Identify data to cache in which layer - PHP/MySQL

Think you are the proud owner of Facebook, then

which data you want to store in app layer [memcached/ APC] and which data in MySQL cache ?

Please explain also why you think so.

[I w开发者_StackOverflow中文版ant to have an idea on which data to cache where]

For memcache, store session data. You have to typically query from a large table or from the filesystem to get it, depending on how it's stored. Putting that on memory removes hitting the disk for a relatively small amount data (that is typically critical to one's web application).

For your database cache, put stuff in there that is not changing so often. We're talking about wall posts, comments, etc. They are queried a lot and rarely change, all things considered. You may also want to consider doing a flat file cache, so you can purge individual files with greater ease, and divide it up as you see fit.

I generally don't directly cache any arbitrary data with APC, usually I will just let it cache stuff automatically and get lessened memory loads.

This is only one way to do it, but as far as the industry goes, this is a somewhat well-used model.