1.2 CORBA Approach to Client/Server Development

The CORBA model provides a more flexible approach to developing distributed applications. The CORBA model:

• Formally separates the client and server portions of the application

  A CORBA client application knows only how to ask for something to be done, and a CORBA server application knows only how to accomplish a task that a client application has requested it to do. Because of this separation, developers can change the way a server accomplishes a task without affecting how the client application asks for the server application to accomplish the task.

• Logically separates an application into objects that can perform certain tasks, called operations

  CORBA is based on the distributed object computing model, which combines the concepts of distributed computing (client and server) and object-oriented computing (based on objects and operations).

  In object-oriented computing, objects are the entities that make up the application, and operations are the tasks that a server can perform on those objects. For example, a banking application could have objects for customer accounts, and operations for depositing, withdrawing, and viewing the balance in the accounts.

• Provides data marshaling to send and receive data with remote or local machine applications

  For example, the CORBA model automatically formats for big or little endian as needed. (Refer to the preceding section for a description of data marshaling.)

• Hides network protocol interfaces from the applications

  The CORBA model handles all network interfaces. The applications see only objects. The applications can run on different machines and, because all the network interface code is handled by the ORB, the application does not require any network-related changes if it is later deployed on a machine that supports a different network protocol.

The CORBA model allows client applications to make requests to server applications, and to receive responses from them without direct knowledge of the information source or its location. In a CORBA environment, applications do not need to include network and operating system information to communicate; instead, client and server applications communicate with the Object Request Broker (ORB). The following figure shows the ORB in a client/server environment.

Figure 1-2 The Object Request Broker (ORB)

CORBA defines the ORB as an intermediary between client and server applications. The ORB delivers client requests to the appropriate server applications and returns the server responses to the requesting client application. Using an ORB, a client application can request a service without knowing the location of the server application or how the server application will fulfill the request.

In the CORBA model, client applications need to know only what requests they can make and how to make the requests; they do not need to be coded with any implementation details of the server or of the data formats. Server applications need only know how to fulfill the requests, not how to return data to the client application.

This means that programmers can change the way a server application accomplishes a task without affecting how the client application asks for the server application to accomplish that task. For example, as long as the interfaces between the client and the server applications do not change, programmers can evolve and create new implementations of a server application without changing the client application; in addition, they can create new client applications without changing the server applications.