FileLock

A fluent Helper to properly handle file locking based on flock().

FileLock offers two locking strategies and several options. Just like flock(), FileLock can either wait until an exclusive lock is acquired (Blocking), or fail immediately (Non Blocking), but it can also try a configurable amount of time to acquire a Non Blocking exclusive lock, and wait a configurable amount of time between each attempts. FileLock can either lock a file (Self Locking) or create a file.lock and lock it instead (External Locking)

External Locking

This locking strategy does not lock the input filePath itself but rather creates a new file $lockFilePath = "$inputFilePath.lock"; and attempts to flock() it instead. This method is preferred in highly concurrent usages, where many processes will try to write the same file at once, such as file caching. Because this allows us to first try to open the .lock file in write mode and fail back to read mode before a flock() is eventually attempted.

By using a separate file for locking, we make sure that every write waiting for the lock (this should be done in Blocking mode) does not hold any handle on the cache file itself while it is most likely already being intensively read. By failing to read mode when write failed, we again lower the write handles to a single one, only when the external .lock file needs to be created. Altogether, this means that after warm up, each process waiting to write on the same file will be holding a read handle on the .lock file, while a single write one is at most used to actually write the cache file. As write handles are costly to open, approximately ten times slower than read handles, doing this can end up making some difference.

External locking can also be useful when you do not want to actually flock() a file (could be already locked or used by some other program/process), or because you just need exclusivity for something as the lock file is then created for you and every php process will be able to check its existence.

Please note that the external and empty .lock file is never deleted by FileLock and that its presence does not necessarily means that the lock is active.

If the $lockFilePath = "$inputFilePath.lock"; version of the .lock file (that is where the $inputFilePath directory) is not writable, it is created in sys_get_temp_dir() instead, with a hashed basedir($inputFilePath) prefix in filename.

$filePath = "/some/dir/some.file.ext";
$lock = new FileLock($filePath, Filelock::LOCK_EXTERNAL); // will create /some/dir/some/file.ext.lock or /tmp/sha1(/some/dir/some)_file.ext.lock

Self Locking

This locking strategy does acquire a lock on the input filePath itself. It provide with more guarantees than the External Locking strategy as the file will be locked for any process, not just the ones checking the lock through FileLock and it is preferred when write sessions are not instant.

$filePath = "/some/dir/some.file.ext";
$lock = new FileLock($filePath, Filelock::LOCK_SELF); // will directtly flock() /some/dir/some/file.ext

It could make sense under specific circumstances to use a double lock, both External and Self, using two FileLock instances.

In practice

In both External and Self locking, once you have an instance you can:

• Acquire a Blocking lock:

$lock->doLock(true);
// we either own the lock or php timed out

• Attempt to acquire a Non Blocking lock:

$lock->doLock();
if ($lock->isLocked()) {
    // we got the lock
}

• Attempt to acquire a Non Blocking lock several time before failing:

$lock->setLockTry(5) // default is 3
    ->setLockWait(0.01) // default is 0.1 second
    ->obtainLock(); // will try 5 times and wait 0.01 second in between
if ($lock->isLocked()) {
    // we got the lock
}

From there, you can get the underlying handle:

$lockHandle = $lock->getHandle();

This is mostly useful when Self locking as you probably need the handle to actually write something.

• Release a lock:

In all cases, locks are either released upon instance destruction or manually:

$lock->unLock(); // doing so also fclose() underlying handle

It is IMPORTANT to notice that when you acquire an Self lock, you need to keep the $lock instance alive until you are done with manipulating the file. Because FileLock is set to release its locks and handles when destroyed. This could happen if you where to acquire a lock in some function without storing the resulting instance outside of its scope.

Open Factory

FileLock comes with an handy factory to ease exclusively and self locked file opening:

    /**
     * @param string     $file
     * @param string     $mode fopen() mode
     * @param int|null   $maxTries 0|null for single non blocking attempt
     *                             1 for a single blocking attempt
     *                             1-N Number of non blocking attempts
     * @param float|null $lockWait Time to wait between attempts in second
     *
     * @return bool|static
     */
    public static function open($file, $mode, $maxTries = null, $lockWait = null)

Usage is pretty similar to fopen() except it returns a FileLock instance upon success (open + lock) instead of a resource.

$filePath = "/some/dir/some.file.ext";
$mode = 'wb'; // any fopen() mode
$fileLock = Filelock::open($filePath, $mode); // returns false or FileLock instance

if ($fileLock) {
    // we got it opened and locked
    $handle = $fileLock->getHandle();
}