how does node.js clustering work?

Question

i am trying to understand the following piece of code that is use to create miltiple servers to make use of a multi core cpu.

 var cluster = require("cluster"); 
 var http = require("http"); 
 var numCPUs = require("os").cpus().length; 
 var port = parseInt(process.argv[2]); 

 if (cluster.isMaster) {
     for (var i = 0; i < numCPUs; i++) { 
         cluster.fork(); 
     } 

     cluster.on("exit", function(worker, code, signal) { 
         cluster.fork();
     }); 
 } else { 
     http.createServer(function(request, response) { 
         console.log("Request for: " + request.url); 
         response.writeHead(200); 
         response.end("hello world\n"); 
     }).listen(port); 
 }

my question is, given every created server listens on same port, what guarantees that a request won't be served by more than one server?

Answer 1

In node v0.10, the OS kernel always chooses which child gets the request. In node v0.11+ and io.js v1.0.0+, manual round-robin scheduling is used (except on Windows for now). This default behavior is configurable by setting an environment variable though.