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 mechanisms... what I've found so far is mysql's GET_LOCK(), and php's flock(). Anything else worth considering?

Update: I've found sem_acquire, but it seems to be a blocking lock.

Answer 1

If you don't mind basing your lock on the filesystem, then you could use fopen() with mode 'x'. Here is an example:

$f = fopen("lockFile.txt", 'x');
if($f) {
    $me = getmypid();
    $now = date('Y-m-d H:i:s');
    fwrite($f, "Locked by $me at $now\n");
    fclose($f);
    doStuffInLock();
    unlink("lockFile.txt"); // unlock        
}
else {
    echo "File is locked: " . get_file_contents("lockFile.txt");
    exit;
}

See www.php.net/fopen

Answer 2

Actually, check to see if this will work better then Peter's suggestion.

http://us2.php.net/flock

use an exclusive lock and if your comfortable with it, put everything else that attempted to lock the file in a 2-3 second sleep. If done right your site will experience a hang regarding the locked resource but not a horde of scripts fighting to cache the samething.

Answer 3

If the point of the lock is to prevent multiple processes from trying to populate an empty cache key, why wouldn't you want to have a blocking lock?

  $value = apc_fetch($KEY);

  if ($value === FALSE) {
      shm_acquire($SEMAPHORE);

      $recheck_value = apc_fetch($KEY);
      if ($recheck_value !== FALSE) {
        $new_value = expensive_operation();
        apc_store($KEY, $new_value);
        $value = $new_value;
      } else {
        $value = $recheck_value;
      }

      shm_release($SEMAPHORE);
   }

If the cache is good, you just roll with it. If there's nothing in the cache, you get a lock. Once you have the lock, you'll need to double-check the cache to make sure that, while you were waiting to get the lock, the cache wasn't repopulated. If the cache was repopulated, use that value & release the lock, otherwise, you do the computation, populate the cache & then release your lock.

Answer 4

What I've found, actually, is that I don't need any locking at all... given what I'm trying to create is a map of all the class => path associations for autoload, it doesn't matter if one process overwrites what the other one has found (it's highly unlikely, if coded properly), because the data will get there eventually anyway. So, the solution turned out to be "no locks".

Answer 5

I realize this is a year old, but I just stumbled upon the question while doing some research myself on locking in PHP.

It occurs to me that a solution might be possible using APC itself. Call me crazy, but this might be a workable approach:

function acquire_lock($key, $expire=60) {
    if (is_locked($key)) {
        return null;
    }
    return apc_store($key, true, $expire);
}

function release_lock($key) {
    if (!is_locked($key)) {
        return null;
    }
    return apc_delete($key);
}

function is_locked($key) {
    return apc_fetch($key);
}

// example use
if (acquire_lock("foo")) {
    do_something_that_requires_a_lock();
    release_lock("foo");
}

In practice I might throw another function in there to generate a key to use here, just to prevent collision with an existing APC key, e.g.:

function key_for_lock($str) {
    return md5($str."locked");
}

The $expire parameter is a nice feature of APC to use, since it prevents your lock from being held forever if your script dies or something like that.

Hopefully this answer is helpful for anyone else who stumbles here a year later.

Answer 6

/*
CLASS ExclusiveLock
Description
==================================================================
This is a pseudo implementation of mutex since php does not have
any thread synchronization objects
This class uses flock() as a base to provide locking functionality.
Lock will be released in following cases
1 - user calls unlock
2 - when this lock object gets deleted
3 - when request or script ends
==================================================================
Usage:

//get the lock
$lock = new ExclusiveLock( "mylock", FALSE);

//lock
if( $lock->lock( ) == FALSE )
    error("Locking failed");
//--
//Do your work here
//--

//unlock
$lock->unlock();
===================================================================
*/
class ExclusiveLock
{
    protected $key   = null;  //user given value
    protected $file  = null;  //resource to lock
    protected $own   = FALSE; //have we locked resource

    function __construct( $key ) 
    {
        $this->key = $key;
        //create a new resource or get exisitng with same key
        $this->file = fopen("$key.lockfile", 'w+');
    }


    function __destruct() 
    {
        if( $this->own == TRUE )
            $this->unlock( );
    }


    function lock( ) 
    {
        if( !flock($this->file, LOCK_EX)) 
        { //failed
            $key = $this->key;
            error_log("ExclusiveLock::acquire_lock FAILED to acquire lock [$key]");
            return FALSE;
        }
        ftruncate($this->file, 0); // truncate file
        //write something to just help debugging
        fwrite( $this->file, "Locked\n");
        fflush( $this->file );

        $this->own = TRUE;
        return $this->own;
    }


    function unlock( ) 
    {
        $key = $this->key;
        if( $this->own == TRUE ) 
        {
            if( !flock($this->file, LOCK_UN) )
            { //failed
                error_log("ExclusiveLock::lock FAILED to release lock [$key]");
                return FALSE;
            }
            ftruncate($this->file, 0); // truncate file
            //write something to just help debugging
            fwrite( $this->file, "Unlocked\n");
            fflush( $this->file );
        }
        else
        {
            error_log("ExclusiveLock::unlock called on [$key] but its not acquired by caller");
        }
        $this->own = FALSE;
        return $this->own;
    }
};