9. Transactions and Concurrency

9.1. Transaction Demarcation

Transaction demarcation is the task of defining your transaction boundaries. Proper transaction demarcation is very important because if not done properly it can negatively affect the performance of your application. Many databases and database abstraction layers like PDO by default operate in auto-commit mode, which means that every single SQL statement is wrapped in a small transaction. Without any explicit transaction demarcation from your side, this quickly results in poor performance because transactions are not cheap.

For the most part, Doctrine 2 already takes care of proper transaction demarcation for you: All the write operations (INSERT/UPDATE/DELETE) are queued until EntityManager#flush() is invoked which wraps all of these changes in a single transaction.

However, Doctrine 2 also allows (and encourages) you to take over and control transaction demarcation yourself.

These are two ways to deal with transactions when using the Doctrine ORM and are now described in more detail.

9.1.1. 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:

<?php
// $em instanceof EntityManager
$user = new User;
$user->setName('George');
$em->persist($user);
$em->flush();

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.

9.1.2. 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:

<?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();
    $em->close();
    throw $e;
}

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 or close the EntityManager, apart from the obvious code reduction. An example that is functionally equivalent to the previously shown code looks as follows:

<?php
// $em instanceof EntityManager
$em->transactional(function($em) {
    //... do some work
    $user = new User;
    $user->setName('George');
    $em->persist($user);
});

The difference between Connection#transactional($func) and EntityManager#transactional($func) is that the latter abstraction flushes the EntityManager prior to transaction commit and also closes the EntityManager properly when an exception occurs (in addition to rolling back the transaction).

9.1.3. 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.

9.2. Locking Support

Doctrine 2 offers support for Pessimistic- and Optimistic-locking strategies natively. This allows to take very fine-grained control over what kind of locking is required for your Entities in your application.

9.2.1. 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.

<?php
class User
{
    // ...
    /** @Version @Column(type="integer") */
    private $version;
    // ...
}

Alternatively a datetime type can be used (which maps to an SQL timestamp or datetime):

<?php
class User
{
    // ...
    /** @Version @Column(type="datetime") */
    private $version;
    // ...
}

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():

<?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!";
}

Or you can use EntityManager#lock() to find out:

<?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!";
}

9.2.1.1. Important Implementation Notes

You can easily get the optimistic locking workflow wrong if you compare the wrong versions. Say you have Alice and Bob accessing a hypothetical bank account:

  • Alice reads the headline of the blog post being “Foo”, at optimistic lock version 1 (GET Request)
  • Bob reads the headline of the blog post being “Foo”, at optimistic lock version 1 (GET Request)
  • Bob updates the headline to “Bar”, upgrading the optimistic lock version to 2 (POST Request of a Form)
  • Alice updates the headline to “Baz”, ... (POST Request of a Form)

Now at the last stage of this scenario the blog post has to be read again from the database before Alice’s headline can be applied. At this point you will want to check if the blog post is still at version 1 (which it is not in this scenario).

Using optimistic locking correctly, you have to add the version as an additional hidden field (or into the SESSION for more safety). Otherwise you cannot verify the version is still the one being originally read from the database when Alice performed her GET request for the blog post. If this happens you might see lost updates you wanted to prevent with Optimistic Locking.

See the example code, The form (GET Request):

<?php
$post = $em->find('BlogPost', 123456);

echo '<input type="hidden" name="id" value="' . $post->getId() . '" />';
echo '<input type="hidden" name="version" value="' . $post->getCurrentVersion() . '" />';

And the change headline action (POST Request):

<?php
$postId = (int)$_GET['id'];
$postVersion = (int)$_GET['version'];

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

9.2.2. Pessimistic Locking

Doctrine 2 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 2 will throw an Exception if you attempt to acquire an pessimistic lock and no transaction is running.

Doctrine 2 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)



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