# Using HTTP/2

HTTP/2 is the second major version of the Hyper Text Transfer Protocol used to transfer web data.

Note

Not all ASGI Servers support HTTP/2. The recommended ASGI server, Hypercorn, does.

To use HTTP/2 in development you will need to create some SSL certificates and run Quart with SSL.

## Server push or push promises

With HTTP/2 the server can choose to pre-emptively push additional responses to the client, this is termed a server push and the response itself is called a push promise. Server push is very useful when the server knows the client will likely initiate a request, say for the css or js referenced in a html response.

Note

Browsers are deprecating support for server push, and usage is not recommended. This section is kept for reference.

In Quart server push can be initiated during a request via the function `make_push_promise()`, for example,

    async def index():
        await make_push_promise(url_for('static', filename='css/minimal.css'))
        return await render_template('index.html')

The push promise will include (copy) header values present in the request that triggers the push promise. These are to ensure that the push promise is responded too as if the request had made it. A good example is the `Accept` header. The full set of copied headers are `SERVER_PUSH_HEADERS_TO_COPY` in the request module.

Note

This functionality is only useable with ASGI servers that implement the `HTTP/2 Server Push` extension. If the server does not support this extension Quart will ignore the push promises (as with HTTP/1 connections). Hypercorn, the recommended ASGI server, supports this extension.

When testing server push,the `QuartClient` `push_promises` list will contain every push promise as a tuple of the path and headers, for example,

    async def test_push_promise():
        test_client = app.test_client()
        await test_client.get("/push")
        assert test_client.push_promises[0] == ("/", {})

## HTTP/2 clients

At the time of writing there aren’t that many HTTP/2 clients. The best option is to use a browser and inspect the network connections (turn on the protocol information). Otherwise curl can be used, if HTTP/2 support is installed, as so,

    $ curl --http2 ...

If you wish to communicate via HTTP/2 in python httpx is the best choice.