jquery_traditional_polling_2_ways.js

AJAX Traditional Polling.js

/**
 * Traditional Polling
 */


// (1) The setInterval Technique

// Now, if you needed to poll your web service, your first instinct
// might be to do something like this with JavaScript and jQuery:

setInterval(function(){
    $.ajax({ url: "server", success: function(data){
        //Update your dashboard gauge
        salesGauge.setValue(data.value);
    }, dataType: "json"});
}, 30000);

// Here, we have the poll ready to execute every thirty (30) seconds. This code is pretty good.
// It's clean and asynchronous. You're feeling confident. Things will work (and they will), 
// but with a catch. What if it takes longer than thirty (30) seconds for the server to return your call?

// That's the gamble with using setInterval. Lag, an unresponsive server or a whole host of network issues
// could prevent the call from returning in its allotted time. At this point, you could end up with a bunch
// of queued Ajax requests that won't necessarily return in the same order.


// (2) The setTimeout Technique

// If you find yourself in a situation where you're going to bust your interval time,
// then a recursive setTimeout pattern is recommend:

(function poll(){
   setTimeout(function(){
      $.ajax({ url: "server", success: function(data){
        //Update your dashboard gauge
        salesGauge.setValue(data.value);

        //Setup the next poll recursively
        poll();
      }, dataType: "json"});
  }, 30000);
})();

// Using the Closure technique, poll becomes a self executing JavaScript function
// that runs the first time automatically. Sets up the thirty (30) second interval.
// Makes the asynchronous Ajax call to your server. Then, finally, sets up the next poll recursively.

// As you can see, jQuery's Ajax call can take as long as it wants to. So, this pattern
// doesn't guarantee execution on a fixed interval per se. But, it does guarantee that
// the previous interval has completed before the next interval is called.

// Both techniques suffer from the same flaw - a new connection to the server must be opened each time
// the $.ajax method is called. To make that connection, your realtime app must gear up and battle through
// hoards of competing network traffic to make it to your server.