Does streaming an http response block nodejs?

Question

I'm trying to understand the limits of nodejs when dishing lots of content under load. Specifically I want to know if streaming a lengthy response to a client will block.

I've created a very simple test setup where node just responds to every request with a stream that's pulling from a 1GB http download. Here's my code:

var http = require('http');
var fs = require('fs');

var iterator = 0;

http.createServer(function(req, res) {

    console.log('req received ', iterator++);

    var url = 'http://download.thinkbroadband.com/1GB.zip';

    http.get(url, bigFile => {

        res.writeHead(200, {
            'content-type': 'application/zip',
            'content-length': bigFile.headers['content-length'],
        });

        bigFile.pipe(res);

    });

}).listen(8003);

So I launched this node server and hit the endpoint with several tabs in my browser. What was interesting was that subsequent responses don't immediately log with the console.log('request received ', iterator++); code. Instead there's a delay of 5 to 10 seconds before that initial event is logged.

This is strange to me because if streaming an http response is blocking then it should wait until the first request is complete before accepting the second. If the streaming doesn't block then I would expect to see all requests logged immediately after they're requested.

Can someone explain this?

I'd also love to hear any thoughts about performance here. Node probably isn't really built for this sort of thing. The download speed really suffers with multiple requests.

score 2 · Accepted Answer · answered Feb 28 '17 at 21:55

After some more research I can see how the different phases of the event loop could cause the results I'm seeing. The node guides explain how the poll phase which executes all the I/O events will block if needed and it also has a maximum stack so it doesn't totally block the main thread for too long.

This would explain the behavior I saw where having lots of I/O events (even if I wasn't listening to them directory with on('data')...) could block your node app temporarily. I saw about a 5 to 10 second delay after the first request before anything else could get through.

Of course there are other things that could be done to improve performance like using the Cluster module to use all available processors on the machine. However, at the end of the day node probably isn't the best solution here. You'd get much better performance from a traditional web server with all the threads you need. Even after using all the processors I was only pushing about 6% CPU utilization with node.

Does streaming an http response block nodejs?

1 Answers1