WebSub (formerly PubSubHubbub) is an open protocol for distributed publish–subscribe communication on the

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...

. Initially designed to extend the

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(and

RSS RSS ( RDF Site Summary or Really Simple Syndication) is a web feed that allows users and applications to access updates to websites in a standardized, computer-readable format. Subscribing to RSS feeds can allow a user to keep track of many di ...

) protocols for data feeds, the protocol can be applied to any data type (e.g.

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, text, pictures, audio, video) as long as it is accessible via HTTP. Its main purpose is to provide real-time notifications of changes, which improves upon the typical situation where a client periodically polls the feed server at some arbitrary interval. In this way, WebSub provides pushed HTTP notifications without requiring clients to spend resources on polling for changes. In October 2017, PubSubHubbub was renamed to WebSub for simplicity and clarity. , the WebSub protocol has been adopted by the W3C as a Recommendation.

Protocol

Under WebSub, there is an ecosystem of publishers, subscribers, and hubs. A subscriber first retrieves content from an HTTP resource (

URL A Uniform Resource Locator (URL), colloquially termed as a web address, is a reference to a web resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identifie ...

) by requesting it from the webserver. The subscriber then inspects the contents of the response, and if it references a hub, the subscriber can subscribe to that resource's URL (it's called a 'topic' by the spec) on that hub. The subscriber needs to run a web accessible server so that hubs can directly notify it when any of its subscribed topics have updated, using a webhook mechanism. Publishers expose their content with the inclusion of hub references in the HTTP headers. They post notifications to those referenced hu