WebSub Inspired Continuous Delivery

The current pull architecture could be enhanced to provide continuous delivery of updated content on change at the source using a flow inspired by the flow in the W3C WebSub recommended protocol. The attached flow diagram departs from the standard in that the content distribution notifications in this proposal are processed by the client (subscriber) and trigger a pull from the server (publisher) rather than delivering either the full content or a diff as standard either expresses or implies. In this proposal, the entity_share_client acts as the protocol subscriber and the entity_share_server acts as publisher and hub. The protocol's topic could be a channel but for our purposes I think it would be better for each entity to be a topic.

The flow in the attached diagram is as follows:

1. Client discovers Hub url advertised by the Server
2. Client sends a POST request to Hub url at Server to subscribe to updates about a topic.
3. Server verifies subscription with a GET request back to the Client
4. On content change, the Server makes a POST request to each Client with notification of the change
5. The Client processes the notification, triggering a sync for that entity using the current process

Foundational classes for this approach are found in core dependencies:

• \Zend\Feed\PubSubHubbub\Publisher
• \Zend\Feed\PubSubHubbub\Subscriber

We would need to extend these and create or find a Hub implementation.