Mumbai University > Information Technology > Sem6 > Distributed System

Marks: 5M

Year: Dec 2015

• CORBA is a standard for Distributed Object Middleware defined by OMG (Object Management Group).
• Conformance to the CORBA’s specifications enables development of heterogeneous application across all major hardware platforms and operating system.
• OMG provides specification to encapsulate application data and business logic within objects called Distributed Components.

• The Major Components of CORBA are as follows:

  • ORB( Object Request Broker):
  1. It provides the communication and activation infrastructure for distributed object applications.
  2. To make a request, client specifies the target object by using an Object Reference (OR).
  3. When a CORBA Object is created, an OR is also created.
  4. It is unique, immutable and opaque.

  5. ORB Facilities communication transparency in terms of:

     I. Object Location: The client has no knowledge of whether the target object is in process or out-process on same machine or different machine.

     II. Object Implementation: Here, the client does not know the language platform or hardware in which object is implemented.

     III. Object Execution State: Here, the client does not know whether the object is active or in-active. ORB transparently activates object when required by client.

     IV. Object Communication Mechanism: ORB can use memory, RPC & TCP/IP for communication.

  • OMG Interface Definition Language (OMG-IDL):

    • An object i/ifs defines the requests that can be made by the object.
    • It is a declarative language which forces clear separation of i/ifs from object implementation.

  • Stubs & Skeletons:

    • A client side stub is a mechanism that effectively creates an issue request on behalf of a client.
    • While server side skeleton is a mechanism that delivers request to the CORBA object implementation.
    • As they are i/f-specific dispatching through stubs and skeletons is called Static Inovocation.

  • IDL Compiler:

    • It generates static stubs and skeletons SI & SSI (Static Invocation/Skeletons from ilf definition)
    • It allows object invocation through SSI & SII.
    • IFR (Interface Repository):
    • It allows system to be accessed and written programmatically at runtime so it is used in dynamic object invocation.
    • DII (Dynamic I/f Invocation ):
    • It allows client to generate request at run time.
    • DLL allows an ORB to deliver request to servants that have no compile-time knowledge of i/f it accesses.

  • Object Adapter(QA):

    • It serves as a Glue between CORBA Object Implementation and ORB.
    • It is an object that add up if of an object to the i/f expected by a caller.

There are two types of QA :

• BOA(Basic QA):

  • BOA defines an object adapter which can be used for most conventional object implementation.
  • POA (Portable QA)
  • POA specifications provide portability for CORBA Server code.

• Inter-ORB Protocol:

  To support interoperability CORBA processes :

• GIOP(General Inter-operable Protocol):

  It defines common mechanism of communication between ORB’s in general forms. It describes CDR (Common Character Data Representation) and message formats between ORB’s.

• IIOP(Internet Interoperable Protocol)

  IIOP provides full duplex, connection-oriented communication channel through TCP/IP Protocol.

  It gives client an ability to access that object while hiding details of that object implementation and status.

  • ORB if:
  • To decouple applications from implementations details if is defined.
  • Thus provides standard operations to initialize and shutdown ORB, convert OR to string and back.
  • IMR (Implementation Repository)
  • It contains information that allows an ORB to activate servers on demand.
  • The IMR maintains table that associates table name with start command.