CS 696 Emerging Technologies: Distributed Objects

1. References
2. Introduction
   1. CORBA Architecture
   2. The Broker Pattern
      1. Dynamics
      2. Variants
      3. Known Uses
      4. Consequences
   3. A Simple CORBA Example using OrbixWeb

Doc 19, CORBA Intro Slide # 2

Object Management Group

Develops CORBA standards, but does not implement

See: http://www.omg.org for more information on OMG and CORBA

http://adams.patriot.net/~tvalesky/freecorba.html

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Provides an infrastructure allowing objects to interact, independent of the specific platforms and languages used to implement the objects.

Standardize the life-cycle management of objects

Services cover such items as

Creating objects

Controlling access to objects

Keeping track of relocated objects,

Controlling the relationship between styles of objects

Object Services provide for application consistency

Doc 19, CORBA Intro Slide # 5

Commercially known as CORBAfacilities

A set of generic application functions such as

printing

document management

database

electronic mail facilities

Include facilities for use over the Internet

Vertical areas that provide functionality of direct interest to end-users in particular application domains

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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

Optional components used for hiding implementation details when two brokers interoperate

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Broker gives the client a communication channel to the server

Client and server interact directly

Many CORBA implementation use this variant

Clients and servers pass messages rather than services (methods)

Clients request a service, not a server

Broker forwards the request to a server that provides the service

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

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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

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Clients (servers) do not care where servers (clients)are located

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

Porting client & servers to a new system usually just requires recompiling the code

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Different broker systems may interoperate if they have a common protocol for the exchange of messages

CORBA defines a general inter-ORB protocol (GIOP) for orb to orb communication

Internet inter--ORB protocol (IIOP) is the TCP/IP version of GIOP

DCOM and CORBA interoperate

DCOM and RMI interoperate

Soon RMI and CORBA will interoperate

In building new clients you can reuse existing services

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

Debugging a Broker system can be difficult

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interface Hello
   {
   string sayHello();
   };

idl Hello.idl

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Client interface to server

Client side proxy for server

Server side proxy

Abstract class to use as parent class to server

This is called the ImplBase approach

Java class which allows server to use composition (delegation) rather than having to inherit from _HelloImplBase

This is called the TIE approach

Used in the TIE approach

Java interface that maps the attributes and operations of the IDL definition to Java methods

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

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

Used for passing Hello objects as parameters

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public interface Hello
    extends org.omg.CORBA.Object
{
    public String sayHello() ;
    public java.lang.Object _deref() ;
}

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 19, CORBA Intro Slide # 18

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

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);
         }
      }
   }
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Java JDK classes

org.omg.CORBA package

IR.Iona.OrbixWeb package

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

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orbixdj -textConsole

putit HelloServer -java HelloServer

java HelloServer

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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 19, CORBA Intro Slide # 22

_HelloStub.java
HelloClient.java

[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

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

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

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