Difference between a normal ajax and long polling

Question

I\'m trying to understand more about long polling to \"manipulate\" a website in real time, saw some videos and I\'m thinking so far:

Say I have an old date that the

Answer 1

Since your initial question was what the difference between the two techniques is, I will start with this:

AJAX polling

Using AJAX polling to update a page will mean, that you send a request in a defined interval to the server, which would look like this:

The client sends a request to the server and the server responses immediately.

A simple example (using jQuery) would look like this:

setInterval(function(){
    $('#myCurrentMoney').load('getCurrentMoney.php');
}, 30000);

The problem with this is, that this will cause a lot of useless requests since there won't be always new things on every request.

AJAX long polling

Using AJAX long polling will mean, that the client sends a request to the server and the server waits for new data to be available before he responds. This would look like this:

The client sends a request and the server responds "irregularly". As soon as the server responds, the client will send a new request to the server.

The client side would look like this:

refresh = function() {
    $('#myCurrentMoney').load('getCurrentMoney.php',function(){
        refresh();
    });
}

$(function(){
    refresh();
});

What this will do is just load the getCurrentMoney.php's output into the current money element and as soon as there is a callback, start a new request.

On the server side you usually use a loop. To solve your question how the server will know, which are new publications: either you pass the timestamp of the newest to the client available publication to the server or you use the time of the "long polling start" as indiactor:

<?
$time = time();

while ($newestPost <= $time) {
    // note that this will not count as execution time on linux and you won't run into the 30 seconds timeout - if you wan't to be save you can use a for loop instead of the while
    sleep(10000);
    // getLatestPostTimestamp() should do a SELECT in your DB and get the timestamp of the latest post
    $newestPost = getLatestPostTimestamp();
}

// output whatever you wan't to give back to the client
echo "There are new posts available";

Here we won't have "useless" requests.