In a web application many jQuery+Ajax+PHP processes not running parallel in the same client

Question

In a web application that interacts with one server, there is a jQuery Ajax call with a PHP script that takes about 20 seconds to execute.

$(document).ready(function() {
    $.ajax({
        type: 'POST',
        dataType: 'json',
        url: 'createPDF.php',
        data: str,
        success: function(response) {
            showPDF(response);
        }
        error: function() {
            console.log('ERROR WHEN CREATING PDF');
        }
    });
});

In the meantime a user ask to the application to get a list (getList.php) through another jQuery Ajax independant from createPdf.

$(document).ready(function() {
    $.ajax({
        type: 'POST',
        dataType: 'json',
        url: 'getList.php',
        data: str,
        success: function(response) {
            showList(response);
        }
        error: function() {
            console.log('ERROR WHEN GETTING LIST');
        }
    });
});

The problem is that getList.php only starts executing once createPDF.php finishes.

The application works with sessions, and tests have been done in separate clients, in which case both requests run parallel, but in the same computer they don't.

In this scenario how to run both requests parallel?

Thank you

I tryed, but it is set by default anyways, so it made no difference — mikl, Mar 06 '15 at 03:52
Are you SURE it's executing only after the subsequent call? By any chance, is `viewPdf()` what makes the call to getList.php? If so, that's your problem. — kylehyde215, Mar 06 '15 at 04:32
They are totally indepoendant processes, they only share the server, and it is happening when different clients work at the same time too, they interfer each other the same way — mikl, Mar 06 '15 at 04:49
By any chance is it a problem caused by the VPS server stacking executions? — mikl, Mar 06 '15 at 04:52
For sure, added it, as mentioned it is pretty much similar to the one of createPdf.php — mikl, Mar 06 '15 at 05:04
A general outline would be (documentready > ajax(saveFile) > ajax(createPdf)) && (documentready > ajax(getList)). Only createPdf is inside another ajax, getList is independant (need getList to execute whilst createPdf does and not only after it finished) — mikl, Mar 06 '15 at 05:16
have you tried removing all the logic from the php-script and temporarily just let the server wait for x seconds to simulate the current situation, just interested if that problem remains or if something else is wrong !? — luk2302, Mar 07 '15 at 23:30
And have you seen [this question/answer](http://stackoverflow.com/questions/6903318/multiple-ajax-requests-delay-each-other?rq=1)? — luk2302, Mar 07 '15 at 23:33

score 1 · Accepted Answer · edited May 23 '17 at 12:28

Basically what is happening is that on your server the session file is locked by the first script, e.g. saveFiles. After that script has completed its task, the session is unlocked again. Unfortunately your script getList wants to access the same session, since it is locked, getList will have to wait until it is unlocked.

One solution for this is to call session_write_close() after all writing to session data has been done, that will cause the session file to be unlocked and be used by the next script. That call should obviously happen as soon as possible after session_start() to minimize the waiting time for other requests.

For a more detailed explanation have a look at https://stackoverflow.com/a/6933294/2442804

In a web application many jQuery+Ajax+PHP processes not running parallel in the same client

1 Answers1