I have a really strange problem. I have a website built on top of wordpress (version 3.6.1, because of plugins compatibility, but with latest security patches applied).
php -v
PHP 5.4.4-14+deb7u12 (cli) (built: Jun 30 2014 13:15:11)
Copyright (c) 1997-2012 The PHP Group
Zend Engine v2.4.0, Copyright (c) 1998-2012 Zend Technologies
apache2 -v
Server version: Apache/2.2.22 (Debian)
Server built: Jul 24 2014 15:34:03
uname -a
Linux p-srv-http02 3.2.0-4-amd64 #1 SMP Debian 3.2.60-1+deb7u3 x86_64 GNU/Linux
It works for couple of weeks and then suddenly, it stops serving http requests, while https requests work fine. htop shows perfectly normal load and https requests are not slower.
I tried restarting apache2, but it did not solve the problem. However, when I restarted whole server, it started working again. If it is not apache, than it must be something with OS or PHP.
I checked apache access logs and access log shows requests from that time, so it looks, that they get to apache, but are not served properly. Usually every request is followed by many requests to wp-content/themes..., but because nothing is displayed, those are not present.
There is nothing in error.log besides some warning (the same for http and https).
I am confused. If this is problem with php website, system restart shouldn't help. If this is problem with OS (memory, IO, sockets ...), then the requests should not be logged into apache. Is it some php process, that can hang behind apache?
What can I do to debug this problem next time it occurs?