WordPress Site Health had been nagging me to use a persistent object cache. I already run Zend OpCache, so I wondered what else I needed.
OpCache is an opcode cache: it stores the compiled bytecode of the PHP source so the engine need not recompile it on every request. The object cache is a different layer, holding the results of database queries and transients. WordPress ships with a non-persistent object cache that lives only for the duration of a single request; making it persistent means backing it with something that survives across requests, such as Redis.
On this site I manage the WordPress tree with svn and do not let PHP write into it, which changes a couple of the steps from the usual click-in-the-dashboard instructions. While I also generally prefer nginx and PHP-FPM, this site is still on Apache with mod_php. However, the installation is basically the same regardless of the web server software, but troubleshooting differs between them. I’m covering both here. (more…)