Does PHP block requests?

I have a tricky problem.

I'm trying to make a progress feedback mechanism to keep users informed about a
slowish process on the server end of a web app. The backend is generating a PDF
file from a bunch of data which extends to many pages. I can keep tabs on the
progress of this generation by working out how many lines of data is present,
and how much of it has been processed. I use that information to set a session
variable with a unique name - so for each step I set

$_SESSION['some-unique-name']=>Array('total'=>$totalLines,'c ount'=>$linesProcessedSoFar);

Now on the front end I'm doing an AJAX call to a script that just encodes this
session variable into a JSON string and sends it back.

The problem is that while the PDF is being generated, the AJAX calls to get the
progress data (on a 1-second interval) are being blocked and I don't get why.
The PDF generation is also triggered by an AJAX call to a script which generates
the PDF in a given file location, then returns a URL to retrieve it with.

So it appears that the problem is that I can't have two AJAX calls to different
PHP scripts at the same time? WTF?

Checking the requests with Wireshark confirms the the browser is certainly
sending the progress calls while the original PDF call is waiting, so it's not
the browser side that's the problem: and once the PDF call is finished the
outstanding progress calls are all serviced (returning 100% completion of course
- not much use!) Different browsers (Firefox, IE, Chrome at least) give the same
result.

For reference, the server is Apache 2.2.10 on a SuSE linux 11.1 box using
mod_php5 and mpm_prefork - is that part of the problem, and is there an alternative?
--
Peter Ford phone: 01580 893333
Developer fax: 01580 893399
Justcroft International Ltd., Staplehurst, Kent

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RE: Does PHP block requests?

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Re: Does PHP block requests?

Ford, Mike wrote:
>> -----Original Message-----
>> From: Peter Ford [mailto:pete@justcroft.com]
>> Sent: 20 November 2009 15:18
>> To: php-general@lists.php.net
>> Subject: [PHP] Does PHP block requests?
>>
>> I have a tricky problem.
>>
>> I'm trying to make a progress feedback mechanism to keep users
>> informed about a
>> slowish process on the server end of a web app. The backend is
>> generating a PDF
>> file from a bunch of data which extends to many pages. I can keep
>> tabs on the
>> progress of this generation by working out how many lines of data is
>> present,
>> and how much of it has been processed. I use that information to set
>> a session
>> variable with a unique name - so for each step I set
>>
>> $_SESSION['some-unique-
>> name']=>Array('total'=>$totalLines,'count'=>$linesProcessedS oFar);
>
> The progress-counter Ajax script uses a session -- but does the PDF generator use the same session? If so, I'm guessing you don't manually close the session at any point and all the progress-checking calls are blocked until the session auto-commits at the end of the generator script. To counter this, you need to manually session_commit() somewhere near the beginning of the generator script.
>
> Cheers!
>
> Mike
> --
>
You're right about trying to use the same session - that was the plan to get the
progress state passed across from one call to the other.
Closing the session on the generator script has the effect of stopping any
further changes to the session variable, so I get the initial value at each
request after that.

I put together a simple "Generator" script to test this:

session_commit();
header("Content-type: text/xml");
$fcode = 'foo';
$_SESSION[$fcode]=Array('total'=>10,'count'=>0);
for ($i=0; $i<10; $i++)
{
sleep(1);
$_SESSION[$fcode]['count']++;
}
echo "

Done

RE: Does PHP block requests?

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