Blocking on http request/response callback?

Question

I'm more or less a node newbie and having a bit of difficulty understanding the the asynchronism aspect of the request / response callback for the http.createServer method.

My understanding was that when a new request is made the anonymouse function would run again for the new client.

However in my testing I've found that the blocking process looks like it effects the response for another requesting client.

I say this because the log message "A new client!" only apears after the first request has finished.

var http = require('http');

http.createServer(function(req, res){

  console.log("A new client!");      

  var startTime = new Date().getTime();
  while (new Date().getTime() < startTime + 4000);

  res.writeHead(200, {"Content-type" : "text/html"});
  res.write("done");
  res.end();

}).listen("8888");

I am testing on localhost:8888 with multiple tabs in my browser.

I suspect strongly that "console.log()" is itself not necessarily synchronous, but I'm not a Node person. What I mean by that is that the visible result of calling "console.log()" is not necessarily seen immediately. That's certainly how the Chrome console works but I don't know whether that fact is really relevant to the Node environment, because they're obviously significantly different. — Pointy
– Pointy, Commented Dec 18, 2011 at 16:43
Of course you could check by including a request parameter in the log message ... — Pointy
– Pointy, Commented Dec 18, 2011 at 16:44
@Pointy console.log is asynchronous, it will only print after the event loop is freed. console.error/console.warn is blocking. — Raynos
– Raynos, Commented Dec 18, 2011 at 17:36

alessioalex · Accepted Answer · 2011-12-18 20:04:44Z

That's not related to blocking, but to the fact that Node.js has an event loop, which means it has events to ready to be executed on the next tick each time.

These events are executed in order and can have callbacks. Your code above starts a web server that has a callback which is run when the request has been made, and after that in the callback you display a message (in the console).

You can check what I've said above (about events being in order) with the following code:

server.js

var counter = 0;
var http = require('http');
http.createServer(function (req, res) {
  console.log(counter++);
  res.writeHead(200, {'Content-Type': 'text/plain'});
  res.end('Hello World\n');
}).listen(1337, "127.0.0.1");
console.log('Server running at http://127.0.0.1:1337/');

Use the Apache Benchmark tool to simulate 100 concurrent connections for that server like so:

ab -n 100 -c 100 server.js

You'll see that you'll get the numbers in order.

Resources:

http://www.slideshare.net/brevoortm/nodejs-async-for-the-rest-of-us
http://code.danyork.com/2011/01/25/node-js-doctors-offices-and-fast-food-restaurants-understanding-event-driven-programming/
http://www.yuiblog.com/blog/2010/12/06/video-yuiconf2010-croucher/

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