The Future of Comet: Part 1, Comet Today: Remember Comet? This came close on the heels of Ajax, and is its twin. Whereas Ajax allows the browser to “reach back” to the server and say “hey, something happened,” Comet does the same for the server. It can now “reach forward” and notify the browser of something.
This article is a good introduction to the complexity and technology behind Comet. Honesty reigns supreme:
Comet is a giant hack. Even when it works flawlessly and efficiently, bringing us real-time interactive applications, deftly weaving around firewalls and browsers, avoiding unfriendly side effects, and cutting latency to near zero, it does so using mechanisms unforeseen by browser vendors, and unspecified by web standards.
It’s a good look at ingenuity, and there’s some information about how a standard is trying to develop around it.