Why is setTimeout blocking my Node.js app?

Question

Take this code, typical node js example of a Http server, to which I've added a delay of 5 seconds to simulate some async work taking place somewhere else:

const http = require('http');

const hostname = '127.0.0.1';
const port = 8080;

http.createServer((req, res) => {
  setTimeout(()=>{
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('Hello World\n');
  },5000);
}).listen(port, hostname, () => {
  console.log(`Server running at http://${hostname}:${port}/`);
});

What I would expect is that when I open 5 tabs, let's say with a delay of half a second between opening each, the server should "respond" to each tab more or less with this timings:

t=0s - I open first tab
t=0.5s - I open second tab
t=1s - I open third tab
...
t=5s - First tab stops loading, server has replied
t=5.5s - Second tab stops loading, server has replied
t=6s - Third tab stops loading, server has replied
t=6.5s - Fourth tab stops loading, server has replied
t=7s - Fifth tab stops loading, server has replied

However, the behaviour I'm seeing is the following:

t=0s - I open first tab
t=0.5s - I open second tab
t=1s - I open third tab
...
t=5s - First tab stops loading, server has replied
t=10s - Second tab stops loading, server has replied
t=15s - Third tab stops loading, server has replied
t=20s - Fourth tab stops loading, server has replied
t=25s - Fifth tab stops loading, server has replied

As if the subsequent requests weren't starting to run until the first one has finished. Am I missing something here? I thought the whole point of Node JS was to be able to run async taks from a single thread?

Answer 1

The problem isn't your code or Node.js -- it's how you set up your test.

You wrongly assumed that your browser would make 5 concurrent requests, which is not happening. Different browsers have different behaviors, but typically browsers limit to a very low number the maximum amount of simultaneous connections to a single origin. The HTTP spec gives a suggest maximum. I was actually quite surprised to see Chrome only opening 1 single connection to localhost, as I know Chrome opens 6 to other origins -- just learned something new!

Use a different tool to run your test, one that you can control and know for sure that it's making concurrent requests. Then you'll see the expected behavior.

As an example, I ran your code and tested with Apache Benchmark as shown below. The parameters indicate: -n 10 is to make 10 requests, -c 10 is to use concurrency 10 (ie, make all the ten requests concurrently). As you can see in the results below, the total time taken for all the requests was ~5s (and "time per request" 0.5s):

~ $ ab -n 10 -c 10 http://127.0.0.1:8080/
This is ApacheBench, Version 2.3 <$Revision: 1663405 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking 127.0.0.1 (be patient).....done


Server Software:
Server Hostname:        127.0.0.1
Server Port:            8080

Document Path:          /
Document Length:        12 bytes

Concurrency Level:      10
Time taken for tests:   5.019 seconds
Complete requests:      10
Failed requests:        0
Total transferred:      1130 bytes
HTML transferred:       120 bytes
Requests per second:    1.99 [#/sec] (mean)
Time per request:       5019.151 [ms] (mean)
Time per request:       501.915 [ms] (mean, across all concurrent requests)
Transfer rate:          0.22 [Kbytes/sec] received

Connection Times (ms)
              min  mean[+/-sd] median   max
Connect:        0    0   0.1      0       0
Processing:  5017 5018   0.3   5018    5018
Waiting:     5008 5008   0.2   5008    5009
Total:       5018 5018   0.2   5019    5019
ERROR: The median and mean for the total time are more than twice the standard
       deviation apart. These results are NOT reliable.

Percentage of the requests served within a certain time (ms)
  50%   5019
  66%   5019
  75%   5019
  80%   5019
  90%   5019
  95%   5019
  98%   5019
  99%   5019
 100%   5019 (longest request)