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WebSub-Socket

Implements a callback URL for [WebSub]1 subscribers, which proxies all incoming webhook notifications to the WebSocket client subscriber.

WebSub-Socket does not initiate Subscribe calls, it does support hub verifications by acknowledging challenge requests.

The hub verifies a subscription request by sending an HTTP (or HTTPS) GET request to the subscriber's callback URL as given in the subscription request.

Example flow:

  • WebSocket client connects using a websocket connection to the Proxy 
    wss://proxy-url.example.com/socket/name-of-endpoint
  • WebSocket client sends a websocket message to the Proxy notifying it of a pendnig subscription 
    {"hub.mode":"subscribe","hub.topic":"whatever","hub.secret":"thesecret"}
  • WebSocket client sends HTTP(s) request to a WebSub HUB, subscribing to a topic 
    POST https://hub.somewhere-on-the-internet.com
{"hub.callback":"http://proxy-url.example.com/hook/name-of-endpoint","hub.mode":"subscribe",...}
  • HUB sends a challenge verification request to the Proxy 
    GET https://proxy-url.example.com/hook/name-of-endpoint?hub.mode=subscribe&hub.challenge=difficult-challenge-token
  • The proxy acknowledges the challenge, and responds with the provided challenge token 
    difficult-challenge-token
  • HUB starts sending notification messages to the callback URL whenever the topic is updated
  • Proxy re-sends these messages to any WebSocket clients registered using a matching endpoint

Reference

  • [WebSub]1 W3C Recommendation - formerly known as PubSubHubbub

Footnotes

  1. https://www.w3.org/TR/websub "WebSub" ↩ ↩2