494 DISTRIBUTED OBJECT-BASED SYSTEMS CHAP. 9
9.1 CORBA
We start our study of distributed object-based systems by taking a look at the Common Object Request Broker Architecture, simply referred to as CORBA. As its name suggests, CORBA is not so much a distributed system but rather the specification of one. These specifications have been drawn up by the Object Management Group( OMG), a nonprofit organization with over 800 members, primarily from industry. An important goal of the OMG with respect to CORBA was to define a distributed system that could overcome many of the interoperability problems with integrating networked applications. The first CORBA specifications became available in the beginning of the 1990s. At present, implementations of CORBA version 2.4 are widely deployed, whereas the first CORBA version 3 systems are becoming available.
Like many other systems that are the result of the work of committees, CORBA has features and facilities in abundance. The core specifications consist of well over 700 pages, and another 1,200 are used to specify the various services that are built on top of that core. And naturally, each CORBA implementation has its own extensions because there is always something that each vendor feels cannot be missed but was not included in the specifications. CORBA illustrates again that making a distributed system that is simple may be a somewhat overwhelmingly difficult exercise.
In the following pages, we will not discuss all the things that CORBA has to offer, but instead concentrate only on the parts that are essential to it as a distributed system and that characterize it with respect to other object-based distributed systems. The specifications of CORBA can be found in( OMG, 2001b), which is publicly available at http:// www. omg. org. A high-level overview of CORBA is described in( Vinoski, 1997), whereas Pope( 1998) provides a more detailed description derived from the original specifications. Information on building applications in C ++ using CORBA can be found in( Baker, 1997) and( Henning and Vinoski, 1999).
9.1.1 Overview of CORBA
The global architecture of CORBA adheres to a reference model of the OMG that was laid down in( OMG, 1997). This reference model, shown in Fig. 9-1, consists of four groups of architectural elements connected to what is called the Object Request Broker( ORB). The ORB forms the core of any CORBA distributed system; it is responsible for enabling communication between objects and their clients while hiding issues related to distribution and heterogeneity. In many systems, the ORB is implemented as libraries that are linked with a client and server application, and that offers basic communication services. We return to the ORB below when discussing CORBA’ s object model.
Besides objects that are built as part of specific applications, the reference