SEC. 9.1 CORBA 495
Application objects |
Vertical facilities( domain specific) |
Horizontal facilities( general purpose) |
Common Object Services |
Object Request Broker
Figure 9-1. The global architecture of CORBA.
model also distinguishes what are known as CORBA facilities. Facilities are constructed as compositions of CORBA services( which we discuss below), and are split into two different groups. Horizontal facilities consist of generalpurpose high-level services that are independent of application domains. Such services currently include those for user interfaces, information management, system management, and task management( which is used to define workflow systems). Vertical facilities consist of high-level services that are targeted to a specific application domain such as electronic commerce, banking, manufacturing, etc.
We will not discuss application objects and CORBA facilities in any detail, but rather concentrate on the basic services and the ORB.
Object Model
CORBA uses the remote-object model that we described in Chap. 2. In this model, the implementation of an object resides in the address space of a server. It is interesting to note that the CORBA specifications never explicitly state that objects should be implemented only as remote objects. However, virtually all CORBA systems support only this model. In addition, the specifications often suggest that distributed objects in CORBA are actually remote objects. Later, when discussing the Globe object model, we show how a completely different model of an object could, in principle, be equally well supported by CORBA.
Objects and services are specified in the CORBA Interface Definition Language( IDL). CORBA IDL is similar to other interface definition languages in that it provides a precise syntax for expressing methods and their parameters. It is not possible to describe semantics in CORBA IDL. An interface is a collection of methods, and objects specify which interfaces they implement.
Interface specifications can be given only by means of IDL. As we shall see later, in systems such as Distributed COM and Globe, interfaces are specified at a lower level in the form of tables. These so-called binary interfaces are by their nature independent of any programming language. In CORBA, however, it is necessary to provide exact rules concerning the mapping of IDL specifications to existing programming languages. At present, such rules have been given for a number of languages, including C, C ++, Java, Smalltalk, Ada, and COBOL.
Given that CORBA is organized as a collection of clients and object servers, the general organization of a CORBA system is shown in Fig. 9-2.