New in Symfony 5.2: Semaphore component
October 22, 2020 • Published by Javier Eguiluz
Warning: This post is about an unsupported Symfony version. Some of this information may be out of date. Read the most recent Symfony Docs.
Contributed by
Grégoire Pineau
in #35780.
Symfony introduced the Lock component in Symfony 3.4, to provide a mechanism to grant exclusive access to a shared resource. In some cases, it's not enough to provide access to a single process and you need to allow access to multiple concurrent processes.
According to the Wikipedia definition, 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. Broadly speaking, a semaphore allows N process to access a resource, and a lock is a semaphore where N = 1.
In Symfony 5.2 we're introducing a new Semaphore component. Instead of using a simple variable, the Semaphore component uses a store (e.g. a Redis server). First, create the store:
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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);
Then, use that factory to actually create a semaphore:
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// ...
$semaphore = $factory->createSemaphore('pdf-invoice-generation', 2);
if ($semaphore->acquire()) {
// The resource "pdf-invoice-generation" is locked.
// You can compute and generate invoice safely here.
$semaphore->release();
}
Given the single-threaded nature of PHP, to fully use the semaphore in concurrent process you must run N workers. In each worker you look for a job to do. If you get a job, try to acquire a semaphore. If you acquire it, process the job; otherwise, try to find another job.
Read the Semaphore component documentation to learn all about it.
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https://symfony.com/doc/current/components/lock.html#semaphorestore