The Semaphore Component
Edit this pageThe Semaphore Component
The Semaphore Component manages semaphores, a mechanism to provide exclusive access to a shared resource.
Installation
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$ composer require symfony/semaphoreNote
If you install this component outside of a Symfony application, you must
require the vendor/autoload.php file in your code to enable the class
autoloading mechanism provided by Composer. Read
this article for more details.
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.
Note
Unlike other implementations, the Semaphore component distinguishes
semaphores instances even when they are created for the same resource. If a
semaphore has to be used by several services, they should share the same
Semaphore instance returned by the SemaphoreFactory::createSemaphore
method.
Tip
If you don't release the semaphore explicitly, it will be released
automatically on instance destruction. In some cases, it can be useful to
lock a resource across several requests. To disable the automatic release
behavior, set the fifth argument of the createSemaphore() method to false.
