Cobra Case Study - Distributed Operating Systems - Lecture Slides, Slides of Computer Science

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CORBA Case Study

History

• The CORBA (Common Object Request Broker

Architecture) specification was developed in 1991

by the Object Management Group (OMG).

• The OMG was founded by eleven corporations to

develop CORBA.

• CORBA 2.0 specification (introduced in 1996)

permit communications between

implementations made by different developers.

Definitions

• CDR – Common Data Representation: A external data representation of all data types that can be used as arguments or return values in remote invocations.
• CORBA – Common Object Request Broker Architecture: An Object Request Broker (ORB) architecture specification for distributed object systems.
• GIOP – General Inter-ORB Protocol: CORBA 2.0 standard that enables different developer implementations to communicate with each other.
• IDL – Interface Definition Language: An interfacing programming language that that allow communication across a heterogeneous distributed system.

Definitions (cont.)

• IIOP – Internet Inter-ORB Protocol: Internet version of GIOP that use the TCP/IP protocol.
• Marshalling: The process of taking a collection of data items and assembling them into a form suitable for transmission in a message.
• Middleware: A software layer that provides a programming abstraction as well as masking the heterogeneity of the underlying networks, hardware, operating systems, and programming languages.
• OMG – Object Management Group: The organization that developed and maintains CORBA
• POS – Persistent Object Service: A persistent object store for CORBA where objects that live between processes activations can be stored.

CORBA RMI Features

• The CORBA RMI consists of the following main

components:

• An interface definition language (IDL)
• An architecture (discussed in Structure )
• The General Inter-ORB Protocol (GIOP)
• The Internet Inter-ORB Protocol (IIOP)

CORBA IDL Features

• Provides an interface consisting of a name and a

set of methods that a client can request.

• IDL supports fifteen primitive types, constructed

types and a special type called Object.

• Primitive types: short, long, unsigned short, unsigned long, float, double, char, boolean, octet , and any.
• Constructed types such as arrays and sequences must be defined using typedefs and passed by value.
• Interfaces and other IDL type definitions can be

grouped into logical units called modules.

CORBA Services

• Set of generic service specifications useful for

distributed applications. Each discussed in Chapter 17.3 and documentation provided @ OMG site.

• CORBA Naming Service - essential to any ORB
• CORBA Event Service - define inferfaces
• CORBA Notification Service - extention of event service
• CORBA Security Service - controls access
• CORBA Trading Service - allows locatation by attribute
• CORBA Transaction and Concurrency Control Docsity.com

CORBA RMI Structure

Client Server ProgramClient^ Proxy for A ORBcore ORBcore Objectadapter

(^) ServantA skeleton

Implementationrepository

repositoryInterface

Request Reply

Structure - Object Adapter

• Object Adapter (server)
  • Bridges the gap between CORBA objects and the programming language interfaces of the servant classes.
  • Creates remoter object references for the CORBA objects
  • Dispatches each RMI to the appropriate servant class via a skeleton, and activates objects.
  • Assigns a unique name to itself and each object
  • Called the Portable Object Adapter in CORBA 2.
    • processes can run on ORB’s produced by different Docsity.com

Structure - Skeletons and Proxies

• Skeletons (server)
  • An IDL compiler generates skeleton classes in the server’s language.
  • Dispatch RMI’s to the appropriate servant class.
• Client Proxies / Stubs
  • Generated by an IDL compiler in the client language.
  • A proxy class is created for object oriented languages
  • Stub procedures are created for procedural

How to use CORBA

• Server must include IDL interfaces in the form

of servant classes.

• An interface compiler generates:
  • the program(Java or C++) interfaces
  • server skeletons for each IDL interface
  • proxy classes (or client stubs) for each IDL interface
  • A Java / C++ class for IDL defined struct
  • helper classes for each IDL defined type

How to use CORBA (cont.)

• Server
  • Creates and initializes the ORB
  • Creates an instance of servant class, which is registered with the ORB. Servant class extends the corresponding skeleton class and implementation methods of an IDL interface.
  • Makes a CORBA object
• Client
  • Creates and initializes the ORB
  • Contacts Naming service to get reference to the server
  • Invokes methods on the server

Significant Points

• Harder to use than a single language RMI such as

JAVA.

• Compatible with various languages and operating

systems.

• CORBA’s IDL is the crucial part because it permits

communication on a heterogeneous distributed

system.

• The OMG consists of several companies that

maintain compatibility and standardization of the

CORBA system.

Summary

• The OMG developed CORBA for distributed

object systems that will work on a heterogeneous

distributed system.

• CORBA consists of many parts and services that

are useful for distributed applications.

• The IDL portion is the crucial part that makes

CORBA a language independent middleware.

• CORBA includes generic service specifications

useful in implementing distributed applications.