Common Object Request Broker Architecture CORBA

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Common Object Request Broker Architecture (CORBA)

• Joe Komar

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CORBA Overview

• Standard distributed object architecture
• Developed by the Object Management Group (OMG)
  consortium
• Open software bus (Object Request Broker(ORB))
• CORBA objects can invoke one another without
  knowing where the objects reside and in what
  language they are implemented

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Interface Definition Language

• Defines interfaces to CORBA objects
• Mappings to Java, C, C, Smalltalk, COBOL, and
  Ada
• Java IDL
• ORB with JDK 1.2
• Allows access to CORBA objects from Java
• Compliant with CORBA/IIOP 2.0 spec and the
  IDL-to-Java Mapping

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Overview of CORBA
Client
Server
Stubs
Skeletons
IIOP
ORB
ORB
Method Request
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CORBA Concepts

• Client obtains an object reference
• Reference points to a stub
• Stub uses the ORB mechanism to forward method
  invocations
• Skeleton code transforms the call and parameters
  to implementation specific format and calls the
  object
• Results returned via the same mechanisms

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Interface Definition Language

• All CORBA objects support an IDL interface
• Interface defines the object type
• Interface can inherit from one or more interfaces
• Syntax is very similar to Java or C
• IDL is mapped to specific languages to provide
  access from that language
• idltojava compiler translates IDL to Java --
  interface, stub, skeleton, and others as needed

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IDL Example
Module HelloApp interface Hello
string sayHello()
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Java Specific CORBA Server

• Write IDL (.idl file)
• Use idltojava compiler and generate .java files
• Write server code to implement interface
• Server implementation compiled and linked to
  other files and the ORB library
• Define a factory object interface and implement
  it for each object type (Only a server may create
  CORBA objects)

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Java Specific Client

• Clients create objects through the factory
• Obtain object references
• from a factory object -- e.g., invoke a
  create method which returns the object
  reference
• from the nameservice
• from a string specifically created from an object
  reference
• Reference is then narrowed using a helper class
  generated by idltojava compiler

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Transient Name Server

• Java IDL Transient Nameservice
• Type tnameserv at the operating system prompt
• Stores object references by name in a tree
  structure
• Clients lookup or resolve object references by
  name
• All names are lost when tnameserv stops running

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Hello World IDL
Module HelloApp interface Hello
string sayHello()
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Idltojava Hello.idl

• Generates the following files in the HelloApp
  subdirectory
• _HelloImplBase.java - server skeleton
• _HelloStub.java - client stub
• Hello.java - Java version of interface (extends
  org.omg.Corba.Object)
• HelloHelper.java - narrow() method
• HelloHolder.java - public instance member of type
  Hello. Provides operations for out and inout
  arguments needed by CORBA

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HelloServer.java

• Implements all the methods of the interface
• main() method must
• Create an ORB instance
• Create a servant instance and tells the ORB about
  it
• Gets a CORBA object reference for a naming
  context
• Registers the new object in the naming context
• Waits for invocations of the new object

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HelloServer.java code
import HelloApp. import org.omg.CosNaming. imp
ort org.omg.CosNaming.NamingContextPackage. impo
rt org.omg.CORBA. class HelloServant extends
_HelloImplBase public String sayHello()
return "\nHello world !!\n"
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HelloServer Code
public class HelloServer public static
void main(String args) try
// create and initialize the ORB
ORB orb ORB.init(args, null) //
create servant and register it with the ORB
HelloServant helloRef new
HelloServant() orb.connect(helloRef)
// get the root naming context
org.omg.CORBA.Object objRef
orb.resolve_initial_references("NameService")
NamingContext ncRef
NamingContextHelper.narrow(objRef)
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HelloServer Code
// bind the Object Reference in Naming
NameComponent nc new NameComponent("Hello",
"") NameComponent path nc
ncRef.rebind(path, helloRef)
// wait for invocations from clients
java.lang.Object sync new java.lang.Object()
synchronized (sync)
sync.wait() catch
(Exception e) System.err.println("ER
ROR " e) e.printStackTrace(System.
out)
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HelloClient.java Implementation

• Creates an ORB
• Obtains a reference to the naming context
• Looks up Hello in the naming context and
  receives a reference to that CORBA object
• Invokes the objects method and prints results

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HelloClient Code
import HelloApp. import org.omg.CosNaming. imp
ort org.omg.CORBA. public class HelloClient
public static void main(String args)

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HelloClient Code
try // create and initialize the ORB
ORB orb ORB.init(args, null)
// get the root naming context
org.omg.CORBA.Object objRef orb.resolve_initial_
references("NameService")
NamingContext ncRef NamingContextHelper.narrow(o
bjRef) // resolve the Object
Reference in Naming NameComponent nc
new NameComponent("Hello", "")
NameComponent path nc Hello
helloRef HelloHelper.narrow(ncRef.resolve(path))

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HelloClient Code
// call the Hello server object and
print results String hello
helloRef.sayHello()
System.out.println(hello) catch
(Exception e) System.out.println("ER
ROR " e) e.printStackTrace(Syste
m.out)
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Building and Running the Example

• idltojava Hello.idl -- to created files
• java .java HelloApp\.java -- to compile all
  programs
• tnameserv -ORBInitialPort 1050 -- to run the
  name server
• java HelloServer -ORBInitialPort 1050 -- to start
  the server
• java HelloClient -ORBInitialPort 1050 -- to run
  the client

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Client/Server Summary

• CORBA is the primary way to link Java to objects
  written in other languages
• RMI can also be used if its all Java
• DCOM is the primary competitor to CORBA, but runs
  on limited platforms
• Sockets are too low level for enterprise use
• CGI is very slow, with servlets being
  substantially faster, but still slow