A simple, open, server-to-server web-hook-based pubsub (publish/subscribe) protocol as an extension to Atom and RSS.

Parties (servers) speaking the PubSubHubbub protocol can get near-instant notifications (via webhook callbacks) when a topic (feed URL) they're interested in is updated.

The protocol in a nutshell is as follows:

• An feed URL (a "topic") declares its Hub server(s) in its Atom or RSS XML file, via <link rel="hub" ...>. The hub(s) can be run by the publisher of the feed, or can be a community hub that anybody can use. (Atom and RssFeeds are supported)

• A subscriber (a server that's interested in a topic), initially fetches the Atom URL as normal. If the Atom file declares its hubs, the subscriber can then avoid lame, repeated polling of the URL and can instead register with the feed's hub(s) and subscribe to updates.

• The subscriber subscribes to the Topic URL from the Topic URL's declared Hub(s).

• When the Publisher next updates the Topic URL, the publisher software pings the Hub(s) saying that there's an update.

• The hub efficiently fetches the published feed and multicasts the new/changed content out to all registered subscribers.

The protocol is decentralized and free. No company is at the center of this controlling it. Anybody can run a hub, or anybody can ping (publish) or subscribe using open hubs.

To bootstrap this, we've provided an open source reference implementation of the hub (the hard part of the protocol) that runs on Google App Engine, and is open for anybody to use.

See the links in the sidebar at right.

Overview