Load balancing and APC

Question

I am interested in a scenario where webservers serving a PHP application is set up with a load balancer.

There will be multiple webservers with APC behind the load balancer. All requests will have to go through the load balancer, which then sends it to one of the web servers to process.

I understand that memcached should be used for distributed caching, but I think having the APC cache on each machine cache things like application configurations and other objects that will NOT be different across any of the servers would yield even better performance.

There is also an administrator area for this application. It is also accessed via the load balancer (for example, site.com/admin). In a case like this, how can I call apc_clear_cache to clear the APC object cache on ALL servers?

Answer 1

Externally in your network you have a public IP you use to route all your requests to your load balancer that distributes load round robin so outside you cannot make a request to clear your cache on each server one at a time because you don't know which one is being used at any given time. However, within your network, each machine has its own internal IP and can be called directly. Knowing this you can do some funny/weird things that do work externally.

A solution I like is to be able to hit a single URL and get everything done such as http://www.mywebsite/clearcache.php or something like that. If you like that as well, read on. Remember you can have this authenticated if you like so your admin can hit this or however you protect it.

You could create logic where you can externally make one request to clear your cache on all servers. Whichever server receives the request to clear cache will have the same logic to talk to all servers to clear their cache. This sounds weird and a bit frankenstein but here goes the logic assuming we have 3 servers with IPs 10.232.12.1, 10.232.12.2, 10.232.12.3 internally:

1) All servers would have two files called "initiate_clear_cache.php" and "clear_cache.php" that would be the same copies for all servers.

2) "initiate_clear_cache.php" would do a file_get_contents for each machine in the network calling "clear_cache.php" which would include itself
for example: 
file_get_contents('http://10.232.12.1/clear_cache.php');
file_get_contents('http://10.232.12.2/clear_cache.php');
file_get_contents('http://10.232.12.3/clear_cache.php');

3) The file called "clear_cache.php" is actually doing the cache clearing for its respective machine.

4) You only need to make a single request now such as http://www.mywebsite/initial_clear_cache.php and you are done.

Let me know if this works for you. I've done this in .NET and Node.js similar but haven't tried this in PHP yet but I'm sure the concept is the same. :)