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Common Object Request Broker Architecture (CORBA)
CORBA is a middeware design that allows application programs to communicate with one another irrespective of their programming languages, their hardware and software platforms, the networks they communicate over and their implementors.
Applications are built from CORBA objects, which implement interfaces defined in
CORBA’s interface definition language, IDL. Clients access the methods in the IDL interfaces of
CORBA objects by means of RMI. The middleware component that supports RMI is called the Object Request Broker or ORB.
The OMG (Object Management Group) was formed in 1989 with a view to encouraging the adoption of distributed object systems in order to gain the benefits of object-oriented
programming for software development and to make use of distributed systems, which were becoming widespread. To achieve its aims, the OMG advocated the use of open systems based on standard object-oriented interfaces. These systems would be built from heterogeneous hardware, computer networks, operating systems and programming languages.
An important motivation was to allow distributed objects to be implemented in any programming language and to be able to communicate with one another. They therefore designed an interface language that was independent of any specific implementation language.
They introduced a metaphor, the object request broker(or ORB), whose role is to help a client to invoke a method on an object. This role involves locating the object, activating the object if necessary and then communicating the client’s request to the object, which carries it out and replies.
In 1991, a specification for an object request broker architecture known as CORBA (Common Object Request Broker Architecture) was agreed by a group of companies. This was followed in 1996 by the CORBA 2.0 specification, which defined standards enabling implementations made by different developers to communicate with one another. These standards are called the General Inter-ORB protocol or GIOP. It is intended that GIOP can be implemented over any transport layer with connections. The implementation of GIOP for the Internet uses the TCP protocol and is called the Internet Inter-ORB Protocol or IIOP [OMG 2004a]. CORBA 3 first appeared in late 1999 and a component model has been added recently.
The main components of CORBA’s language-independent RMI framework are the following:
An interface definition language known as IDL,
The GIOP defines an external data representation, called CDR. It also defines specific formats for the messages in a request-reply protocol. In addition to request and reply messages, it specifies messages for enquiring about the location of an object, for cancelling requests and for reporting errors.
The IIOP, an implementation of GIOP defines a standard form for remote object references,
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