Ajax long polling (comet) + php on lighttpd v1.4.22 multiple instances problem.

Question

Hi,

I am new to this site, so I really hope I will provide all the necessary information regarding my question.

I've been trying to create a "new message arrived notification" using long polling. Currently I am initiating the polling request by window.onLoad event of each page in my site.

On the server side I have an infinite loop:

while(1){
 if(NewMessageArrived($current_user))break;
 sleep(10);
}
echo $newMessageCount;

On the client side I have the following (simplified) ajax functions:

poll_new_messages(){
 xmlhttp=GetXmlHttpObject();
 //...
 xmlhttp.onreadystatechange=got_new_message_count;
 //...
 xmlhttp.send();
}

got_new_message_count(){
 if (xmlhttp.readyState==4){
  updateMessageCount(xmlhttp.responseText);
  //...
  poll_new_messages();
 }
}

The problem is that with each page load, the above loop starts again. The result is multiple infinite loops for each user that eventually make my server hang.

*The NewMessageArived() function queries MySQL DB for new unread messages.

*At the beginning of the php script I run start_session() in order to obtain the $current_user value.

I am currently the only user of this site so it is easy for me to debug this behavior by writing time() to a file inside this loop. What I see is that the file is being written more often than once in 10 seconds, but it starts only when I go from page to page.

Please let me know if any additional information might help.

Thank you.

Answer 1

You might try checking connection_aborted() periodically. Note that connection_aborted() might not pick up on the fact that the connection has in fact been aborted until you've written some output and done a flush().

In fact, just producing some output periodically may be sufficient for php to notice the connection close itself, and automatically kill your script.

Answer 2

It seems common for long-polling requests to timeout after 30 seconds. So in your while loop you could echo 'CLOSE' after 30 seconds.

while(!$new_message && $timer < 30){
    $new_message = NewMessageArrived($current_user);
    if(!$new_message) {
     sleep(10);
     $timer += 10;
    }
}
if($newMessageCount) {
    echo $newMessageCount;
} else {
    echo 'CLOSE';
}

---

In the Javascript, you can listen for the CLOSE.

function poll_new_messages(){
    xmlhttp=GetXmlHttpObject();
    //...
    xmlhttp.onreadystatechange=got_new_message_count;
    //...
    xmlhttp.send();
}

function got_new_message_count(){
    if (xmlhttp.readyState==4){
     if(xmlhttp.responseText != 'CLOSE') {
      updateMessageCount(xmlhttp.responseText);
     }
     //...
     poll_new_messages();
    }
}

Now, the PHP will return a response within 30 seconds, no matter what. If you use stays on the page, and you receive a CLOSE, you just don't update the count on the page, and re-ask.

If the user moves to a new page, your PHP instance will stop the loop regardless within 30 seconds, and return a response. Being on a new page though, the XHR that cared about that connection no longer exists, so it won't start up another loop.

Answer 3

I think I found a solution to my problem. I would appreciate if anyone could tell, if this is the technique that is being used in COMET and how scalable this solution.

I used a user based semaphore like this:

$sem_id = sem_get($current_user);
sem_acquire($sem_id);
while(1){
 if(NewMessageArrived($current_user))break;
 sleep(10);
}
sem_release($sem_id);
echo $newMessageCount;