PubSubHubbub

PubSubHubbub is a real-time publish-subscribe protocol that extends RSS and Atom feeds with a hub-based notification model. It allows feed publishers to push updates to subscribers via a central hub, rather than relying on polling the feed at regular intervals. The goal is to deliver new content more quickly and efficiently to interested readers and applications.

How it works

Publishers advertise their feed's hub and topic URLs by including link relations in the feed itself, typically rel="hub" and rel="self". A subscriber expresses interest by sending a subscribe request to the hub, including parameters such as hub.mode=subscribe, hub.topic (the feed URL), hub.callback (the subscriber’s endpoint), and an optional hub.lease_seconds. The hub verifies the subscription through a challenge-response handshake, where it sends hub.challenge to the callback and expects it echoed back. Once verified, the hub maintains the subscription and will push updates to the subscriber when the topic changes. When the publisher updates the feed, the hub notifies all active subscribers, often by a POST to the subscriber’s callback URL, prompting the subscriber to fetch the latest content or receive a payload directly.

History and evolution

PubSubHubbub was developed in the late 2000s as an open mechanism to enable near real-time syndication. It is commonly described as a precursor to the WebSub protocol, which modernized and standardized the same general approach. In later years, WebSub became the more current terminology and standard, while PubSubHubbub remains a reference to the original protocol.

Usage

The protocol has been used to accelerate delivery of updates for RSS and Atom feeds across various blogging platforms and feed readers, and influenced subsequent real-time feed technologies. It emphasizes decoupling publishers from subscribers and reducing unnecessary polling.