Question

I want to create some kind of AJAX script or call that continuously will check a MySQL database if any new messages has arrived. When there is a new message in the database, the AJAX script should invoke a kind of alert box or message box.

I’m not quite a AJAX expert (yet anyway) and have Googled around to find a solution but I’m having a hard time to figure out where to begin. I imagine that it is kind of the same method that an AJAX chat is using to see if any new chat-message has been send.

I’ve also tried to search for AJAX (httpxmlrequest) call in a continuously and infinity loop but still haven’t got a solution yet.

I hope there is someone, which can help me with such a AJAX script or maybe nudge me in the right direction.

Thanks
Sincerely
Mestika

Answer 1

Step 1 – You need a server-side page that you can call that checks to see if something new has arrived.

Step 2 – You could adapt the sequential AJAX request script from here (it uses jQuery to simplify the AJAX requests):

http://www.stevefenton.co.uk/Content/Blog/Date/201004/Blog/AJAX-and-Race-Conditions/

Currently, this script is for queuing a list of sequential AJAX requests, but you could use it to continually check by changing it like this…

var InfiniteAjaxRequest = function (uri) {
    $.ajax({
        url: uri,
        success: function(data) {
            // do something with "data"
            if (data.length > 0) {
                alert(data);// Do something sensible with it!
            }
            InfiniteAjaxRequest (uri);
        },
        error: function(xhr, ajaxOptions, thrownError) {
            alert(thrownError);
        }
    });
};

InfiniteAjaxRequest ("CheckForUpdate.php");

What are the benefits of using this script?

Well, rather than checking every “x” seconds, it will only check once the previous request has been received, so it chains the requests. You could add in a delay to throttle this constant request, which I would highly recommend you do – otherwise you will be hitting your site with way too much traffic. You would add that delay in AFTER you’ve done something with the response, but BEFORE you call back into “InfiniteAjaxRequest”.