I read that RSS and ATOM are HTTP-based using XML format. So how an RSS or Atom reader get informed about content updates if HTTP doesn't support connections? Do they need to poll the server periodically? I'm interested in using it in a real time web application but I heard that "polling doesn't scale".
This is pretty similar to this question. Please read my answer there.
There is no currently long polling for RSS feeds. The technology you're looking for is PubSubHubbub : it will allow the feed publisher to push you the content when it's available.
There are two answers.
Today most readers and aggregators poll for most feeds.
There are several approaches under development for realtime notification of updates.
There is a provision for realtime notification in the RSS 2.0 spec, called the <cloud> element. (I wanted to include a link to the spec text, but Stack Overflow will only let me include one link per post.)
We've recently done work to provide a walkthrough of an implementation, that's up and running on one of my servers.
Google is sponsoring a project called PubSubHubbub that does realtime notification of updates.
I have a project going in the scripting.com community for long-polling notification of updates for RSS and Atom feeds called "FeedHose" -- you can find pointers at: http://feedho.se/.
So how RSS or atom reader are informed about content updates if HTTP doesn't support connections?
The client checks the RSS feed frequently for new items. That's the only way to go here, I know of no "long polling" method in the area. It's why caching is so important when delivering RSS.
来源：https://stackoverflow.com/questions/3952882/how-rss-and-atom-inform-client-about-updates-long-polling-or-polling-or-somethi