From IndieWeb

pubsub is a generic name for a protocol, system, or service that routes messages between producers and consumers. It usually refers to one of three things:

Subscribing to updates to a URL (ie the first definition above) is an extremely useful building-block for the indieweb. The rest of this page discusses that context.

Generalised Flow

  1. Subscriber -> Publisher: Please let me know when you update
  2. Publisher -> Subscriber: Okay, now Iโ€™ve verified youโ€™re not spam Iโ€™ll let you know

Later, after updating

  1. Publisher -> Subscriber: I updated, hereโ€™s its new content

Problems with this flow

  • Lots of load on the publisher: Subscription management, sending updates. Too much complexity, too low a value/pain ratio for most creators to implement
    • Solution: Hubs as the middle man between subscribers and publishers

Previous/Current Work

  • PubSubHubbub is a decentralised, hub-based approach
    • Widely implemented and suitably easy for publishers to use, but difficult to test
    • Lots of complexity for subscribers
    • Reliance on ATOM format, semantics and DRY-violating duplication
  • RSS Cloud


Ideal solution: a content-agnostic, pure-HTTP hub-based approach.

Publishers create content on the web, and let hub(s) know that it exists/when itโ€™s updated via a POST request. Content is published with a Last-modified header.

Hubs periodically poll the content for changes (HEAD request and Last-modified header inspection) just in case they missed any updates/to enable pubsub of content published by people who for whatever reason canโ€™t notify the hub.

Publishers publicly declare which hub manages notifications for their content via a Link header or HTML/XML element with rel=hub.

When a subscriber wants to subscribe to changes to a URL, they discover the hub as described above, and send a POST request to some endpoint (/subscribe?) with the URL theyโ€™re subscribing to and the URL they want notifications sent to.

  • We need some sort of auth at this point to weed out spammers and enable the authenticity of future notifications.
  • TODO: spec this stage out in more detail, take inspiration from current PuSH work

Then, when a publisher updates their content and either a) sends a notification to the hub or b) the hub polls the URL and sees the Last-modified header change, the hub sends POST requests out to all the subscribers for that URL.

  • Possibly with the URL content as the POST body to prevent the Thundering Herd problem

Benefits over current solutions

  • Eliminates DRY-violating duplication
  • Content-agnostic, not tied to ATOM feeds but can be used with *any* content served over HTTP
  • Extremely low barrier to entry for publishers โ€” occasional hub polling of content eliminates the need to even send notifications to the hub to begin with

See Also