Does the Fastmail app rely entirely on OS implementations of push notifications for this? Or is there some kind of push functionality in JMAP?
The JMAP spec provides for eventsource and webhooks, but obviously an individual provider can offer more than that via an extension. When we do switch over to JMAP, we'll continue to use our custom method to register platform notifications.
> If the client sent some data down the connection to the server. Unfortunately the eventsource specification doesn’t provide any way to do this;
Solution: use websocket, which the client can actively ping the server.
You will need to detect this and apply a fallback mechanism.
Which goes back to the point of this post: easy in theory. Much harder in practice.
Where I work we ended up using Microsoft's signalr-library to avoid having to learn and tackle all of these issues ourselves.
Yet we still needed to add some additional Band-Aids around the libraries provided to be able to support all cases we wanted seamlessly.