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How to prevent php-fpm from overriding your PHP-based caching logic

A while ago I implemented ETag support to the dynamic preview generator in Pepperminty Wiki. While I thought it worked during testing, for some reason on a private instance of Pepperminty Wiki I discovered recently that my browser didn't seen to be taking advantage of it. This got me curious, so I decided to do a little bit of digging to find out why.

It didn't take long to find the problem. For some reason, all the responses from the server had a Cache-Control: no-cache, no-store, must-revalidate header attached to them. How strange! Even more so that it was in capital letters - my convention in Pepperminty Wiki is to always make the headers lowercase.

I checked the codebase with via the Project Find feature of Atom just to make sure that I hadn't left in a debugging statement or anything (I hadn't), and then I turned my attention to Nginx (engine-x) - the web server that I use on my server. Maybe it had added a caching header?

A quick grep later revealed that it wasn't responsible either - which leaves just one part of the system unchecked - php-fpm, the PHP FastCGI server that sits just behind Nginx that's responsible for running the various PHP scripts I have that power this website and other corners of my server. Another quick grep returned a whole bunch of garbage, so after doing some research I discovered that php-fpm, by default, is configured to send this header - and that it has to be disabled by editing your php.ini (for me it's in /etc/php/7.1/fpm/php.ini), and changing

;session.cache_limiter = nocache

to be explicitly set to an empty string, like so:

session.cache_limiter = ''

This appears to have solved by problem for now - allowing me to regain control over how the resources I send back via PHP are cached. Hopefully this saves someone else the hassle of pulling their entire web server stack apart and putting it back together again the future :P

Found this helpful? Still having issues? Got a better way of solving the problem? Post a comment below!

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