It does nothing to explain why app servers range in price from free to tens of thousands of dollars per CPU, why an app server may bring critical capabilities to a development project, and why choosing an app server can lead to unexpected technological lock-in. Also, the generational model did not capture the benefits of object-oriented programming, which provides powerful tools to abstract both data and process. But the generational model does map well into the creation of Web applications.
Web services are typically more centered on data flow than on user-interface manipulation.
The success of XML is largely based on the removal of user-interface concerns from the realm of the software engineer. Just as 4GLs had wildly variant pricing, so too does the open-source Zope Web server project compare well against some of the most expensive tools. Finally, the collapse of the 4GL industry has important lessons for app server vendors, managers and developers. My definition of a WebGL goes beyond programming language syntax. While the .NET platform is the best move Microsoft has made in years, it will be several years before it becomes mature enough to be a serious contender.