Expose active WebSocket connection count in proxy

[romainvai]

Hello,

First of all, thank you for your work on this project — I've recently upgraded to your version of http-proxy and really appreciate the improvements.

I'm currently facing a challenge with managing WebSocket connections in my setup. I manually handle the upgrade using proxy.ws to track the number of active WebSocket clients. The idea is to start a stream API when at least one client is connected, and shut it down when there are none, as the stream is quite CPU-intensive.

To count clients, I rely on the "open" and "close" events from the proxy. However, this approach has proven unreliable in some edge cases:

• When a connection is abruptly terminated (e.g., unplugging the Ethernet cable), the proxy "close" event is not triggered.
• Restarting the stream API sometimes triggers the error event, but not consistently the close event.

As a workaround, I’ve tried forcibly calling socket.end() on all sockets when the proxy "error" event occurs, hoping it would trigger proxy "close" event — but I’m unsure if this is a robust solution.

Would it be possible for the proxy to expose the current number of active WebSocket connections directly? This would help avoid manual counting and reduce the risk of miscounting connections.

I’d be happy to share my code if needed, although I’m not sure it would add much value to the issue.

Thanks again for your work and support!

Best regards.

[williamstein]

Is this helpful:

http-proxy-3/lib/test/websocket/websocket-proxy.test.ts

Line 85 in 28af6f7

await wait({ until: () => httpProxy.numOpenSockets() == 0 });

There's a numOpenSockets() method already, which might be helpful. It's the very first thing I added when I forked http-proxy, since I was trying to fix a socket leak that was causing a lot of trouble for JupyterHub and CoCalc.

[williamstein]

I’d be happy to share my code if needed, although I’m not sure it would add much value to the issue.

Awesome -- please check to see how close numOpenSockets is to what you need...

[romainvai]

Hello,

Thank you for your reply!
You're right — I hadn't noticed the numOpenSockets function, which does seem helpful for addressing my issue, at least partially.

In my case, I have multiple proxies for different paths under the same http-proxy-3 instance, as shown in the example code below.

// Create proxy servers

const proxy1 = httpProxy.createProxyServer({
    target: 'http://127.0.0.1:8000',
    xfwd: true,
});

const proxy2Ws = httpProxy.createProxyServer({
    target: 'http://127.0.0.1:8001',
    xfwd: true,
    ws: true,
});

const proxy3Ws = httpProxy.createProxyServer({
    target: 'http://127.0.0.1:8002',
    xfwd: true,
    ws: true,
});

// Paths

const proxy1Path = '/proxy-1';
const proxy2PathWs = '/proxy-2-ws';
const proxy3PathWs = '/proxy-3-ws';

// Start express server (including proxies and static interfaces)

const app = express();

// Handle HTTP requests

app.use((req, res, next) => {
    if (req.url.startsWith(proxy1Path)) {
        proxy1.web(req, res);
    } else if (req.url.startsWith(proxy2PathWs)) {
        proxy2Ws.web(req, res);
    } else if (req.url.startsWith(proxy3PathWs)) {
        proxy3Ws.web(req, res);
    } else {
        next();
    }
});

// Handle websocket connections

const handleWebsocketUpgrade = (req, socket, head) => {
    if (req.url.startsWith(proxy2PathWs)) {
		// Here we count clients to start/stop the service behind the proxy
        proxy2Ws.ws(req, socket, head);
    } else if (req.url.startsWith(proxy3PathWs)) {
        proxy3Ws.ws(req, socket, head);
    }
};

// Start proxy http server

const httpServer = http.createServer(app).listen(80, function (err) {});
httpServer.on('upgrade', handleWebsocketUpgrade);

However, I need to count the clients connected to only one specific proxy (proxy2Ws). From what I understand, numOpenSockets returns the total number of clients across all proxies (including proxy3Ws in my case).

I'm going to try importing http-proxy-3 multiple times like this:

function createHttpProxy() {
    delete require.cache[require.resolve('http-proxy-3')];
    return require('http-proxy-3');
}

const httpProxy1 = createHttpProxy();
const httpProxy2Ws = createHttpProxy(); 
const httpProxy3Ws = createHttpProxy();

// Create proxy servers

const proxy1 = httpProxy1.createProxyServer({
    target: 'http://127.0.0.1:8000',
    xfwd: true,
});

const proxy2Ws = httpProxy2Ws.createProxyServer({
    target: 'http://127.0.0.1:8001',
    xfwd: true,
    ws: true,
});

const proxy3Ws = httpProxy3Ws.createProxyServer({
    target: 'http://127.0.0.1:8002',
    xfwd: true,
    ws: true,
});

But ideally, it would be great to have a way to count clients per proxy instance if that's feasible.

EDIT: It seems that the socket number is still shared between httpProxy2Ws & httpProxy3Ws so my second code is not working.

Thanks again for your help!