There are two answers.
Today, most readers and aggregators surveyed most channels.
There are several approaches to developing real-time update notifications.
The RSS 2.0 specification has a real-time notification notification called an element <cloud>. (I wanted to include a link to the specification text, but Stack Overflow will allow me to include only one link per message.)
We have recently done work to ensure that an implementation passes that works and works on one of my servers.
Google PubSubHubbub, .
, scripting.com RSS Atom "FeedHose" - : http://feedho.se/.