Is this a true long polling?

Question

After a lot of trials, I am successfully able to maintain continuous server connection with a database.

Now code keep checking and show the messages, if there are new in database.

Please review and tell:

if there is true long polling technique used in this code? If it is not, then please suggest, where I am wrong(deviating from long polling) and how this can be made a true long polling.

Currently, I am getting these errors. However still it maintains the continous connection with database.

**each time only one message is pulled instead of all **(I used each loop but it stops the long polling)

After every 10/15 seconds, token error appeares (Parse error (syntax error=unexpected token)).

var last_msg_id = 2;

function load_msgs() {
  $.ajax({
    type:"Post",
    url:"getdata.php",
    data:{
      last_msg_id:last_msg_id
    },
    dataType:"json",
    async:true,
    cache:false,
    success:function(data) {
      var json = data;
      $("#commidwin").append(json['msg']);
      last_msg_id = json["last_msg_id_db"];
      setTimeout("load_msgs()", 1000);
    },
    error:function(XMLhttprequest, textstatus, errorthrown) {
      alert("error:" + textstatus + "(" + errorthrown + ")");
      setTimeout("load_msgs()", 15000);
    }
  });
}

Php file is here

$last_msg_id=$_POST['last_msg_id'];
$last_msg_id_db=1;

while($last_msg_id>$last_msg_id_db){
    usleep(10000);
    clearstatcache();

    $sql=mysqli_query($db3->connection,"SELECT * FROM chat_com where id>'$last_msg_id' ORDER by id ASC");

    $sql_m=mysqli_query($db3->connection,"SELECT max(id) as maxid  FROM chat_com");
    $row_m=mysqli_fetch_array($sql_m);
    $last_msg_id_db=$row_m['maxid'];

    while($row=mysqli_fetch_array($sql)){
        $textt=$row['mesg'];

        $last_msg_id_db=$last_msg_id_db;
        $response=array();
        $response['msg']=$textt;
        $response['last_msg_id_db']=$last_msg_id_db;
    }
}

echo json_encode($response);

@jgauffin i have updated the error name.plz review.Is this a true long polling and scalable(upto 10000 users?) — Aana Saeed, Dec 07 '12 at 10:24

score 1 · Answer 1 · answered Dec 07 '12 at 15:46

Polling is a bit harder than a simple while : just because generally all things you output to the browser will be interpreted when complete. Your example is quite clear :

success:function(data) {
    var json = data;
    $("#commidwin").append(json['msg']);
    last_msg_id = json["last_msg_id_db"];
    setTimeout("load_msgs()", 1000);
},

jQuery will wait until the response is complete to build your data variable and then will call your success callback.

One way to create long-polling is to have a task and a follower :

the task is the "infinite" loop, it displays nothing but just catch and trigger events, put in a "box".
the follower is an ajax call made every X seconds, it looks inside the "box" filled by the task, and immediately act inside the page.

Here is an example of long-polling, there is no follower, just an event (release) that stops the poll, but you'll get the idea :

<?php

// For this demo
if (file_exists('poll.txt') == false)
{
    file_put_contents('poll.txt', '');
}

// If this variable is set, a long-polling is starting...    
if (isset($_GET['poll']))
{

    // Don't forget to change the default time limit
    set_time_limit(120);

    date_default_timezone_set('Europe/Paris');
    $time = time();

    // We loop until you click on the "release" button...
    $poll = true;
    $number_of_tries = 1;
    while ($poll)
    {
        // Here we simulate a request (last mtime of file could be a creation/update_date field on a base)
        clearstatcache();
        $mtime = filemtime('poll.txt');

        if ($mtime > $time)
        {
            $result = htmlentities(file_get_contents('poll.txt'));
            $poll = false;
        }

        // Of course, else your polling will kill your resources!
        $number_of_tries++;
        sleep(1);
    }

    // Outputs result
    echo "Number of tries : {$number_of_tries}<br/>{$result}";
    die();
}

// Here we catch the release form
if (isset($_GET['release']))
{
    $data = '';
    if (isset($_GET['data']))
    {
        $data = $_GET['data'];
    }
    file_put_contents('poll.txt', $data);
    die();
}
?>

<!-- click this button to begin long-polling -->
<input id="poll" type="button" value="Click me to start polling" />

<br/><br/>

Give me some text here :
<br/>
<input id="data" type="text" />
<br/>

<!-- click this button to release long-polling -->
<input id="release" type="button" value="Click me to release polling" disabled="disabled" />

<br/><br/>

Result after releasing polling :
<div id="result"></div>

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js"></script>
<script type="text/javascript">

    // Script to launch polling
    $('#poll').click(function() {
        $('#poll').attr('disabled', 'disabled');
        $('#release').removeAttr('disabled');
        $.ajax({
            url: 'poll.php',
            data: {
                poll: 'yes' // sets our $_GET['poll']
            },
            success: function(data) {
                $('#result').html(data);
                $('#poll').removeAttr('disabled');
                $('#release').attr('disabled', 'disabled');
            }
        });
    });

    // Script to release polling
    $('#release').click(function() {
        $.ajax({
            url: 'poll.php',
            data: {
                release: 'yes', // sets our $_GET['release']
                data: $('#data').val() // sets our $_GET['data']
            }
        });
    });

</script>

Is this a true long polling?

1 Answers1

Demonstration : here.

Linked