PHP APC, educate me

Question

I'm currently implementing memcached into my service but what keeps cropping up is the suggestion that I should also implement APC for caching of the actual code.

I have looked through the few tutorials there are, and the PHP documentation as well, but my main question is, how do I implement it on a large scale? PHP documentation talks about storing variables, but it isn't that detailed.

Forgive me for being uneducated in this area but I would like to know where in real sites this is implemented. Do I literally cache everything or only the parts that are used often, such as functions?

Thanks!

Answer 1

APC is both an opcode cache and a general data cache. The latter works pretty much like memcached, whereas the opcode cache works by caching the parsed php-files, so that they won't have to be parsed on each request. That can generally speed up execution time up quite a bit.

Answer 2

As you know PHP is an interpreted language, so everytime a request arrives to the server it need to open all required and included files, parse them and execute them. What APC offers is to skip the require/include and parsing steps (The files still have to be required, but are stored in memory so access is much much faster), so the scripts just have to be executed. On our website, we use a combination of APC and memcached. APC to speed up the above mentioned steps, and memcached to enable fast and distributed storing and accessing of both global variables (precomputed expensive function calls etc that can be shared by multiple clients for a certain amount of time) as well as session variables. This enables us to have multiple front end servers without losing any client state such as login status etc.

When it comes to what you should cache... well, that really depends on your application. If you have a need for multiple frontends somewhere down the line, I would try to go with memcached for such caching and storing, and use APC as an opcode cache.

Answer 3

You don't have to implement the opcode caching features of APC, you just enable them as a php module.

APC cache size and other configuration information is here.