The problem

The HTTP/2 spec supports full-duplex streams between client and server. Accordingly, if you use the Node.js http2 module on client and server, you get a Duplex stream on both sides.

However, you won’t get a full-duplex stream in a browser client.

The Fetch API specification does provide for duplex streams:

• You can specify a ReadableStream when making a request (here’s the PR which added it to the spec).

• You can call getReader on response bodies to get a ReadableStream.

The latter works fine in browsers — you can read data sent from the server using the ReadableStream class.

But streaming data from the browser to the server using Fetch doesn’t work.

To quote this comment from the Chromium team:

It won’t be full-duplex. You’ll have to send everything before receiving anything (anything sent by the server before that will be buffered).

And to quote this article from a Chrome developer advocate:

In Chrome’s current implementation, you won’t get the response until the body has been fully sent.

Further, it looks like streaming upoad support has now been dropped:

Thank you very much for participate in the origin trial. We worked with a parter but we failed to show benefits of the feature, so we’re giving up shipping the feature.

The issue can be followed here and here.

Solution

browser-http2-duplex emulates a full-duplex Node.js Duplex stream in the browser over HTTP/2 using the Fetch API.

• Each data chunk your application writes to the Duplex is sent to the server in a separate POST request (over a single HTTP/2 connection).

• Data from the initial response body’s ReadableStream is pushed to the Duplex for your application to read.

On the server, browser-http2-duplex marries up the separate POST requests with the initial reponse and presents a Duplex stream to your application.

UPDATE: The new WebTransport W3C standard supports bidirectional streams between browser and server. If you can live with HTTP/3 only then you might want to check it out.

Example

Here’s a server which echoes data it receives on a duplex stream back to clients.

server.js

import fs from 'fs';
import { join } from 'path';
import { createSecureServer } from 'http2';
import { Http2DuplexServer } from 'http2-duplex';
const { readFile } = fs.promises;
const cert_dir = join(__dirname, 'certs');

(async function () {
    const http2_server = createSecureServer({ // 
        key: await readFile(join(cert_dir, 'server.key')),
        cert: await readFile(join(cert_dir, 'server.crt'))
    });

    const http2_duplex_server = new Http2DuplexServer( // 
        http2_server,
        '/example'
    );

    http2_duplex_server.on('duplex', function (stream) { // 
        stream.pipe(stream);
    });

    http2_duplex_server.on('unhandled_stream', function (stream, headers) { // 
        const path = headers[':path'];
        if (path === '/client.html') {
            return stream.respondWithFile(
                join(__dirname, path.substr(1)),
                { 'content-type': 'text/html' });
        }
        if ((path === '/client.js') ||
            (path === '/bundle.js')) {
            return stream.respondWithFile(
                join(__dirname, path.substr(1)),
                { 'content-type': 'text/javascript' });
        }
        stream.respond({ ':status': 404 }, { endStream: true });
    });

    http2_server.listen(7000, () =>
        console.log('Please visit https://localhost:7000/client.html'));
})();

• Create a standard Node.js HTTP/2 server.

• Create a server to communicate with clients using full-duplex emulation.

• When a client creates a new duplex, the server gets a duplex event.

• Other requests raise an unhandled_stream event. Here we return the client files to the browser.

Note you can just Control-C the server to stop it. If you wanted to stop the server in code, you would do something like this:

http2_duplex_server.detach(); // 
await promisify(http2_server.close.bind(http2_server))();

• This destroys all active sessions.

Here’s a client which sends keypresses to the server and writes the echoed response to the page:

client.js

export default async function () {
    const duplex = await http2_client_duplex_bundle.make( // 
        'https://localhost:7000/example');

    document.addEventListener('keypress', ev => { // 
        duplex.write(ev.key);
    });

    duplex.on('readable', function () { // 
        let buf;
        do {
            buf = this.read();
            if (buf !== null) {
                document.body.appendChild(document.createTextNode(buf.toString()));
            }
        } while (buf !== null);
    });
}

• Connect to the server and emulate a new full-duplex stream.

• When the user presses a key, write the character to the stream.

• Read characters the server echoes back from the stream and append them to the document body.

That’s a simple example of setting up duplex emulation between a browser and a server. You’ll also need an HTML page and to bundle up the client-side library (e.g. using Webpack). You can find all these files in the example directory. To run the example:

grunt --gruntfile Gruntfile.cjs example

and then point your browser to https://localhost:7000/client.html.

Installation

npm install http2-duplex

Licence

MIT

Test

grunt --gruntfile Gruntfile.cjs test

Lint

grunt --gruntfile Gruntfile.cjs lint

Coverage

grunt --gruntfile Gruntfile.cjs coverage

Istanbul results are available here.

Coveralls page is here.