Installation

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$ composer require symfony/semaphore

Usage

In computer science, a semaphore is a variable or abstract data type used to control access to a common resource by multiple processes in a concurrent system such as a multitasking operating system. The main difference with locks is that semaphores allow more than one process to access a resource, whereas locks only allow one process.

Create semaphores with the SemaphoreFactory class, which in turn requires another class to manage the storage:

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use Symfony\Component\Semaphore\SemaphoreFactory;
use Symfony\Component\Semaphore\Store\RedisStore;

$redis = new Redis();
$redis->connect('172.17.0.2');

$store = new RedisStore($redis);
$factory = new SemaphoreFactory($store);

The semaphore is created by calling the createSemaphore() method. Its first argument is an arbitrary string that represents the locked resource. Its second argument is the maximum number of processes allowed. Then, a call to the acquire() method will try to acquire the semaphore:

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// ...
$semaphore = $factory->createSemaphore('pdf-invoice-generation', 2);

if ($semaphore->acquire()) {
    // The resource "pdf-invoice-generation" is locked.
    // Here you can safely compute and generate the invoice.

    $semaphore->release();
}

If the semaphore can not be acquired, the method returns false. The acquire() method can be safely called repeatedly, even if the semaphore is already acquired.