title: What does "xhr-polling" config do in socket.io?
body: <p>I have a node.js server with socket.io:</p>
  
  <pre><code>var io = require('socket.io').listen(app);
  
  // assuming io is the Socket.IO server object
  io.configure(function () { 
        io.set("transports", ["xhr-polling"]); 
        io.set("polling duration", 10); 
      });
  
  io.sockets.on('connection', function(socket){
    console.log('connected: %s', socket.id);
    ...
  }
  </code></pre>
  
  <p>With xhr-polling and a polling duration of 10 seconds, does this mean that a new connection will be invoked every 10 seconds? If so, how can I keep track of users if they keep disconnecting? I'm running node.js on heroku.</p>