What happens when a single request takes a long time with these non-blocking I/O servers?

Question

With Node.js, or eventlet or any other non-blocking server, what happens when a given request takes long, does it then block all other requests?

Example, a request c

Answer 1

Here is some code to actually see the blocking / non-blocking in action:

• With this example (long CPU-computing task, non I/O):

  var net = require('net');
  handler = function(req, res) {
      console.log('hello');
      for (i = 0; i < 10000000000; i++) { a = i + 5;  }
  }
  net.createServer(handler).listen(80);
  

  if you do 2 requests in the browser, only a single hello will be displayed in the server console, meaning that the second request cannot be processed because the first one blocks the Node.js thread.

• If we do an I/O task instead (write 2 GB of data on disk, it took a few seconds during my test, even on a SSD):

  http = require('http');
  fs = require('fs');
  buffer = Buffer.alloc(2*1000*1000*1000);
  first = true;
  done = false;
  
  write = function() {
      fs.writeFile('big.bin', buffer, function() { done = true; });
  }
  
  handler = function(req, res) {
      if (first) {
          first = false;
          res.end('Starting write..')
          write();      
          return;
      }
      if (done) { 
          res.end("write done."); 
      } else {  
          res.end('writing ongoing.'); 
      }
  }
  
  http.createServer(handler).listen(80);
  

  here we can see that the a-few-second-long-IO-writing-task write is non-blocking: if you do other requests in the meantime, you will see writing ongoing.! This confirms the well-known non-blocking-for-IO features of Node.js.