PubSubHubbub is an open protocol for distributed publish/subscribe communication on the Internet. The protocol extends the Atom (and RSS) protocols for data feeds. The main purpose is to provide near-instant notifications of change updates, which would improve on the typical situation where a client periodically polls the feed server at some arbitrary interval.
In essence it provides for pushed Atom/RSS update notifications instead of requiring clients to poll whole feeds.
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Under PubSubHubbub, there is an ecosystem of publishers, subscribers, and hubs.[1]
A subscriber initially pulls the Atom (or RSS) feed in the conventional way, i.e. by requesting it from the feed server. The subscriber then inspects the feed, and if it references a hub, the subscriber can subscribe to the feed URL topic on that hub. The subscriber runs a server so that hubs can directly notify it when any of its subscribed topics have updated.
Publishers expose their content as Atom (or RSS) feeds, but with the inclusion of hub references. They post notifications to those referenced hubs whenever they publish something. Thus, when a publication event occurs, the publisher calls its hubs and the hubs call their subscribers.
It is expected that the protocol will support any kind of arbitrary content, effectively allowing subscription to any web resource. Similarly, work is being done to allow support for private/protected data.