Q:

How long polling works

I am studying the ajax long polling but I am confused. what is different in traditional ajax calls and long polling 
   var lpOnComplete = function(response) {
   alert(response);
   // do more processing
   lpStart();
  };

    var lpStart = function() {
    $.post('/path/to/script', {}, lpOnComplete, 'json');
    };

    $(document).ready(lpStart);

this example is just calling in recursive manner to the server.. what is different than the traditional call in setInterval..

A:

As the name suggest Long Polling means polling something for a long time.
$.post('/path/to/script', {}, lpOnComplete, 'json');

Here is what the actual process starts, You make an ajax call to some script on server, in this case its /path/to/script , You need to make your server script(php for example) smart enough so that it only respond to request's when required data is available, the script should wait for a specified time period(for example 1 minute) and if no data available upto 1 minute then it should return without data.
As soon as server return something, in your callback function you again make an ajax call to the same script and the server script again continues the process.
Consider a chat application, In conventional way you are polling the server say every 2 second's and the server return's even if no messages are available.If upto one minute server get's no new messages for you, you end up hitting the server 30 times in last one minute.
Now consider Long Polling way, you set your server script to wait for one minute for the new messages. From the client, you make a single ajax call to your script and say no messages are arriving for next one minute, server will not respond until 1 minute. And you have hit the server just one time in last 1 minute. Can you imagine 30 Hit Vs 1 Hit

A:

In theory with setinterval, you could have overlapping processing,
so if one oncomplete handler takes particularly long, it may overlap with the next call, slowing down your system or resolving in unpredictable ways.
by explicitly starting the next poll after the first one has completed, you get less regular calls with the advantage that you're guaranteed to only doing as one unit of work at a time as a byproduct.

