The distribution middleware is commonly referred as today's middleware. Most important developments in field are in this layer as we have discussed previously. Schantz and Schmidt conclude this layer middleware as the one defining higher-level distributed programming models whose reusable APIs and components automate and extend the native OS network programming capabilities encapsulated by host infrastructure middleware [9]. One advantage that is most cited for this layer middleware is that it enables programming distributed applications much like stand-alone applications, and thus simplifies the development process.
There are many choices in this layer on current market. However, three major developments so far have dominated most of the design issues, namely CORBA, RMI, and DCOM. As we have cleared out at the beginning, CORBA is rather an architecture specification which can be implemented by a specific vendor. Many middleware products are implemented following CORBA specifications today. RMI is distribution middleware that enables developers to create distributed Java-to-Java applications. It is also widely used in programming field as Java becoming one of the most popular OO language at present. It also takes the advantage from the underlying JVM, which is platform-independent. DCOM captures Microsoft's vision of distribution middleware. Theoretically, DCOM can be implemented with any language under any platform. However, because Microsoft mainly addresses its Windows system, DCOM is implemented primarily on Windows platforms. SOAP is a lightweight and simple XML-based protocol that allows applications to exchange structured and typed information on the Web as we have seen in Figure 3. It is key to the .NET platform's Web service. We have seen growing acceptance of SOAP in web integration throughout our studies.
We now take CORBA, RMI, and DCOM as our primarily subjects, and describe them from different perspectives below.