WebSub

WebSub is an open, standardized protocol for real-time publication updates on the Web. It defines a publish/subscribe model in which publishers announce changes to a topic, subscribers express interest via a hub, and the hub distributes notifications to subscribers when the topic changes.

The core actors are publishers, hubs, and subscribers. A topic is a URL identifying a resource. A hub is an intermediary that relays notifications to subscribed endpoints. Topic and hub discovery happens through HTTP link headers embedded in the resource, typically a rel="hub" link to a hub and a rel="self" link to the topic.

Subscriptions are initiated by the subscriber contacting the hub and requesting delivery for a specific topic, providing a callback URL where notifications should be delivered. The hub verifies the request (often via a challenge) and, after successful verification, keeps the subscription. When the publisher updates the topic, the hub delivers a notification by an HTTP POST to each callback URL with information about the update.

WebSub is a W3C Recommendation and has been adopted by various platforms to deliver near real-time updates without frequent polling. It is designed to be interoperable with existing feed formats such as RSS and Atom and complements other web standards. Critics cite reliance on hubs for scalability and privacy concerns.