WebSub

WebSub, formerly known as PubSubHubbub, is a W3C Recommendation that defines an open publish/subscribe protocol layered directly on top of HTTP. Instead of subscribers constantly polling a resource to see if anything changed, a publisher advertises a hub, subscribers register their interest with that hub, and the hub pushes content updates out the moment they happen. It is one of the oldest and simplest ways to make the web — and APIs — behave in real time.

• Hub-mediated fan-out - A publisher offloads delivery to a hub, so it never has to track or notify subscribers itself.
• Discovery via link relations - Publishers advertise their hub and topic URLs with Link headers or feed elements, so subscription is self-describing.
• HTTP callbacks - Notifications arrive as ordinary HTTP POSTs to a subscriber’s callback URL, the same shape as webhooks.
• Verification and signatures - The subscribe handshake and optional HMAC signing guard against spoofed or hijacked subscriptions.

In API operations WebSub shows up wherever a provider wants to eliminate polling and give consumers push-based updates without inventing a bespoke webhook scheme. It grew out of the feed world but maps cleanly onto event-driven APIs, sitting alongside webhooks and AsyncAPI as a standardized option for the notification layer. For the agentic turn, a well-advertised hub means an automated consumer can discover, subscribe to, and react to changes in an API’s resources without a human wiring up integration glue.