Notifications topology

[hzbarcea]

The spec says nothing about topology, and it's hard to read between the lines what the envisioned topology should be. This issue is only relevant in the context of massive scale.

Notifications at scale are extremely costly. For that reason, pretty much all services we today use email for notifications (or SMS in some cases). Both scale extremely well, with the limitations and issues we know. These are asynchronous communication technologies backed by serious infrastructure.

For instance, the current spec seems to imply that the RS is aware of the notification sender capable of delivering notifications (which in my experience has a big impact on scalability). With a small number of notification receivers, not an issue, but at scale it's very costly and unsustainable (e.g. let's say we have a video sharing app, like youtube, and millions of followers need to be notified of the availability of new content).

I am not sure this aspect is suitable for addressing in one github issue, so I see this more of an umbrella issue, conversation starter, that may lead to the creation of smaller, actionable issues. It is possible for some aspects to be considered out of scope as well.