[prodigal_erik]

The docs talk about "event types" like contact.update which do not exist in HTTP and require knowing the semantics of a particular group of resources. To truly be RESTful, I would want "please notify me of any PUT, PATCH, or DELETE to this list of URLs, or POST creating a URL that matches this pattern." And if you're going to include a copy of the resource (I don't know what else a payload would be) you need the media type the subscriber expects as if you were doing content negotiation for a GET request from them. Or maybe the payload could just be the current result of a HEAD request so the subscriber can invalidate their caches, and they can send the appropriate GET request(s) when and if they care to.

There should be some standard link relation like <link rel="subscription"> so clients have some hint that this is available and where to request it.

I'd also want some way to manage the freshness/load tradeoff, like "please notify me within one hour but no more than every five minutes".