PHP request file caching in load-balanced server environment

Question

I'm looking to write a basic PHP file caching driver in a PHP application that routes all traffic to a front controller. For example's sake, assume the following simplified setup using apache mod_proxy_balancer:

In a single-server environment I would cache request responses on disk in a directory structure matching the request URI. Then, simple apache rewrite rules like the following could allow apache to return static cache files (if they exist) and avoid the PHP process altogether:

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /front_controller.php [L]

Obviously, this is problematic in a load-balanced environment because the cache file would only be written to disk on the specific PHP server where the request was served up and the results cached.

Solving the issue ...

So, to solve this problem, I figured I could knock out some code to have the individual back-end PHP servers write/delete cache data to the load balancer. However, being mostly ignorant as to the capabilities of mod_proxy_balancer (and any other load balancing options, really), I need some outside verification for the following questions:

And the questions ...

1. Is it possible to do some form of checking like the above RewriteRules to have the front-facing load balancer serve up a static file before sending off requests to one of the backend servers?
2. Is this even advisable? Should the load balancer be allowed to route traffic exclusively and not be bothered with serving up static content?
3. Would it be better to just use an acceptable TTL on the cached files at the PHP server level and deal with an accepted level of stale cache overlap?

Finally, apologies if this is is too broad or has already been answered; I'm not really sure what to search for as a result of my aforementioned ignorance on the load-balancing subject.

Answer 1

This sounds like something Nginx could do easily, and would remove the need to write to files on disk.

Nginx can do the load balancing and caching, here's a tutorial on it:

http://nathanvangheem.com/news/nginx-with-built-in-load-balancing-and-caching

Answer 2

For the simplest solution, you can use NFS. Mount a file system via NFS on all of the PHP servers and it acts like local storage, but is the same for all servers. To get a little more sophisticated, use something like Nginx or Varnish that can cache what is on the NFS file system.

Using memcache is also a viable alternative, which is a distributed memory based storage system. The nice thing about memcache is that you don't need to manage cache clearing or purging if you don't want to. You can set TTL for each cached item, or if memcache gets full, it automatically purges cached items.