What's with web applications sending notifications via delaying loads?

I've noticed on a lot of applications recently, such as Google Docs and Facebook, that JavaScript is being used to load a file which doesn't load until the website needs to send you a notification.

Essentially, they're using an HTTP request to a file, and then the server which handles the file doesn't respond to the request (or may respond but not send the content) until the server wants to send a notification (such as a new IM or a document update) to the user.

I haven't been able to find much on the web about the best practic开发者_开发百科es for doing this. It seems to be a relatively new technique.

I'm looking into doing this myself and would like to read more on it. Does this have a name? Are there established best practices for doing this?

You could start reading about Comet technology. And here is a good Comet Server, APE: Ajax Push Engine