Common Object Request Broker Architecture

(CORBA)

By:

Sunil Gopinath

David Watkins

Introduction

• What is the purpose / goals of CORBA?

• How to meet goals?

• Example

• Conclusion

What is the purpose / goals of CORBA?

• Enable the building of plug and play component software environment

• Enable the development of portable, object oriented, interoperable code that is hardware, operating system, network, and programming language independent

How to meet goals?

• Interface Definition Language (IDL)

• Object Request Broker (ORB)

Interface Definition Language (IDL)

• Language Independence

• Defines Object Interfaces

• Hides underlying object implementation

• Language mappings exist for C, C++, Java, Cobol, Smalltalk, and Ada

Interface Definition Language (IDL)

module <identifier>{interface <identifier> [:inheritance]

{<type declarations>;<constant declarations>;<exception declarations>;<attribute declarations>;

[<op_type>] <identifier>(<parameters>)

[raises exception][context];}

}

Defines a CORBA object

Defines a method

Defines a container (namespace)

IDL Compiler

IDL Definitions

IDL Compiler

Stubs Skeletons

1. Define objects using IDL

2. Run IDL file through IDL compiler

3. Compiler uses language mappings to generate programming language specific stubs and skeletons

Object Request Broker (ORB)

• What is it?

• Architecture

• Request Handling

• Scenario

• CORBA Services

• CORBA Facilities

Application

What is it?

• Implementation of CORBA specification

• Middleware product

• Conceptual Software Bus

• Hides location and

implementation details

about objects

Hardware

OS Drivers

Middleware

Client / Object Interaction

Client Obj Impl

IDL IDL

ORB

Client Obj Impl

IDL IDL

ORB

Network

TC

P/I

P

OS

I

AT

M

IPX

TC

P/I

P

OS

I

AT

M

IPX

ORB Architecture

Interface Repository

IDL Compiler

Implementation Repository

Client Object (servant)

ORB Core

IDL Skeleton

DSIORB

InterfaceIDL Stub

DII

OBJ Ref

GIOP/IOOP

Object Adapter

Interface Repository

Interface Repository

Object Adapter

• Database of object definitions• Contains metadata about each object• Allows for introspection• Allows clients to discover interfaces at run-time• Used in support of dynamic invocations

IDL Compiler

IDL Compiler

Object Adapter

• Compiles IDL definition into stubs and skeletons

• Uses OMG specified language mappings to translate IDL into a language specific implementation

Implementation Repository

Implementation Repository

Object Adapter

• Contains information that allows the ORB to locate and activate object implementations

• Provides information about the classes a server supports, the objects that are instantiated, and their IDs

ORB Core

ORB CoreGIOP/IOOP

Object Adapter

• Provides mechanism for transparently communicating client requests to target object implementations

• Makes client requests appear to be local procedure calls

• GIOP – General Inter-ORB Protocol• IIOP – Internet Inter-ORB Protocol

ORB Interface

Client Object (servant)

ORB Interface Object Adapter

• Provides helper functions• Converting object

references to strings• Creating argument

lists for requests made through DII

IDL Stub

• Static invocation interface (SII)

• Marshals application data into a common packet-level representation– Network byte order

(little-endian or big- endian)

– Size of data types

Client

IDL Stub

IDL Skeleton

Object (servant)

IDL Skeleton

• Demarshals the packet-level representation back into typed data that is meaningful to an application– Network byte order

(little-endian or big- endian)

– Size of data types

Dynamic Invocation Interface

Client

DIIObject Adapter

• Dynamically issue requests to objects without requiring IDL stubs to be linked in

• Clients discover interfaces at run-time and learn how to call them

Steps:1. Obtain interface name

2. Obtain method description (from interface repository)

3. Create argument list

4. Create request

5. Invoke request

Dynamic Skeleton Interface

Object (servant)

DSI

Object Adapter

• Server side analogue to DII• Allows an ORB to deliver

requests to an object implementation that does not have compile-time knowledge of the type of object it is implementing

Object Adapter

Object (servant)

IDL Skeleton

DSI

Object Adapter

• Accept requests for service on behalf of the server’s objects

• Demultiplexes requests to the correct servant

• Dispatches the appropriate operation upcall on the servant

• Registers classes it supports and their run-time instances with the implementation repository

• Portable Object Adapter (POA)

• policies control object behavior (ie. LifespanPolicy)

• Instance of the adapter design pattern

Object Reference

Client Object (servant)OBJ Ref

Object Adapter

• Interoperable Object Reference (IOR)• Uniquely identifies each object• Contents

• Type Name (repository ID)• Protocol and Address Details• Object Key (object adaptor name, object name)

Request Handling

ORB POA

Server Application

Servants

POA

POA

Incoming Request

Scenario

Interface Repository

IDL Compiler

Implementation Repository

Client Object (servant)OBJ Ref

ORB CoreGIOP/IOOP

IDL Stub

DIIObject Adapter

ORB Interface

IDL Skeleton

DSI

CORBA Services

• Provide basic infrastructure functionality• Currently there are 15 defined services

– Naming - maps human names to object references (White Pages)

– Event - provides both a push and pull event model

– Object Trader - discover objects based on the services they provide (Yellow Pages)

– Transactions – allows distributed objects to participate in atomic transactions

CORBA Facilities

• Provide higher-level functionality at the application level

• Provide standard components that can be used “off-the-shelf”

• Two Categories– Horizontal – user interface, information management,

systems management, and task management

– Vertical – domain based, telecommunications, financial services

Example

Conclusion

• Distributed object, component architecture

• Real world examples of design patterns– TAO – freeware ORB

• Adapter, Factory, Reactor, Strategy

• Plugable Transports

• CORBA Beans

References

• www.omg.org• www.cs.wustl.edu/~schmidt/corba.html• http://www.infosys.tuwien.ac.at/Research/

Corba/OMG/arch2.htm• Communications of the ACM, October 1998• Jeri Edwards, Dan Harkey, and Robert Orfali.

Instant CORBA. New York: Wiley Computer Publishing, 1997.