AJAX long-polling : How do I make this code long-polling?

Question

Originally before I understood long-polling, I had this code:

     var updateMessages = function() {
        var conv_id = [];
        var lastMessages = [];

        $('.user').each(function(){
            conv_id.push($(this).find('p').attr('id'));
        });
        if($('.rightP').find('.msg .msg_block').length!=0){
            $('.rightP').find('.msg .msg_block').each(function(){
                if(($('.rightP').find('.msg .msg_block p').length)==0){
                }else {
                    lastMessages.push($(this).find('p:last-child')[0].dataset.created_at);
                }
            });
        }
        $.ajax({
            type: "POST",
            url: 'create/populate',
            data: {
                'from': lastMessages,
                'conv_id':conv_id
            },
            success: function(messages) {
                console.log(messages);
                $.each(messages, function() {
                    appendMessage(this);
                });
            },
            error: function(xhr,textStatus,errorThrown) {
                console.log(xhr.responseText);
            },
            complete: function() {
                window.setTimeout(updateMessages, 2000);
            },
            dataType: 'json'
        });
    };
updateMessages();

However, one person commented that this code isn't long-polling. So I researched and adjusted some codes above like so :

 ...
     complete: function() {
                updateMessages(); //also tried updateMessages;
            },
     timeout:30000,
     dataType: 'json'
...

but it ran into problems such as not polling at all and the messages won't update. How can I adjust my original code to do long-polling? Improvement of code is a bonus. Thank you!

gente note : I don't use web sockets bcoz of legacy browser compatibility issues. I also don't use nodejs because my shared-server does not allow long-running processes.

---

PHP code (in my controller)

public function index()
    {
        $timestamps = Input::get('from'); //timestamp of latest message
        $conv_id = Input::get('conv_id');
        $allMessages = Messages::whereIn('conv_id',$conv_id);
        if(is_null($timestamps)){
           $messages = $allMessages->orderBy('created_at','desc')->take(10);
        }else{
           asort($timestamps);
           $messages = $allMessages->where('created_at','>',end($timestamps));
        }
        return $messages->get()->reverse();
    }

Answer 1

Note that the code below only tries to demonstrate how jQuery + PHP long-polling could work. And may have issues such as performance with concurrency, etc. I'm trying to demonstrate a simple long-polling script based on the question.

To be able to scale and support long-polling properly, I would suggest using technologies like Web Sockets that Darren mentioned, Socket.io, and possibly ReactPHP for alternative to node.js.

Client-side javascipt:

var updateMessages = function() {
    var conv_id = [];
    var lastMessages = [];

    // your code to get conv_id and lastMessages here

    $.ajax({
        type: "POST",
        url: 'create/populate',
        timeout: 30000,
        dataType: 'json',
        data: {
            'from': lastMessages,
            'conv_id': conv_id
        },
        success: function(messages) {
            // your success code here
        },
        error: function(xhr,textStatus,errorThrown) {
            console.log(xhr.responseText);
        },
        complete: function() {
            window.setTimeout(updateMessages, 2000);
        },
    });
};

updateMessages();

Your PHP controller:

public function populate()
{
    $timeout_in_seconds = 20;
    $interval_in_seconds = 5;

    $start_time = time();
    $timeout = false;

    $timestamps = Input::get('from'); //timestamp of latest message
    $conv_id = Input::get('conv_id');

    // While we don't have any new messages, and haven't reached timeout yet.
    while (empty($messages) && !$timeout) {

        $allMessages = Messages::whereIn('conv_id', $conv_id)
            ->orderBy('created_at','desc');

        if (empty($timestamps)) {
           $messages = $allMessages->take(10)->get()->reverse();
        } else {
           asort($timestamps);
           $messages = $allMessages->where('created_at', '>', end($timestamps))->get()->reverse();
        }

        $timeout = (time() - $start_time) > $timeout_in_seconds;

        // If there is message or timeout, break out of the while loop and return the messages immediately.
        if (!empty($messages) || $timeout) {
            break;
        }

        sleep($interval_in_seconds);
    }

    return $messages;
}