The format initially used for the transport of data in the upstream and downstream communication between the web server and the script code executed on the clients machine was XML at the beginning. With the evolvement of Web 2.0 applications did the type of requests and responses change to make the use of XML less favorable. XML is because of its descriptive nature creating much more code to wrap around the actual information than needed most of the time.
Formats like Json or proprietary formats developed for the different AJAX frameworks require less code and are more comon for the use of AJAX enabled web applications today. XML (respectively SOAP) based communication is largely limited to the use within companies intranet applications where bandwith and volume of overall data is not such a big problem.
Below are resources for web application developers to learn more about AJAX and AJAX frameworks to create content rich, multi-media enabled, interactive and user friendly websites and to aid in the development of them.
(Programmer to Programmer)
by Nicholas C. Zakas,
Jeremy McPeak, and Joe Fawcett
Ajax Patterns and Best Practices
by Christian Gross
Ajax in Action
by Dave Crane ,Eric Pascarello
and Darren James
Foundations of Ajax
by Ryan Asleson , Nathaniel T. Schutta