Ajax/PHP long-polling results in 503 error - php

Question

I'm playing around with AJAX long-polling and trying to read/update a simple counter (number) value in an MySQL cell by clicking on a button.
The PHP creates an infinite while loop and is checking if the value in the according cell has been modified (MySQL "current_timestamp", UNIX). If it has and the current_timestamp value is bigger then the timestamp the AJAX call was made, it's breaking the loop and sending the updated value and the updated current_timestamp to the client. The AJAX processes the data.
The Problem: It works but after a while I get a 503 error. I guess it's obviously the while loop or other open connections through multiple windows in other browsers (for testing).
PHP-File text.php:
// Connect to database
$con = mysql_connect('XX', 'XX', 'XX');
if (!$con)
{
die('Error' . mysql_error());
}
mysql_select_db('X', $con);
// Get data
$query = mysqli_query("SELECT counter_value, last_modified FROM content WHERE X = 'X' ORDER BY X DESC");
// Start infinite loop
set_time_limit(0);
while (true)
{
// The timestamp of the last ajax call = the last modified timestamp
$last_ajax_call = $_GET['timestamp'];
clearstatcache();
// Get the value of the counter and the last modified timestamp
while($row = mysql_fetch_array($query))
{
$counter_value = $row['counter_value'];
$last_modified= strtotime($row['last_modified']);
}
// If the time of the last modified timestamp is bigger/later than the last ajax call
if ($last_modified > $last_ajax_call)
{
$result = array(
'counter_value' => $counter_value,
'timestamp' => $last_modified
);
$json = json_encode($result);
echo $json;
break;
// If not, try again in 3 seconds
} else
{
sleep(3);
continue;
}
}
// Close database
mysql_close($con);
AJAX Part in js-File:
function getContent()
{
// get timestamp of last modified stored in attribute. The initial/first timestamp attribute is set beforehand.
var timestamp = $('#timestamp').attr('data-timestamp');
$.ajax(
{
type: 'GET',
url: 'test.php',
async: true,
cache: false,
data: {timestamp:timestamp},
success: function(data){
var obj = jQuery.parseJSON(data);
$("#counter").text(obj.counter_value);
$("#timestamp").attr("data-timestamp", obj.timestamp);
getContent();
}
}
);
}
getContent();
So the result is a 503 error which goes away after ca. 10 Minutes and it's working again.
(Any typos/formatting might be the result of cleaning up the code.)
I just started learning PHP and JS, so there might be a few newbie mistakes or weird lines in there, please be nice. Any advice on optimising the code is very appreciated!

Answer 1

It dies because PHP doesn't work the way you think it does.
You have intentionally put an infinite loop into your php in the assumption it will make your code keep looping around and rechecking the next GET request on each loop.
The reality is that the code is executed once for each request, and until execution of the code completes the server doesn't respond.
AJAX long polling requires no special handling in php, it's just an AJAX request in a loop. You might want to include a slight delay in your AJAX code otherwise your server will be hammered with requests.
To be honest this isn't what long polling is for, the idea of it is to update pages without any user interaction to display unread message notifications etc.
If you want to monitor user events like button clicks then bind your AJAX function to the clicking of the button.