How does RSS reader know that a feed is updated?

Just learning about this via youtube but could not find answer to my question of how reader knows there 开发者_开发知识库is an update.

Is it like a Push in blackberry?

RSS is a file format source and doesn't actually know anything about where it gets the entries from. The answer really is: "how can an http request get only the newest results from a server" and the answer is Conditional GET source. Http also supports Conditional PUT.

This is an article about using this feature of http to specifically support rss hackers.

RSS is a pull technology. The reader re-fetches the RSS feed now and then (for example two times per hour, or more often if the reader learns that it's an often updated feed).

The feed is served through regular HTTP and consists of a simple XML file. It is always fetched from the same URL.

It just check the feed for update regularly.

Recently there is a new protocol called pubsubhubbub to make feed push to the listener. But it requires the publishers support it.

Here is a list of web services support real-time RSS pushing, including Google Reader, Blogger, FeedBurner, FriendFeed, MySpace, etc.

Let's summarize :

• Usually, a client knows that an RSS feed has been updated through polling, that is regular pull (HTTP GET request on the feed URL)
• Push doesn't exist on the web, at least, not with HTTP until HTML5 websocket is fixed.
• However, some blog frameworks like Wordpress, Google and others, now support the pubsubhubbub convention. In this mode, you would "subscribe" to the updates of an RSS flow. The "hub" will call an URL on YOUR site (callback URL) to send you updates : that is a push.

Push or pull, in both cases you still need to write some piece of code to update the RSS list on your site, database or wherever you store/display it.

And, as a side question, it is not necessary to request the whole XML at every pull to see if the content has changed : using a standard that is not linked to RSS, but global to the whole HTTP protocol (etag and last-modified headers), you can know if the RSS page was modified after a given date, and grab the whole XML only if modified.

It's a pull. That's why you have to configure your reader how often it should refresh the feed.