LockHandler

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LockHandler¶

What is a Lock?¶

File locking is a mechanism that restricts access to a computer file by allowing only one user or process access at any specific time. This mechanism was introduced a few decades ago for mainframes, but continues being useful for modern applications.

Symfony provides a LockHelper to help you use locks in your project.

Usage¶

Caution

The lock handler only works if you're using just one server. If you have several hosts, you must not use this helper.

A lock can be used, for example, to allow only one instance of a command to run.

use Symfony\Component\Filesystem\LockHandler;

$lockHandler = new LockHandler('hello.lock');
if (!$lockHandler->lock()) {
    // the resource "hello" is already locked by another process

    return 0;
}

The first argument of the constructor is a string that it will use as part of the name of the file used to create the lock on the local filesystem. A best practice for Symfony commands is to use the command name, such as acme:my-command. LockHandler sanitizes the contents of the string before creating the file, so you can pass any value for this argument.

Tip

The .lock extension is optional, but it's a common practice to include it. This will make it easier to find lock files on the filesystem. Moreover, to avoid name collisions, LockHandler also appends a hash to the name of the lock file.

By default, the lock will be created in the system's temporary directory, but you can optionally select the directory where locks are created by passing it as the second argument of the constructor.

Tip

Another way to configure the directory where the locks are created is to define a special environment variable, because PHP will use that value to override the default temporary directory. On Unix-based systems, define the TMPDIR variable. On Windows systems, define any of these variables: TMP, TEMP or USERPROFILE (they are checked in this order). This technique is useful for example when deploying a third-party Symfony application whose code can't be modified.

The lock() method tries to acquire the lock. If the lock is acquired, the method returns true, false otherwise. If the lock() method is called several times on the same instance it will always return true if the lock was acquired on the first call.

You can pass an optional blocking argument as the first argument to the lock() method, which defaults to false. If this is set to true, your PHP code will wait indefinitely until the lock is released by another process.

Caution

Be aware of the fact that the resource lock is automatically released as soon as PHP applies the garbage-collection process to the LockHandler object. This means that if you refactor the first example shown in this article as follows:

use Symfony\Component\Filesystem\LockHandler;

 if (!(new LockHandler('hello.lock'))->lock()) {
    // the resource "hello" is already locked by another process

    return 0;
}

Now the code won't work as expected because PHP's garbage collection mechanism removes the reference to the LockHandler object and thus, the lock is released just after it's been created.

Another alternative way to release the lock explicitly when needed is to use the release() method.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.