Distributed Programming - CORBA

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Distributed Programming - CORBA

• Marc Conrad
• D104 (Park Square Building)
• Marc.Conrad_at_luton.ac.uk
• See also www.perisic.com for CORBA links.

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The Common Object Request Broker Architecture

CORBA
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Today

• Overview
• Acronyms (CORBA, ORB, OMG)
• One more acronym (IDL)
• An extensive example of an IDL to Java mapping
  (part one)

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The Common Object Request Broker Architecture

• CORBA is not a programming language but an
  architecture.
• CORBA is an elegant and sophisticated way to
  describe and link the use of objects in a
  distributed environment.
• The "dirty work", the implementation of the
  behavior of the objects is out of the scope of
  CORBA.

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The Common Object Request Broker Architecture

• CORBA is in the context of Object Oriented
  Programming.

BUT The objects can be located on different
machines all over the world and implemented in a
variety of languages!
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The Common Object Request Broker Architecture

• The Object Request Broker (ORB) is itself an
  object which provides the services for accessing
  remote objects and transmitting data.
• Note that the implementation of the ORB depends
  on the specific vendor. The CORBA standard only
  defines the interface.

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The Common Object Request Broker Architecture
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The Common Object Request Broker Architecture

• CORBA is a standard which has been devloped by
  the OMG, the Object Management Group.
• The OMG is the world's largest computer industry
  consortium with over 800 members.
• See http//cgi.omg.org/cgi-bin/membersearch.pl

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The History of the OMG

• Founded 1989 by eleven companies as non profit
  organisation.
• Aims on standards for object oriented software
  production.
• Other projects
• MDA http//www.omg.org/mda/
• UML http//www.omg.org/uml/
• CWM http//www.omg.org/cwm/

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OMG technology

• The framework within which all OMG adopted
  technology fits is the OMA the Object
  Management Architecture.
• So, learning CORBA is also on learning about
  Object Oriented technologies from a language
  independent view.

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OMG goals

• The goals of the OMG are promotion of the
  object-oriented approach to software engineering
  and development of a common architectural
  framework for writing distributed object-oriented
  applications based on interface specifications
  for the object in the application.

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OMG - structure

• See http//www.omg.org/news/about/omg_technology_p
  lenary.htm
• Platform Technology Committee
• This committee is concerned with infrastructure
  issues, e.g. the ORB.
• Domain Technology Committee
• This committee is concerned with technologies to
  support application development,e.g. electronic
  commerce.

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The Common Object Request Broker Architecture

• CORBA is a standard which has been developed by
  the OMG, the Object Management Group (OMG).
• The OMG is the world's largest computer industry
  consortium with over 800 members.
• See http//cgi.omg.org/cgi-bin/membersearch.pl

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The Common Object Request Broker Architecture

• But Common also means something else
• CORBA is a architecture which has the power to
  integrate legacy code written in languages as
  Java, C, C, Smalltalk, COBOL, Lisp, Python.

We focus here mainly on Java
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A Collection Of Rather Boring Acronyms.

CORBA
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A Collection Of Rather Bulky Acronyms.

• OMG - Object Management Group
• ORB - Object Request Broker
• IDL - Interface Definition Language
• IOR - Interoperable Object Reference
• POA - Portable Object Adapter
• IIOP - Internet Inter-ORB Protocol

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The Interface Definition Language (IDL)

• The IDL provides a class definition for objects
  similar to a C header file or a Java interface.
• However IDL does not provide an implementation.

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The CORBA Development Process

• Write some IDL that describes the interfaces to
  the object or objects that will be used or
  implemented.
• Compile the IDL file.
• Identify the IDL compiler generated classes and
  interfaces.
• Write additional code.
• Run the application.

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IDL and Java

• Warning!
• For simplicity we discuss in this lecture only
  the IDL to Java mapping, that means using an IDL
  for producing Java classes which are used in a
  Java environment.
• Other languages which are similarly supported are
  C, Lisp, Smalltalk, ...

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IDL Overview.
(diagram Java Programming with CORBA, OMG press,
page 143)
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IDL Example

• module Example
• interface Hello
• string sayHello()

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IDL Example

• module Example
• interface Hello
• string sayHello()

This is an example of an IDL which provides one
method, namely hello(). This code is saved in a
file hello.idl
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Understanding the IDL

• OMG IDL is a purely declarative language designed
  for specifying programming-language-independent
  operational interfaces for distributed
  applications. OMG specifies a mapping from IDL to
  several different programming languages,
  including C, C, Smalltalk, COBOL, Ada, and
  Java. When mapped, each statement in OMG IDL is
  translated to a corresponding statement in the
  programming language of choice.
• from http//java.sun.com/j2se/1.3/docs/guide/idl/
  tutorial/GSIDL.html

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Understanding the IDL

• You can use the tool idlj (provided by SUN) to
  map an IDL interface to Java and implement the
  client class. When you map the same IDL to C
  and implement the server in that language, the
  Java client and C server interoperate through
  the ORB as though they were written in the same
  language.
• from http//java.sun.com/j2se/1.3/docs/guide/idl/
  tutorial/GSIDL.html

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IDL Example

• module Example
• interface Hello
• string sayHello()

A module is a namespace similar to C namespaces
or Java packages It acts as a container for
related interfaces and declarations.
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IDL Example

• module Example
• interface Hello
• string sayHello()

Declares the application interface of the class.
Similar to C header files or Java interfaces.
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IDL Example

• module Example
• interface Hello
• string sayHello()

An operation. Operations are mapped to (Java,C,
...) methods.
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Mapping Hello.idl to Java(Here with idlj)

• Go to a command line prompt and type in the
  following command idlj -fall
  Hello.idl

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Mapping Hello.idl to Java(Here with idlj)
The name of your Compiler. The idlj is offered as
part of the JDK 1.3. Other vendors use other
compiler, e.g. idl2java by VisiBroker

• Go to a command line prompt and type in the
  following command idlj -fall
  Hello.idl

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Mapping Hello.idl to Java(Here with idlj)

• Go to a command line prompt and type in the
  following command idlj -fall
  Hello.idl

The name of the IDL file you have specified.
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Mapping Hello.idl to Java(Here with idlj)
This option (specific to idlj) means that files
should be generated both for a server and a
client implementation. (Default Client only)

• Go to a command line prompt and type in the
  following command idlj -fall
  Hello.idl

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Mapping Hello.idl to Java(Here with idlj)

• The command idlj -fall Hello.idl produces 6
  files

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Mapping Hello.idl to Java(Here with idlj)

• The command idlj -fall Hello.idl produces 6
  files

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Mapping Hello.idl to Java(Here with idlj)

• The command idlj -fall Hello.idl produces 6
  files
• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

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Mapping Hello.idl to Java(Here with idlj)

• This abstract class is the server skeleton,
  providing basic CORBA functionality for the
  server. It implements the Hello.java interface.
  The server class HelloServant extends
  _HelloImplBase.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

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Mapping Hello.idl to Java(Here with idlj)

• This class is the client stub, providing CORBA
  functionality for the client. It implements the
  Hello.java interface

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

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Mapping Hello.idl to Java(Here with idlj)

• This signature interface contains the Java
  version of our IDL interface. The Hello.java
  interface extends org.omg.CORBA.Object, providing
  standard CORBA object functionality.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

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Mapping Hello.idl to Java(Here with idlj)

• This operations interface contains the single
  method sayHello(). The IDL-to-Java mapping puts
  all of the operations defined on the IDL
  interface into this file.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

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Mapping Hello.idl to Java(Here with idlj)

• This final class provides auxiliary
  functionality, notably the narrow() method
  required to cast CORBA object references to their
  proper types.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java

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Mapping Hello.idl to Java(Here with idlj)

• This final class holds a public instance member
  of type Hello. It provides operations for out and
  inout arguments, which CORBA allows, but which do
  not map easily to Java's semantics.
• Note that CORBA allows call-by-value method
  parameters which are not directly available in
  Java.

• _HelloImplBase.java
• _HelloStub.java
• Hello.java
• HelloHelper.java
• HelloHolder.java
• HelloOperations.java