HTTP/2 Push JSON Payload

Question

The most use cases for http/2 server push is to pre-emptively push assets files (such as javascript and css files) to browser. I am wondering can http/2 server push be used to send dynamic payload such as JSON documents to client application? From the http2-spec, it doesn't mention anything about this. Can anyone elaborate more on this? Why or why not?

Erfert

Answer 1

HTTP/2 is not intended as a replacement of websockets in that you make a request (e.g. a web page) and may get several resources back (e.g. The web page, the CSS needed to display the webpage, JavaScript needed to run that webpage... etc.).

HTTP/2 is therefore not truly bidirectional in that it still responds to an initial request.

So if you're intending to send the JSON request in response to the initial request then that's fine - it's just another resource in much the same as CSS and javascript.

However if you're intending to keep the channel open to continually send further JSON payloads to keep your page up to date then that's not what HTTP/2 is intended for. That's what websockets are for.

This question has some further details on HTTP/2 versus websockets: Does HTTP/2 make websockets obsolete?

Pentarchy

This answer to the question Does HTTP/2 make websockets obsolete? says "I think that HTTP/2 does obsolete websockets for most use cases, but maybe not all" and further "... whether HTTP/2 does a better job of enabling bidirectional streaming in the browser, and I think that yes, it does". – Taking

It's an interesting way of pushing HTTP/2 beyond it's original intention, and may work in certain scenarios (gRPC is an interesting conceptual version of such an application), however I would say it's probably overkill for most applications compared to a lighter websocket style implementation. Also this all would have to be built on top of HTTP/2 so, getting back to the original question, there is no easy way to push dynamic payloads such as JSON documents natively in any HTTP/2 client or server implementations after the initial response. – Pentarchy

Well I should say “any current HTTP/2 client and server implementations that I’m aware of” :-) – Pentarchy

Answer 2

Yes, you can use HTTP/2 Push to send any type of assets. But keep in mind the following:

• As BazzaDP said, HTTP/2 Push is not a push notification mechanism. But HTTP/2 is excellent for doing long polling, and then you have content-encoding compression, encryption, HTTP headers and flow control, so for 90% of the cases you can and probably should skip websockets anyway. Notice that when doing long polling, HTTP/2 Push is not required.Also notice that there is something called Server Sent Events which sort of is the browser-endorsed version of long polling.

• HTTP/2 Push is so far transparent to the application. Meaning that you have to do the Push and do a request for the resource from your application.

• For now, only cacheable things are made available to the application. That means that you need to set cache headers in the dynamically generated JSON response. Probably you can set a short expiration time, or a long one but under a dynamic URL.

Couvade