CORBA is the Common Object Request Broker Architecture. It is an ongoing effort by the Object Management Group (OMG) to define interfaces for interoperable software. Work on CORBA started several years ago, and there are now over 700 members of OMG including virtually all of the computer manufacturers and most of the major software vendors.

If you are using CORBA, communications between two computers is performed using an Object Request Broker (ORB). An object in a program running on some computer (the client) can use the ORB to access the public attributes of another object in some other program perhaps on a different computer (the server) that is also using an ORB. The client object can:

• either hard-code the name of the computer and the name of the object that it wants to contact (Static Method Invocation);
• or it can use the ORB to discover which objects have the required interface (Dynamic Invocation Interface).

In this document, we will only be using Static Method Invocation.

Although an ORB can be running on a single computer, it is more likely to be used to aid communication between one or more client computers and a server computer. If an ORB is unable to satisfy a request from a client, then, if it is connected to other ORBs (elsewhere in the world), the request may be satisfied by an object known to a remote ORB.

One of the key aspects of CORBA is the Interface Definition Language (IDL). This is the language that is used to describe the interface that an object has. With RMI, we had to provide the interface in Java: whereas, with CORBA, the interface has to be provided in IDL. It is a language that looks something like C, C++ or Java. For example, it has function declarations and interfaces; it has types such as boolean, char, string, short, long, float and double.

Having produced an IDL file that describes the interface that an object has, we also need some code for the server that creates an object that implements that interface, and we will also need codes for clients that wish to access the object. These codes need to be written in real programming languages. In order for an interface written in IDL to be usable with existing languages, there needs to be a mapping that says what each IDL feature is in each language. For example, the IDL-to-Java mapping defines that an IDL long is an int in Java, an IDL string is a String in Java, ... . Currently, there are bindings for Ada, C, C++, Java and Smalltalk.

With RMI, both the client and the server have to be Java applications/applets (running in some Java environment). However, with CORBA, the client and server programs may be in different languages. Each program can run on any operating system written in any language so long as you have a compiler to translate the interface from IDL into that language on that operating system.

There are many vendors providing ORBs. In this document, we will look at two products: JavaIDL from Sun Microsystems and OmniBroker from Object Oriented Concepts. Some other ORBs are listed in the Further information about some other ORBs section given later.