Transactions and Concurrency

Approach 1: Implicitly

The first approach is to use the implicit transaction handling provided by the Doctrine ORM EntityManager. Given the following code snippet, without any explicit transaction demarcation:

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<?php
// $em instanceof EntityManager
$user = new User;
$user->setName('George');
$em->persist($user);
$em->flush();
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Since we do not do any custom transaction demarcation in the above code, EntityManager#flush() will begin and commit/rollback a transaction. This behavior is made possible by the aggregation of the DML operations by the Doctrine ORM and is sufficient if all the data manipulation that is part of a unit of work happens through the domain model and thus the ORM.

Approach 2: Explicitly

The explicit alternative is to use the Doctrine\DBAL\Connection API directly to control the transaction boundaries. The code then looks like this:

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<?php
// $em instanceof EntityManager
$em->getConnection()->beginTransaction(); // suspend auto-commit
try {
    // ... do some work
    $user = new User;
    $user->setName('George');
    $em->persist($user);
    $em->flush();
    $em->getConnection()->commit();
} catch (Exception $e) {
    $em->getConnection()->rollBack();
    throw $e;
}
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Explicit transaction demarcation is required when you want to include custom DBAL operations in a unit of work or when you want to make use of some methods of the EntityManager API that require an active transaction. Such methods will throw a TransactionRequiredException to inform you of that requirement.

A more convenient alternative for explicit transaction demarcation is the use of provided control abstractions in the form of Connection#transactional($func) and EntityManager#transactional($func). When used, these control abstractions ensure that you never forget to rollback the transaction, in addition to the obvious code reduction. An example that is functionally equivalent to the previously shown code looks as follows:

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<?php
// $em instanceof EntityManager
$em->transactional(function($em) {
    // ... do some work
    $user = new User;
    $user->setName('George');
    $em->persist($user);
});
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The difference between Connection#transactional($func) and EntityManager#transactional($func) is that the latter abstraction flushes the EntityManager prior to transaction commit and rolls back the transaction when an exception occurs.

Exception Handling

When using implicit transaction demarcation and an exception occurs during EntityManager#flush(), the transaction is automatically rolled back and the EntityManager closed.

When using explicit transaction demarcation and an exception occurs, the transaction should be rolled back immediately and the EntityManager closed by invoking EntityManager#close() and subsequently discarded, as demonstrated in the example above. This can be handled elegantly by the control abstractions shown earlier. Note that when catching Exception you should generally re-throw the exception. If you intend to recover from some exceptions, catch them explicitly in earlier catch blocks (but do not forget to rollback the transaction and close the EntityManager there as well). All other best practices of exception handling apply similarly (i.e. either log or re-throw, not both, etc.).

As a result of this procedure, all previously managed or removed instances of the EntityManager become detached. The state of the detached objects will be the state at the point at which the transaction was rolled back. The state of the objects is in no way rolled back and thus the objects are now out of synch with the database. The application can continue to use the detached objects, knowing that their state is potentially no longer accurate.

If you intend to start another unit of work after an exception has occurred you should do that with a new EntityManager.

Optimistic Locking

Database transactions are fine for concurrency control during a single request. However, a database transaction should not span across requests, the so-called "user think time". Therefore a long-running "business transaction" that spans multiple requests needs to involve several database transactions. Thus, database transactions alone can no longer control concurrency during such a long-running business transaction. Concurrency control becomes the partial responsibility of the application itself.

Doctrine has integrated support for automatic optimistic locking via a version field. In this approach any entity that should be protected against concurrent modifications during long-running business transactions gets a version field that is either a simple number (mapping type: integer) or a timestamp (mapping type: datetime). When changes to such an entity are persisted at the end of a long-running conversation the version of the entity is compared to the version in the database and if they don't match, an OptimisticLockException is thrown, indicating that the entity has been modified by someone else already.

You designate a version field in an entity as follows. In this example we'll use an integer.

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<?php
class User
{
    // ...
    /** @Version @Column(type="integer") */
    private $version;
    // ...
}
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<doctrine-mapping>
  <entity name="User">
    <field name="version" type="integer" version="true" />
  </entity>
</doctrine-mapping>
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User:
  type: entity
  fields:
    version:
      type: integer
      version: true
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Alternatively a datetime type can be used (which maps to a SQL timestamp or datetime):

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<?php
class User
{
    // ...
    /** @Version @Column(type="datetime") */
    private $version;
    // ...
}
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<doctrine-mapping>
  <entity name="User">
    <field name="version" type="datetime" version="true" />
  </entity>
</doctrine-mapping>
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User:
  type: entity
  fields:
    version:
      type: datetime
      version: true
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Version numbers (not timestamps) should however be preferred as they can not potentially conflict in a highly concurrent environment, unlike timestamps where this is a possibility, depending on the resolution of the timestamp on the particular database platform.

When a version conflict is encountered during EntityManager#flush(), an OptimisticLockException is thrown and the active transaction rolled back (or marked for rollback). This exception can be caught and handled. Potential responses to an OptimisticLockException are to present the conflict to the user or to refresh or reload objects in a new transaction and then retrying the transaction.

With PHP promoting a share-nothing architecture, the time between showing an update form and actually modifying the entity can in the worst scenario be as long as your applications session timeout. If changes happen to the entity in that time frame you want to know directly when retrieving the entity that you will hit an optimistic locking exception:

You can always verify the version of an entity during a request either when calling EntityManager#find():

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<?php
use Doctrine\DBAL\LockMode;
use Doctrine\ORM\OptimisticLockException;

$theEntityId = 1;
$expectedVersion = 184;

try {
    $entity = $em->find('User', $theEntityId, LockMode::OPTIMISTIC, $expectedVersion);

    // do the work

    $em->flush();
} catch(OptimisticLockException $e) {
    echo "Sorry, but someone else has already changed this entity. Please apply the changes again!";
}
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Or you can use EntityManager#lock() to find out:

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<?php
use Doctrine\DBAL\LockMode;
use Doctrine\ORM\OptimisticLockException;

$theEntityId = 1;
$expectedVersion = 184;

$entity = $em->find('User', $theEntityId);

try {
    // assert version
    $em->lock($entity, LockMode::OPTIMISTIC, $expectedVersion);

} catch(OptimisticLockException $e) {
    echo "Sorry, but someone else has already changed this entity. Please apply the changes again!";
}
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<?php
$post = $em->find('BlogPost', 123456);

echo '<input type="hidden" name="id" value="' . $post->getId() . '" />';
echo '<input type="hidden" name="version" value="' . $post->getCurrentVersion() . '" />';
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$postId = (int)$_GET['id'];
$postVersion = (int)$_GET['version'];

$post = $em->find('BlogPost', $postId, \Doctrine\DBAL\LockMode::OPTIMISTIC, $postVersion);
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Pessimistic Locking

Doctrine ORM supports Pessimistic Locking at the database level. No attempt is being made to implement pessimistic locking inside Doctrine, rather vendor-specific and ANSI-SQL commands are used to acquire row-level locks. Every Entity can be part of a pessimistic lock, there is no special metadata required to use this feature.

However for Pessimistic Locking to work you have to disable the Auto-Commit Mode of your Database and start a transaction around your pessimistic lock use-case using the "Approach 2: Explicit Transaction Demarcation" described above. Doctrine ORM will throw an Exception if you attempt to acquire an pessimistic lock and no transaction is running.

Doctrine ORM currently supports two pessimistic lock modes:

• Pessimistic Write (Doctrine\DBAL\LockMode::PESSIMISTIC_WRITE), locks the underlying database rows for concurrent Read and Write Operations.
• Pessimistic Read (Doctrine\DBAL\LockMode::PESSIMISTIC_READ), locks other concurrent requests that attempt to update or lock rows in write mode.

You can use pessimistic locks in three different scenarios:

1. Using EntityManager#find($className, $id, \Doctrine\DBAL\LockMode::PESSIMISTIC_WRITE) or EntityManager#find($className, $id, \Doctrine\DBAL\LockMode::PESSIMISTIC_READ)
2. Using EntityManager#lock($entity, \Doctrine\DBAL\LockMode::PESSIMISTIC_WRITE) or EntityManager#lock($entity, \Doctrine\DBAL\LockMode::PESSIMISTIC_READ)
3. Using Query#setLockMode(\Doctrine\DBAL\LockMode::PESSIMISTIC_WRITE) or Query#setLockMode(\Doctrine\DBAL\LockMode::PESSIMISTIC_READ)