Re: [fetch] HTTP/2 server push support (#51)

This bug report has a lot of information about use-cases. Copying it here for convenience: https://www.w3.org/Bugs/Public/show_bug.cgi?id=28083

http://lists.w3.org/Archives/Public/public-whatwg-archive/2015Feb/0030.html

The use-case I have in mind is MPEG DASH or HLS live streaming:

A server updates the manifest (MPD or m3u8) and pushes it. The client browser will cache it, but JavaScript applications don’t know that it exists. For newly generated segments, we need the MPD to use them anyway, but for adaptive streaming we need to know when the request started and when it finished so we can estimate bandwidth. If a server is pushing MPD updates, we want a way to tell it to stop (RST_STREAM on a pushed stream to end the transfer, or RST_STREAM on the original client request to prevent future pushes for that request).

The obvious question to ask is “why not just poll the server”? The answer its that live streaming latency depends (among other things) on how quickly you poll. Unless you can perfectly predict when the server will have an update available, you need to either poll slightly late (introducing latency) or poll significantly more often than the server creates updates. Using server push is equivalent to to polling infinitely fast, while simultaneously reducing load on the server by making fewer requests (win/win).

I imagine that this would be useful for JavaScript implementations of HTTP/2 REST API’s, especially API’s using something like ATOM or RSS. Firefox has an API for addons to detect pushed streams, so presumably they think there is some use for this (but they didn’t propose this as a public API, so maybe not?).

What's needed:

  - Event when PUSH_PROMISE is detected, containing a new Response.
  - Ability to signal to a UA that a stream should left in the "half closed" state (to allow server pushes).

I'm not sure fetch is the best API for this, since there's currently no way to cancel fetches or use a partial response before it finishes, but it seems to be the only API anyone wants to make changes to.

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