AJAX manipulates the local representation of a Web page using JavaScript and updates it with content (usually XML or JSON) that is fetched dynamically using HTTP. AJAX is becoming very popular because it avoids page refreshes and provides a more dynamic interface but one of its challenges is how to avoid stale data; there are a number of techniques to address this, including polling, piggybacking and long-lived HTTP connections. These approaches have been around for a while in one way or another but, like AJAX, are becoming more popular and are being given different names. For example, the collection of technologies is called Reverse AJAX in DWR 2.0 (blog, Intro@Prokata). And the usef of long-lived HTTP connections, also called HTTP Streaming is also called Comet. Comet is particularly interesting in that it has an impact in the programming model and in the implementation. There is beginning to be enough experience with the model (AsyncWeb, LightStreamer, Jetty's@TSS) to start thinking of standarization and GregW and others have advocated along this direction. One of the interesting developments is that the Grizzly's machinery provides a very good foundation for Comet -- see for example the report from AgileAjax, and Jean-Francois and others are talking about using it more widely in that direction. I hope this will make it into one of our future releases of GlassFish.

PS. The name Comet is a takeoff on AJAX. This is possibly more of a US-centered joke, so check... AJAX's site (a brand of Colgate-Palmolive) and Comet's site (a brand of Prestige Brands, Inc).