://schemas.xmlsoap.org/soap/encoding/	SOAP 1.1Encoding
soapenv	http://schemas.xmlsoap.org/soap/envelope/	SOAP 1.1 Envelope
xsi	http://www.w3.org/2000/10/XMLSchema-instance	XSD Instance namespace
xsd	http://www.w3.org/2000/10/XMLSchema	Schema namespace
tns	Varies	This namespace


Doc 14, IDL, RMI, CORBA, WSDL Slide # 34

Example

<?xml version="1.0"?>
<definitions name="BabelFishService" 
   xmlns:tns="http://www.xmethods.net/sd/BabelFishService.wsdl" 
   targetNamespace="http://www.xmethods.net/sd/BabelFishService.wsdl" 
   xmlns:xsd="http://www.w3.org/1999/XMLSchema" 
   xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/" 
   xmlns="http://schemas.xmlsoap.org/wsdl/">
   <message name="BabelFishRequest">
      <part name="translationmode" type="xsd:string"/>
      <part name="sourcedata" type="xsd:string"/>
   </message>
   <message name="BabelFishResponse">
      <part name="return" type="xsd:string"/>
   </message>
   <portType name="BabelFishPortType">
      <operation name="BabelFish">
         <input message="tns:BabelFishRequest" name="BabelFish"/>
         <output message="tns:BabelFishResponse" 
            name="BabelFishResponse"/>
      </operation>
   </portType>
   <binding name="BabelFishBinding" type="tns:BabelFishPortType">
      <soap:binding style="rpc" 
         transport="http://schemas.xmlsoap.org/soap/http"/>
      <operation name="BabelFish">
         <soap:operation soapAction="urn:xmethodsBabelFish#BabelFish"/>
         <input>
            <soap:body use="encoded" namespace="urn:xmethodsBabelFish" 
            encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
         </input>
Doc 14, IDL, RMI, CORBA, WSDL Slide # 35
         <output>
            <soap:body use="encoded" namespace="urn:xmethodsBabelFish" 
            encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"/>
         </output>
      </operation>
   </binding>
   <service name="BabelFish">
      <documentation>
            Translates text of up to 5k in length, between a variety of languages.
      </documentation>
      <port name="BabelFishPort" binding="tns:BabelFishBinding">
         <soap:address 
            location="http://services.xmethods.net:80/perl/soaplite.cgi"/>
      </port>
   </service>
</definitions>

Previous visitors since 13-Mar-03 Next