如何解决在PHP后端未同时执行的Ajax调用的问题？

Question

I am developing a web app that needs to call functions from a web service using SOAP.

I have coded a simple PHP backend to access the SOAP functions. The backend mainly consists of classes with static "helper" methods (I use __autoload() to load the classes only when needed).

At some point in the app, I need to call two SOAP functions at the same time. Since I want to speed up the process, I use two simultaneous ajax calls.

The thing is, those calls don't always run simultaneously. The requests are sent at the same time, but the second call lasts longer than the first one (they should have roughly the same run time). If the first call lasts 600 ms, the second one lasts 1400-1600 ms.

I have setup a simple test, where I call both functions first in parallel and then, when both requests are complete, in serial one after the other.

I have tried profiling the PHP side (both using microtime() and xdebug), but I've consistently found that the calls have the same execution time on PHP (about 500ms).

What is REALLY driving me insane, though, is the fact that the very same code seems to work and break in an apparently random fashion. Sometimes I reload the page and the calls are executed simultaneously. Then I reload again, and the second call takes longer, once more.

PHP Session lock should not be an issue, I open the session only when needed and close it with session_write_close() afterwards.

The SOAP server called by the php backend supports multiple connections, and besides my microtime() profiling has shown me that it takes about 400ms to complete a SOAP request, so I don't think the web service is to blame.

On the client side, the requests are sent at the same time. I tried adding a slight delay to the second call, to no avail.

But then again, this "random" behaviour is baffling me. I have tested the app both on local (WAMP) and on two different servers (shared hosting). In all of the environments, PHP 5.5 was used.

What could be causing this discrepancy, and how could one try to isolate the problem, if the behaviour is not 100% consistent?

Answer 1

I've solved the mystery.

In my local environment, once I disabled xdebug, the ajax calls ran flawlessly in parallel.

In remote, I still had the same problem (and no xdebug at all). So first of all, I've tried using one single AJAX call to a PHP script that used curl_multi_exec() to run the SOAP requests in parallel, basically a similar setup but mainly server-side.

I found that I had the same discrepancies in the timings using this technique as well, with a very similar pattern.

I've contacted the hosting provider explaining my problem, and they told me that it might be due to the shared environment having limited resources depending on the overall load.

So in the end while it's possible to achieve this behaviour, in my specific case I had to change my strategy and use two serial AJAX calls, updating the view after the first call to give the impression of a faster response while waiting for the second call to complete.