clustering in node.js is not working. Only one worker is always responding

Question

I am practising clustering in node.js I am having a two core CPU. I created two workers and each worker runs a simple http server. Server response callback will block for 5

Answer 1

TIL on the surface node clustering does not appear to do what it says it does (at least to me).

I was in the same situation you were, observing the same worker process always being given a request when I spawned them synchronously from a browser. A colleague of mine used fiddler to replay ~20 requests at once. When all of those requests hit the server very fast, faster than the cluster manager can pass them off to workers (I'm guessing), then you will see additional workers being invoked with requests.

It seems that once the manager hands off the request to a worker, it doesn't know/care about the worker blocking. It just knows that there is only one request in the pipe and there is no need to give that request to anyone other than the first worker because, as far as the manager knows, he's available.

Answer 2

The while loop you have in your response code could be causing some serious problems. You should be using a setTimeout there if you want to simulate a long running request.

Try this for your worker instead:

http.createServer(function(req, res) {
    console.log('worker:' + cluster.worker.id + " going to send response ");
    setTimeout(function() {
       res.writeHead(200);
       res.end("hello world. worker: " + cluster.worker.id);
    }, 5000);
}).listen(8000);

That being said, what @dandavis said in the comments is true: cluster does not do round-robin load balancing so, as long as Worker 1 is available for requests, it will handle them. Using setTimeout like I suggested will actually make your workers more available to handle requests so it's likely that you'll only see Worker 1 handle requests if you're just hitting the server manually from a browser. You might need some sort of load testing script to see both workers handling requests.

As for the difference you're seeing between F5 and CTRL-F5, my best guess is that your browser is keeping the connection to the server alive so, when you just use F5, it uses the same connection which will always go to the same worker. When you use CTRL-F5 it's actually closing the connection that it had before and therefor could connect to either worker on the next request.