best way to obtain a lock in php

Question

I\'m trying to update a variable in APC, and will be many processes trying to do that.

APC doesn\'t provide locking functionality, so I\'m considering using other me

Answer 1

You can use the apc_add function to achieve this without resorting to file systems or mysql. apc_add only succeeds when the variable is not already stored; thus, providing a mechanism of locking. TTL can be used to ensure that falied lockholders won't keep on holding the lock forever.

The reason apc_add is the correct solution is because it avoids the race condition that would otherwise exist between checking the lock and setting it to 'locked by you'. Since apc_add only sets the value if it's not already set ( "adds" it to the cache ), it ensures that the lock can't be aquired by two calls at once, regardless of their proximity in time. No solution that doesn't check and set the lock at the same time will inherently suffer from this race condition; one atomic operation is required to successfully lock without race condition.

Since APC locks will only exist in the context of that php execution, it's probably not the best solution for general locking, as it doesn't support locks between hosts. Memcache also provides an atomic add function and thus can also be used with this technique - which is one method of locking between hosts. Redis also supports atomic 'SETNX' functions and TTL, and is a very common method of locking and synchronization between hosts. Howerver, the OP requests a solution for APC in particular.