It’s a way for RSS/Atom feed consumers (feed readers etc) to be instantly updated and notified when an RSS is updated.
In a nutshell, the RSS publisher notifies a specific hub when it has a new item. The hub then notifies – instantly – any subscribers who have requested the hub to contact them when there is an update.
This is all automatic and requires no special setup by users. Once the feed producer has set up PubSubHubbub, and specified a particular, the RSS feed has an extra entry in the feed itself telling subscribing client systems that they can use a specific hub for this feed. Clients which do not understand this line will just ignore and carry on as normal.Those that are compatible with PubSubHubbub can then contact the hub and ask to be notified when there are updates.
It has been developed by Google, and they’ve implemented it in to various Google services such as Google Reader and Blogger. This should help give it momentum (which is also crucial for this sorts of things). In a video on Joss’ post (linked to above) the developers demonstrate posting an article and showing Google Reader instantly update the article count for that feed (in fact, before the blog software has even finished loading the page after the user has hit ‘publish’). It reminds be of the speed of Friendfeed, I will often see by friendfeed stream webpage update with my latest tweet before I see it sent from twirl.
I’ve installed a PubSubHubbub WordPress plugin for this blog. Let’s hope it takes off
UPDATE: I’ve just looked at the source of my feed ( http://www.nostuff.org/words/feed/ ) and saw the following line:
<atom:link rel="hub" href="http://pubsubhubbub.appspot.com"/>