If the subscription is confirmed, the hub will respond with a 204 status.

Published at 2017-04-25T03:21:23Z

Topic: An Atom [RFC4287] or RSS [RSS20] feed URL [RFC3986]. The unit to which one can subscribe to changes. This spec currently only addresses feed URLs that require no additional authorization headers. Hub ("the hub"): The server (URL [RFC3986]) which implements both sides of this protocol. We have implemented this and are running a server at http://pubsubhubbub.appspot.com that is, at least for now, open to anybody for use, as either a publisher or subscriber. Any hub MAY implement its own policies on who can use it. Publisher: An owner of a topic. Notifies the hub when the topic feed has been updated. It just notifies that it has been updated, but not how. As in almost all pubsub systems, the publisher is unaware of the subscribers, if any. Other pubsub systems might call the publisher the "source". Subscriber: An entity (person or program) that wants to be notified of changes on a topic. The subscriber must be directly network-accessible and is identified by its Subscriber Callback URL. Subscription: A unique relation to a topic by a subscriber that indicates it should receive updates for that topic. A subscription's unique key is the tuple (Topic URL, Subscriber Callback URL). Subscriptions may (at the hub's decision) have expiration times akin to DHCP leases which must be periodically renewed. Subscriber Callback URL: The URL [RFC3986] at which a subscriber wishes to receive notifications. Event: An event that causes updates to multiple topics. For each event that happens (e.g. "Brad posted to the Linux Community."), multiple topics could be affected (e.g. "Brad posted." and "Linux community has new post"). Publisher events cause topics to be updated and the hub looks up all subscriptions for affected topics, sending out notifications to subscribers. Notification: A payload describing how a topic's contents have changed. This difference (or "delta") is computed by the hub and sent to all subscribers. The format of the notification will be an Atom or RSS feed served by the publisher with only those entries present which are new or have changed. The notification can be the result of a publisher telling the hub of an update, or the hub proactively polling a topic feed, perhaps for a subscriber subscribing to a topic that's not pubsub-aware. Note also that a notification to a subscriber can be a payload consisting of updates for multiple topics. Hubs MAY choose to send multi-topic notifications as an optimization for heavy subscribers, but subscribers MUST understand them. See Section 7.3 for format details.

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