is progress bar possible with php and javascript using ajax

Question

I was wondering how to make progress bar like gmail.

I tried

<script src="jquery.js"></script>
<script>
$(function (){
    $.ajax({
        url: 'index.php',
        success: function(data) {
            $('#bar').html(data);
        }
    });
})
</script>
<div id="bar"></div>

And on index.php

[EDIT]: by sleep() i just meant to simulate continuous stream of output like multithreaded programs which is not supported in php.

<?php

for($i=0; $i<=10; $i++)
{
    sleep(1);
    echo "$i";
}

it seems that output is echoed out at once so i get result 012345678910 at once.

also i tried

setInterval(function (){
        $.ajax({
            url: 'index.php',
            success: function(data) {
                $('#bar').html(data);
            }
        });
    }, 1000);

Instead, i had trouble maintaining value of 'progress', so i did

<?php

session_start();

if(isset($_SESSION['value'])){
    if($_SESSION['value'] >= 10)
    {
        unset($_SESSION['value']);
    }
    else
    {
        $_SESSION['value']++;
    }
}
else
{
    $_SESSION['value'] = 0;
}

echo $_SESSION['value'];

as part of my php. But it seems that, i am calling ajax function on continuous interval.

My Question is,

1. How does google use progress bar, while loginng in gmail. Do they get continuos 'stream' of data like i tried on my first example or send (regularly) request on some url (though not through ajax .. through JSONP or whatever) and upadate the page like second ?

2. Can I do same with php , even if not with php, can I do it using javascript and other server side scripting language where multithreading is supported?

Answer 1

I'm not sure what exactly Gmail does for the progressbar, but you can achieve something similar in PHP, although it may be a bit tricky.

First, to explain why your examples don't work:

If you echo and sleep, like your first example, it will never work. Ajax performs a full request - that is, if the response does not finish, it will wait. When you loop and sleep, the request is not "closed" until the PHP script has finished executing.

If you use session, like in the other example, the problem becomes the session store. The store is typically locked during script execution, and it will not update itself to allow for the type of progress check you want.

What you could do is write progress to a file (or to a database) manually. For example:

file_put_contents('progress.txt', 1);

Then, have another script read the file and output the contents.

This should work, because file_put_contents opens, writes and closes the file.

Using some other language than PHP would make it easier. Being multithreaded would possibly make it easier, but is not a requirement. However, having a continuously running process would make it simpler (PHP only runs a process for the duration of your request and then exits)