[PHP / AJAX] Simultaneous AJAX requests

[smani]

Hello.
I am trying to make two AJAX requests work simultaneously, a very simplified code is the following:
Server side:


session_start();

function abort(){
$_SESSION['abort']=true;
}

function someTask(){
while(workToDo=true && $_SESSION['abort']==false){
do_some_task();
}
}

Client side:


<script type="text/javascript">

new AJAXRequest().GET('page.php?task=someTask',callbac kfz);
abortBtn.onclick=function(){
new AJAXRequest().GET('page.php?task=abort',callbackfz );
}
</script>


Now, it seems as if the webserver only processes the requests sequentially, i.e. the abort request is only processed once the someTask request has completed... Firefox should allow up to 6 simultaneous http connections with a server, is it therefore a problem/limitation with apache / php ? Also I was wondering, should the AJAX calls be indeed processed simultaneously, are session variables dynamic inside the php instance? I.e. if another instance changes a session variable, will it also be changed immediately in any other running instance?

Thanks for any inputs!
smani

[DocForbin]

Pretty sure the session thing isn't going to work for the reason you already suspect. You could set a flag in a database or write a temp file to check for (then delete the file after the do work loop).

[DocForbin]

or write the temp file first, add a check for it in your loop, and delete it in your abort function.

[smani]

Thanks for your answer! That issue seems tackable, though I am still in the dark on why the requests are processed serially. :S

[DocForbin]

what makes you think they are?

[smani]

My complete abort function is the following:


if($task=="abort"){
$taskID=$_GET['taskID'];
if(!isset($_SESSION["t$taskID"])){echo "0\nThe specified operation does not exist";}
else if($_SESSION["t$taskID"]['working']!=true){unset($_SESSION["t$taskID"]);echo "0\n";}
else{
$_SESSION["t$taskID"]['aborted']=true;
$timeout=0; // timeout for 15 seconds
while($_SESSION["t$taskID"]['working']==true && $timeout<15){
sleep(1);
++$timeout;}
if($timeout==15){echo "51\nTimeout trying to abort task.";}
else{
unset($_SESSION["t$taskID"]);
echo "0\n";}}
exit(0);
}


If I look at the return text, I get "The specified operation does not exist", which can only happen after the work-task has ended, as I designed tasks to work as follows:
- get a TaskID via AJAX and return it to the client:


if($task=="getid"){
$taskID=rand();
while(isset($_SESSION["t$taskID"])){$taskID=rand();}
$_SESSION["t$taskID"]['aborted']=false;
$_SESSION["t$taskID"]['working']=true;
$_SESSION["t$taskID"]['completed']=0;
echo "0\n".$taskID;
exit(0);
}

- start the task itself identified by the taskID by with another AJAX request, example:


if($task=="sometask"){
$taskID=$_GET['taskID'];
$items=json_decode($_POST['items']);
for($i=0;$i<count($items->{'items'});++$i){
if($_SESSION["t$taskID"]['aborted']==true){$_SESSION["t$taskID"]['working']=false;echo "52\nAborted";exit(0);}
//Some task
}
echo "0\n";
unset($_SESSION["t$taskID"]);
exit(0);
}

- a progess window is shown on the client side with a cancel button, which, if pressed, executes the above abort function with a separate AJAX request.
- when the task ends on server side, all session variables concerning that taskID are unset.

Therefore even if session variables were not dynamic in the sense I meant above, the taskID would exist from when the ID is created to when the task has ended, hence the only possibility I see is that the requests run sequentially. Also, if I try to load any page on the same server while the task is executing, it won't load until the task is complete.

[DocForbin]

Probably its just a problem with your code/testing, but I guess you could have the MaxClients directive set to 1 in apache.

[smani]

cat apache2.conf | grep Max
# MaxKeepAliveRequests: The maximum number of requests to allow
MaxKeepAliveRequests 100
# MaxSpareServers: maximum number of server processes which are kept spare
# MaxClients: maximum number of server processes allowed to start
# MaxRequestsPerChild: maximum number of requests a server process serves
MaxSpareServers 10
MaxClients 150
MaxRequestsPerChild 0
# MaxClients: maximum number of simultaneous client connections
# MaxSpareThreads: maximum number of worker threads which are kept spare
# MaxRequestsPerChild: maximum number of requests a server process serves
MaxClients 150
MaxSpareThreads 75
MaxRequestsPerChild 0

Seems okay .. I'll do some more testing tomorrow, in the meantime thanks for your replies!

[DocForbin]

I haven't looked at your code, but write a test script that takes a while to finish, e.g

<?php
sleep(120);
?>

and access a 2nd test script, e.g.

<?php
phpinfo();
?>

while the first is running. If the 2nd waits for the first to complete then it is some strange configuration issue. Otherwise the problem is your code/testing...

[smani]

Okay, got it!
Seems like session variables make AJAX request run sequentially, i.e. my test code:
Client:


...
<script type="text/javascript">
function AJAX1(el){
AJAXFetch(new AJAXObj(), 'GET',true,'test.php?task=task1',null,function(tex t){el.value=text;});
}
function AJAX2(el){
AJAXFetch(new AJAXObj(), 'GET',true,'test.php?task=task2',null,function(tex t){el.value=text;});
}
</script>
...
<input type="button" value="AJAX Request 1" onclick="AJAX1(this);"/>
<input type="button" value="AJAX Request 2" onclick="AJAX2(this);"/>

Server:


<?php
session_start();
$task=$_GET['task'];
if($task=="task1"){
$_SESSION['test']=1;
sleep(10);
echo "Task 1 OK, ".$_SESSION['test'];}
else if($task=="task2"){
$_SESSION['test']=2;
echo "Task 2 OK, ".$_SESSION['test'];}
?>

Now, if I comment out the three session-related statements (session_start and the two $_SESSION['test'] assignments), the requests run in parallel as expected. As soon as there is a session related statement, they run sequentially, and the result is indeed that the variables are not synchronized, i.e. AJAX1 returns "Test 1 OK, 1", AJAX2 returns "Test 2 OK, 2".
Good news that there is an explanation, bad news because I sort of need session variables for the scripts to work, even basic ones such as $_SESSION['workdir']...

[smani]

Right, in case someone want's more info on the topic, here is a good explanation: http://ch2.php.net/manual/en/ref.session.php#64525