[1] Initially designed to extend the Atom (and RSS) protocols for data feeds, the protocol can be applied to any data type (e.g. HTML, 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.
Publishers expose their content with the inclusion of hub references in the HTTP headers.
The hub immediately sends a verification message to the subscriber with the URL of the topic and the above code.