Dealing with Concurrency with Locks

Warning: You are browsing the documentation for Symfony 6.1, which is no longer maintained.

Read the updated version of this page for Symfony 7.1 (the current stable version).

Dealing with Concurrency with Locks

When a program runs concurrently, some part of code which modify shared resources should not be accessed by multiple processes at the same time. Symfony's Lock component provides a locking mechanism to ensure that only one process is running the critical section of code at any point of time to prevent race conditions from happening.

The following example shows a typical usage of the lock:

1
2
3
4
5
6
7
8
9
$lock = $lockFactory->createLock('pdf-creation');
if (!$lock->acquire()) {
    return;
}

// critical section of code
$service->method();

$lock->release();

Installing

In applications using Symfony Flex, run this command to install the Lock component:

1
$ composer require symfony/lock

Configuring

By default, Symfony provides a Semaphore when available, or a Flock otherwise. You can configure this behavior by using the lock key like:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
# config/packages/lock.yaml
framework:
    lock: ~
    lock: 'flock'
    lock: 'flock:///path/to/file'
    lock: 'semaphore'
    lock: 'memcached://m1.docker'
    lock: ['memcached://m1.docker', 'memcached://m2.docker']
    lock: 'redis://r1.docker'
    lock: ['redis://r1.docker', 'redis://r2.docker']
    lock: 'zookeeper://z1.docker'
    lock: 'zookeeper://z1.docker,z2.docker'
    lock: 'zookeeper://localhost01,localhost02:2181'
    lock: 'sqlite:///%kernel.project_dir%/var/lock.db'
    lock: 'mysql:host=127.0.0.1;dbname=app'
    lock: 'pgsql:host=127.0.0.1;dbname=app'
    lock: 'pgsql+advisory:host=127.0.0.1;dbname=app'
    lock: 'sqlsrv:server=127.0.0.1;Database=app'
    lock: 'oci:host=127.0.0.1;dbname=app'
    lock: 'mongodb://127.0.0.1/app?collection=lock'
    lock: '%env(LOCK_DSN)%'

    # named locks
    lock:
        invoice: ['semaphore', 'redis://r2.docker']
        report: 'semaphore'

6.1

The CSV support (e.g. zookeeper://localhost01,localhost02:2181) in ZookeeperStore DSN was introduced in Symfony 6.1.

Locking a Resource

To lock the default resource, autowire the lock factory using LockFactory:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
// src/Controller/PdfController.php
namespace App\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\Lock\LockFactory;

class PdfController extends AbstractController
{
    #[Route('/download/terms-of-use.pdf')]
    public function downloadPdf(LockFactory $factory, MyPdfGeneratorService $pdf)
    {
        $lock = $factory->createLock('pdf-creation');
        $lock->acquire(true);

        // heavy computation
        $myPdf = $pdf->getOrCreatePdf();

        $lock->release();

        // ...
    }
}

Caution

The same instance of LockInterface won't block when calling acquire multiple times inside the same process. When several services use the same lock, inject the LockFactory instead to create a separate lock instance for each service.

Locking a Dynamic Resource

Sometimes the application is able to cut the resource into small pieces in order to lock a small subset of processes and let others through. The previous example showed how to lock the $pdf->getOrCreatePdf() call for everybody, now let's see how to lock a $pdf->getOrCreatePdf($version) call only for processes asking for the same $version:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
// src/Controller/PdfController.php
namespace App\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\Lock\LockFactory;

class PdfController extends AbstractController
{
    #[Route('/download/{version}/terms-of-use.pdf')]
    public function downloadPdf($version, LockFactory $lockFactory, MyPdfGeneratorService $pdf)
    {
        $lock = $lockFactory->createLock('pdf-creation-'.$version);
        $lock->acquire(true);

        // heavy computation
        $myPdf = $pdf->getOrCreatePdf($version);

        $lock->release();

        // ...
    }
}

Naming Locks

If the application needs different kind of Stores alongside each other, Symfony provides named lock:

1
2
3
4
5
# config/packages/lock.yaml
framework:
    lock:
        invoice: ['semaphore', 'redis://r2.docker']
        report: 'semaphore'

An autowiring alias is created for each named lock with a name using the camel case version of its name suffixed by LockFactory.

For instance, the invoice lock can be injected by naming the argument $invoiceLockFactory and type-hinting it with LockFactory:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
// src/Controller/PdfController.php
namespace App\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\Lock\LockFactory;

class PdfController extends AbstractController
{
    #[Route('/download/terms-of-use.pdf')]
    public function downloadPdf(LockFactory $invoiceLockFactory, MyPdfGeneratorService $pdf)
    {
        // ...
    }
}
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
TOC
    Version