.. index:: single: Lock single: Components; Lock
The Lock Component creates and manages locks, a mechanism to provide exclusive access to a shared resource.
$ composer require symfony/lock
Locks are used to guarantee exclusive access to some shared resource. In Symfony applications, you can use locks for example to ensure that a command is not executed more than once at the same time (on the same or different servers).
Locks are created using a :class:`Symfony\\Component\\Lock\\LockFactory` class, which in turn requires another class to manage the storage of locks:
use Symfony\Component\Lock\LockFactory; use Symfony\Component\Lock\Store\SemaphoreStore; $store = new SemaphoreStore(); $factory = new LockFactory($store);
The lock is created by calling the :method:`Symfony\\Component\\Lock\\LockFactory::createLock` method. Its first argument is an arbitrary string that represents the locked resource. Then, a call to the :method:`Symfony\\Component\\Lock\\LockInterface::acquire` method will try to acquire the lock:
// ...
$lock = $factory->createLock('pdf-invoice-generation');
if ($lock->acquire()) {
// The resource "pdf-invoice-generation" is locked.
// You can compute and generate invoice safely here.
$lock->release();
}
If the lock can not be acquired, the method returns false. The acquire()
method can be safely called repeatedly, even if the lock is already acquired.
Note
Unlike other implementations, the Lock Component distinguishes locks
instances even when they are created for the same resource. If a lock has
to be used by several services, they should share the same Lock instance
returned by the LockFactory::createLock method.
Tip
If you don't release the lock 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
third argument of the createLock() method to false.
By default, when a lock cannot be acquired, the acquire method returns
false immediately. To wait (indefinitely) until the lock
can be created, pass true as the argument of the acquire() method. This
is called a blocking lock because the execution of your application stops
until the lock is acquired.
Some of the built-in Store classes support this feature. When they don't,
they can be decorated with the RetryTillSaveStore class:
use Symfony\Component\Lock\LockFactory;
use Symfony\Component\Lock\Store\RedisStore;
use Symfony\Component\Lock\Store\RetryTillSaveStore;
$store = new RedisStore(new \Predis\Client('tcp:https://localhost:6379'));
$store = new RetryTillSaveStore($store);
$factory = new LockFactory($store);
$lock = $factory->createLock('notification-flush');
$lock->acquire(true);
Locks created remotely are difficult to manage because there is no way for the
remote Store to know if the locker process is still alive. Due to bugs,
fatal errors or segmentation faults, it cannot be guaranteed that release()
method will be called, which would cause the resource to be locked infinitely.
The best solution in those cases is to create expiring locks, which are
released automatically after some amount of time has passed (called TTL for
Time To Live). This time, in seconds, is configured as the second argument of
the createLock() method. If needed, these locks can also be released early
with the release() method.
The trickiest part when working with expiring locks is choosing the right TTL.
If it's too short, other processes could acquire the lock before finishing the
job; if it's too long and the process crashes before calling the release()
method, the resource will stay locked until the timeout:
// ...
// create an expiring lock that lasts 30 seconds
$lock = $factory->createLock('charts-generation', 30);
$lock->acquire();
try {
// perform a job during less than 30 seconds
} finally {
$lock->release();
}
Tip
To avoid letting the lock in a locking state, it's recommended to wrap the job in a try/catch/finally block to always try to release the expiring lock.
In case of long-running tasks, it's better to start with a not too long TTL and then use the :method:`Symfony\\Component\\Lock\\LockInterface::refresh` method to reset the TTL to its original value:
// ...
$lock = $factory->createLock('charts-generation', 30);
$lock->acquire();
try {
while (!$finished) {
// perform a small part of the job.
// renew the lock for 30 more seconds.
$lock->refresh();
}
} finally {
$lock->release();
}
Tip
Another useful technique for long-running tasks is to pass a custom TTL as
an argument of the refresh() method to change the default lock TTL:
$lock = $factory->createLock('charts-generation', 30);
// ...
// refresh the lock for 30 seconds
$lock->refresh();
// ...
// refresh the lock for 600 seconds (next refresh() call will be 30 seconds again)
$lock->refresh(600);
This component also provides two useful methods related to expiring locks:
getExpiringDate() (which returns null or a \DateTimeImmutable
object) and isExpired() (which returns a boolean).
Locks that are acquired for the first time are owned by the Lock instance that acquired
it. If you need to check whether the current Lock instance is (still) the owner of
a lock, you can use the isAcquired() method:
if ($lock->isAcquired()) {
// We (still) own the lock
}
Because of the fact that some lock stores have expiring locks (as seen and explained above), it is possible for an instance to lose the lock it acquired automatically:
// If we cannot acquire ourselves, it means some other process is already working on it
if (!$lock->acquire()) {
return;
}
$this->beginTransaction();
// Perform a very long process that might exceed TTL of the lock
if ($lock->isAcquired()) {
// Still all good, no other instance has acquired the lock in the meantime, we're safe
$this->commit();
} else {
// Bummer! Our lock has apparently exceeded TTL and another process has started in
// the meantime so it's not safe for us to commit.
$this->rollback();
throw new \Exception('Process failed');
}
Caution!
A common pitfall might be to use the isAcquired() method to check if
a lock has already been acquired by any process. As you can see in this example
you have to use acquire() for this. The isAcquired() method is used to check
if the lock has been acquired by the current process only!
| [1] | Technically, the true owners of the lock are the ones that share the same instance of Key,
not Lock. But from a user perspective, Key is internal and you will likely only be working
with the Lock instance so it's easier to think of the Lock instance as being the one that
is the owner of the lock. |
Locks are created and managed in Stores, which are classes that implement
:class:`Symfony\\Component\\Lock\\PersistingStoreInterface` and, optionally,
:class:`Symfony\\Component\\Lock\\BlockingStoreInterface`.
The component includes the following built-in store types:
| Store | Scope | Blocking | Expiring |
|---|---|---|---|
| :ref:`FlockStore <lock-store-flock>` | local | yes | no |
| :ref:`MemcachedStore <lock-store-memcached>` | remote | no | yes |
| :ref:`MongoDbStore <lock-store-mongodb>` | remote | no | yes |
| :ref:`PdoStore <lock-store-pdo>` | remote | no | yes |
| :ref:`RedisStore <lock-store-redis>` | remote | no | yes |
| :ref:`SemaphoreStore <lock-store-semaphore>` | local | yes | no |
| :ref:`ZookeeperStore <lock-store-zookeeper>` | remote | no | no |
The FlockStore uses the file system on the local computer to create the locks. It does not support expiration, but the lock is automatically released when the PHP process is terminated:
use Symfony\Component\Lock\Store\FlockStore;
// the argument is the path of the directory where the locks are created
// if none is given, sys_get_temp_dir() is used internally.
$store = new FlockStore('/var/stores');
Caution!
Beware that some file systems (such as some types of NFS) do not support locking. In those cases, it's better to use a directory on a local disk drive or a remote store based on PDO, Redis or Memcached.
The MemcachedStore saves locks on a Memcached server, it requires a Memcached
connection implementing the \Memcached class. This store does not
support blocking, and expects a TTL to avoid stalled locks:
use Symfony\Component\Lock\Store\MemcachedStore;
$memcached = new \Memcached();
$memcached->addServer('localhost', 11211);
$store = new MemcachedStore($memcached);
Note
Memcached does not support TTL lower than 1 second.
.. versionadded:: 5.1
The ``MongoDbStore`` was introduced in Symfony 5.1.
The MongoDbStore saves locks on a MongoDB server >=2.2, it requires a
\MongoDB\Collection or \MongoDB\Client from mongodb/mongodb or a
MongoDB Connection String.
This store does not support blocking and expects a TTL to
avoid stalled locks:
use Symfony\Component\Lock\Store\MongoDbStore;
$mongo = 'mongodb:https://localhost/database?collection=lock';
$options = [
'gcProbablity' => 0.001,
'database' => 'myapp',
'collection' => 'lock',
'uriOptions' => [],
'driverOptions' => [],
];
$store = new MongoDbStore($mongo, $options);
The MongoDbStore takes the following $options (depending on the first parameter type):
| Option | Description |
|---|---|
| gcProbablity | Should a TTL Index be created expressed as a probability from 0.0 to 1.0 (Defaults to 0.001) |
| database | The name of the database |
| collection | The name of the collection |
| uriOptions | Array of uri options for MongoDBClient::__construct |
| driverOptions | Array of driver options for MongoDBClient::__construct |
When the first parameter is a:
MongoDB\Collection:
$options['database']is ignored$options['collection']is ignored
MongoDB\Client:
$options['database']is mandatory$options['collection']is mandatory
MongoDB Connection String:
$options['database']is used otherwise/pathfrom the DSN, at least one is mandatory$options['collection']is used otherwise?collection=from the DSN, at least one is mandatory
Note
The collection querystring parameter is not part of the MongoDB Connection String definition.
It is used to allow constructing a MongoDbStore using a Data Source Name (DSN) without $options.
The PdoStore saves locks in an SQL database. It requires a PDO connection, a Doctrine DBAL Connection, or a Data Source Name (DSN). This store does not support blocking, and expects a TTL to avoid stalled locks:
use Symfony\Component\Lock\Store\PdoStore; // a PDO, a Doctrine DBAL connection or DSN for lazy connecting through PDO $databaseConnectionOrDSN = 'mysql:host=127.0.0.1;dbname=lock'; $store = new PdoStore($databaseConnectionOrDSN, ['db_username' => 'myuser', 'db_password' => 'mypassword']);
Note
This store does not support TTL lower than 1 second.
Before storing locks in the database, you must create the table that stores the information. The store provides a method called :method:`Symfony\\Component\\Lock\\Store\\PdoStore::createTable` to set up this table for you according to the database engine used:
try {
$store->createTable();
} catch (\PDOException $exception) {
// the table could not be created for some reason
}
A great way to set up the table in production is to call the createTable()
method in your local computer and then generate a
:ref:`database migration <doctrine-creating-the-database-tables-schema>`:
$ php bin/console doctrine:migrations:diff
$ php bin/console doctrine:migrations:migrate
The RedisStore saves locks on a Redis server, it requires a Redis connection
implementing the \Redis, \RedisArray, \RedisCluster or
\Predis classes. This store does not support blocking, and expects a TTL to
avoid stalled locks:
use Symfony\Component\Lock\Store\RedisStore;
$redis = new \Redis();
$redis->connect('localhost');
$store = new RedisStore($redis);
The SemaphoreStore uses the PHP semaphore functions to create the locks:
use Symfony\Component\Lock\Store\SemaphoreStore; $store = new SemaphoreStore();
The CombinedStore is designed for High Availability applications because it manages several stores in sync (for example, several Redis servers). When a lock is being acquired, it forwards the call to all the managed stores, and it collects their responses. If a simple majority of stores have acquired the lock, then the lock is considered as acquired; otherwise as not acquired:
use Symfony\Component\Lock\Store\CombinedStore;
use Symfony\Component\Lock\Store\RedisStore;
use Symfony\Component\Lock\Strategy\ConsensusStrategy;
$stores = [];
foreach (['server1', 'server2', 'server3'] as $server) {
$redis = new \Redis();
$redis->connect($server);
$stores[] = new RedisStore($redis);
}
$store = new CombinedStore($stores, new ConsensusStrategy());
Instead of the simple majority strategy (ConsensusStrategy) an
UnanimousStrategy can be used to require the lock to be acquired in all
the stores.
Caution!
In order to get high availability when using the ConsensusStrategy, the
minimum cluster size must be three servers. This allows the cluster to keep
working when a single server fails (because this strategy requires that the
lock is acquired in more than half of the servers).
The ZookeeperStore saves locks on a ZooKeeper server. It requires a ZooKeeper
connection implementing the \Zookeeper class. This store does not
support blocking and expiration but the lock is automatically released when the
PHP process is terminated:
use Symfony\Component\Lock\Store\ZookeeperStore;
$zookeeper = new \Zookeeper('localhost:2181');
// use the following to define a high-availability cluster:
// $zookeeper = new \Zookeeper('localhost1:2181,localhost2:2181,localhost3:2181');
$store = new ZookeeperStore($zookeeper);
Note
Zookeeper does not require a TTL as the nodes used for locking are ephemeral and die when the PHP process is terminated.
The component guarantees that the same resource can't be lock twice as long as the component is used in the following way.
Remote stores (:ref:`MemcachedStore <lock-store-memcached>`,
:ref:`MongoDbStore <lock-store-mongodb>`,
:ref:`PdoStore <lock-store-pdo>`,
:ref:`RedisStore <lock-store-redis>` and
:ref:`ZookeeperStore <lock-store-zookeeper>`) use a unique token to recognize
the true owner of the lock. This token is stored in the
:class:`Symfony\\Component\\Lock\\Key` object and is used internally by
the Lock, therefore this key must not be shared between processes (session,
caching, fork, ...).
Caution!
Do not share a key between processes.
Every concurrent process must store the Lock in the same server. Otherwise two
different machines may allow two different processes to acquire the same Lock.
Caution!
To guarantee that the same server will always be safe, do not use Memcached behind a LoadBalancer, a cluster or round-robin DNS. Even if the main server is down, the calls must not be forwarded to a backup or failover server.
Expiring stores (:ref:`MemcachedStore <lock-store-memcached>`, :ref:`MongoDbStore <lock-store-mongodb>`, :ref:`PdoStore <lock-store-pdo>` and :ref:`RedisStore <lock-store-redis>`) guarantee that the lock is acquired only for the defined duration of time. If the task takes longer to be accomplished, then the lock can be released by the store and acquired by someone else.
The Lock provides several methods to check its health. The isExpired()
method checks whether or not its lifetime is over and the getRemainingLifetime()
method returns its time to live in seconds.
Using the above methods, a more robust code would be:
// ...
$lock = $factory->createLock('invoice-publication', 30);
$lock->acquire();
while (!$finished) {
if ($lock->getRemainingLifetime() <= 5) {
if ($lock->isExpired()) {
// lock was lost, perform a rollback or send a notification
throw new \RuntimeException('Lock lost during the overall process');
}
$lock->refresh();
}
// Perform the task whose duration MUST be less than 5 minutes
}
Caution!
Choose wisely the lifetime of the Lock and check whether its remaining
time to leave is enough to perform the task.
Caution!
Storing a Lock usually takes a few milliseconds, but network conditions
may increase that time a lot (up to a few seconds). Take that into account
when choosing the right TTL.
By design, locks are stored in servers with a defined lifetime. If the date or time of the machine changes, a lock could be released sooner than expected.
Caution!
To guarantee that date won't change, the NTP service should be disabled and the date should be updated when the service is stopped.
By using the file system, this Store is reliable as long as concurrent
processes use the same physical directory to stores locks.
Processes must run on the same machine, virtual machine or container. Be careful when updating a Kubernetes or Swarm service because for a short period of time, there can be two running containers in parallel.
The absolute path to the directory must remain the same. Be careful of symlinks that could change at anytime: Capistrano and blue/green deployment often use that trick. Be careful when the path to that directory changes between two deployments.
Some file systems (such as some types of NFS) do not support locking.
Caution!
All concurrent processes must use the same physical file system by running on the same machine and using the same absolute path to locks directory.
By definition, usage of FlockStore in an HTTP context is incompatible
with multiple front servers, unless to ensure that the same resource will
always be locked on the same machine or to use a well configured shared file
system.
Files on the file system can be removed during a maintenance operation. For instance,
to clean up the /tmp directory or after a reboot of the machine when a directory
uses tmpfs. It's not an issue if the lock is released when the process ended, but
it is in case of Lock reused between requests.
Caution!
Do not store locks on a volatile file system if they have to be reused in several requests.
The way Memcached works is to store items in memory. That means that by using the :ref:`MemcachedStore <lock-store-memcached>` the locks are not persisted and may disappear by mistake at anytime.
If the Memcached service or the machine hosting it restarts, every lock would be lost without notifying the running processes.
Caution!
To avoid that someone else acquires a lock after a restart, it's recommended to delay service start and wait at least as long as the longest lock TTL.
By default Memcached uses a LRU mechanism to remove old entries when the service needs space to add new items.
Caution!
The number of items stored in Memcached must be under control. If it's not possible, LRU should be disabled and Lock should be stored in a dedicated Memcached service away from Cache.
When the Memcached service is shared and used for multiple usage, Locks could be
removed by mistake. For instance some implementation of the PSR-6 clear()
method uses the Memcached's flush() method which purges and removes everything.
Caution!
The method flush() must not be called, or locks should be stored in a
dedicated Memcached service away from Cache.
Caution!
The locked resource name is indexed in the _id field of the lock
collection. Beware that in MongoDB an indexed field's value can be
a maximum of 1024 bytes in length inclusive of structural overhead.
A TTL index must be used to automatically clean up expired locks. Such an index can be created manually:
db.lock.ensureIndex(
{ "expires_at": 1 },
{ "expireAfterSeconds": 0 }
)Alternatively, the method MongoDbStore::createTtlIndex(int $expireAfterSeconds = 0)
can be called once to create the TTL index during database setup. Read more
about Expire Data from Collections by Setting TTL in MongoDB.
Tip
MongoDbStore will attempt to automatically create a TTL index.
It's recommended to set constructor option gcProbablity = 0.0 to
disable this behavior if you have manually dealt with TTL index creation.
Caution!
This store relies on all PHP application and database nodes to have
synchronized clocks for lock expiry to occur at the correct time. To ensure
locks don't expire prematurely; the lock TTL should be set with enough extra
time in expireAfterSeconds to account for any clock drift between nodes.
writeConcern, readConcern and readPreference are not specified by
MongoDbStore meaning the collection's settings will take effect. Read more
about Replica Set Read and Write Semantics in MongoDB.
The PdoStore relies on the ACID properties of the SQL engine.
Caution!
In a cluster configured with multiple primaries, ensure writes are synchronously propagated to every nodes, or always use the same node.
Caution!
Some SQL engines like MySQL allow to disable the unique constraint check.
Ensure that this is not the case SET unique_checks=1;.
In order to purge old locks, this store uses a current datetime to define an expiration date reference. This mechanism relies on all server nodes to have synchronized clocks.
Caution!
To ensure locks don't expire prematurely; the TTLs should be set with enough extra time to account for any clock drift between nodes.
The way Redis works is to store items in memory. That means that by using the :ref:`RedisStore <lock-store-redis>` the locks are not persisted and may disappear by mistake at anytime.
If the Redis service or the machine hosting it restarts, every locks would be lost without notifying the running processes.
Caution!
To avoid that someone else acquires a lock after a restart, it's recommended to delay service start and wait at least as long as the longest lock TTL.
Tip
Redis can be configured to persist items on disk, but this option would slow down writes on the service. This could go against other uses of the server.
When the Redis service is shared and used for multiple usages, locks could be removed by mistake.
Caution!
The command FLUSHDB must not be called, or locks should be stored in a
dedicated Redis service away from Cache.
Combined stores allow to store locks across several backends. It's a common
mistake to think that the lock mechanism will be more reliable. This is wrong
The CombinedStore will be, at best, as reliable as the least reliable of
all managed stores. As soon as one managed store returns erroneous information,
the CombinedStore won't be reliable.
Caution!
All concurrent processes must use the same configuration, with the same amount of managed stored and the same endpoint.
Tip
Instead of using a cluster of Redis or Memcached servers, it's better to use
a CombinedStore with a single server per managed store.
Semaphores are handled by the Kernel level. In order to be reliable, processes must run on the same machine, virtual machine or container. Be careful when updating a Kubernetes or Swarm service because for a short period of time, there can be two running containers in parallel.
Caution!
All concurrent processes must use the same machine. Before starting a concurrent process on a new machine, check that other process are stopped on the old one.
Caution!
When running on systemd with non-system user and option RemoveIPC=yes
(default value), locks are deleted by systemd when that user logs out.
Check that process is run with a system user (UID <= SYS_UID_MAX) with
SYS_UID_MAX defined in /etc/login.defs, or set the option
RemoveIPC=off in /etc/systemd/logind.conf.
The way ZookeeperStore works is by maintaining locks as ephemeral nodes on the server. That means that by using :ref:`ZookeeperStore <lock-store-zookeeper>` the locks will be automatically released at the end of the session in case the client cannot unlock for any reason.
If the ZooKeeper service or the machine hosting it restarts, every lock would be lost without notifying the running processes.
Tip
To use ZooKeeper's high-availability feature, you can setup a cluster of multiple servers so that in case one of the server goes down, the majority will still be up and serving the requests. All the available servers in the cluster will see the same state.
Note
As this store does not support multi-level node locks, since the clean up of intermediate nodes becomes an overhead, all locks are maintained at the root level.
Changing the configuration of stores should be done very carefully. For instance, during the deployment of a new version. Processes with new configuration must not be started while old processes with old configuration are still running.