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Transactions and Concurrency

Transactions

As per the documentation, MongoDB write operations are "atomic on the level of a single document".

Even when updating multiple documents within a single write operation, though the modification of each document is atomic, the operation as a whole is not and other operations may interleave.

Transaction support

MongoDB supports multi-document transactions on replica sets (starting in MongoDB 4.2) and sharded clusters (MongoDB 4.4). Standalone topologies do not support multi-document transactions.

Transaction Support in Doctrine MongoDB ODM

Transaction support in MongoDB ODM was introduced in version 2.7.

You can instruct the ODM to use transactions when writing changes to the databases by enabling the useTransactionalFlush setting in your configuration:

1 $config = new Configuration();
$config->setUseTransactionalFlush(true);
// Other configuration

$dm = DocumentManager::create(null, $config);
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From then onwards, any call to DocumentManager::flush will start a transaction, apply the write operations, then commit the transaction.

To enable or disable transaction usage for a single flush operation, use the withTransaction write option when calling DocumentManager::flush:

1 // To explicitly enable transaction for this write
$dm->flush(['withTransaction' => true]);

// To disable transaction usage for a write, regardless of the ``useTransactionalFlush`` config:
$dm->flush(['withTransaction' => false]);
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Please note that transactions are only used for write operations executed during the flush operation. For any other operations, e.g. manually executed queries or aggregation pipelines, transactions will not be used and you will have to rely on the MongoDB driver's transaction mechanism.

Lifecycle Events and Transactions

When using transactional flushes, either through the configuration or explicitly, there are a couple of important things to note regarding lifecycle events. Due to the way MongoDB transactions work, it is possible that ODM attempts write operations multiple times. However, to preserve the expectation that lifecycle events are only triggered once per flush operation, lifecycle events will not be dispatched when the transaction is retried. This maintains current functionality when a lifecycle event modifies the unit of work, as this change is automatically carried over when the transaction is retried.

Lifecycle events now expose a MongoDB\Driver\Session object which needs to be used if it is set. Since MongoDB transactions are not tied to the connection but only to a session, any command that should be part of the transaction needs to be told about the session to be used. This does not only apply to write commands, but also to read commands that need to see the transaction state. If a session is given in a lifecycle event, this session should always be used regardless of whether a transaction is active or not.

Other Concurrency Controls

Multi-Document transactions provide certain guarantees regarding your database writes and prevent two simultaneous write operations from interfering with each other. Depending on your use case, this is not enough, as the transactional guarantee will only apply once you start writing to the database as part of the DocumentManager::flush() call. This could still lead to data loss if you replace data that was written to the database by a different process in between you reading the data and starting the transaction. To solve this problem, optimistic and pessimistic locking strategies can be used, allowing for fine-grained control over what kind of locking is required for documents in your application.

Optimistic Locking

Approach

Doctrine has integrated support for automatic optimistic locking via a version field. Any document that should be protected against concurrent modifications during long-running business transactions gets a version field. When changes to the document are persisted, the expected version and version increment are incorporated into the update criteria and modifiers, respectively. If this results in no document being modified by the update (i.e. expected version did not match), a LockException is thrown, which indicates that the document was already modified by another query.

| Versioning can only be used on root (top-level) documents.

Only types implementing the \Doctrine\ODM\MongoDB\Types\Versionable interface can be used for versioning. Following ODM types can be used for versioning: int, decimal128, date, and date_immutable.

Document Configuration

The following example designates a version field using the int type:

1 <?php
/** @Version @Field(type="int") */
private $version;
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1 <field field-name="version" version="true" type="int" />

Or with decimal128 type:

1 <?php
/** @Version @Field(type="decimal128") */
private $version;
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1 <field field-name="version" version="true" type="decimal128" />

Alternatively, the date type may be used:

1 <?php
/** @Version @Field(type="date") */
private $version;
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1 <field field-name="version" version="true" type="date" />

Or its immutable counterpart date_immutable:

1 <?php
/** @Version @Field(type="date_immutable") */
private $version;
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1 <field field-name="version" version="true" type="date_immutable" />

Choosing the Field Type

When using the date-based type in a high-concurrency environment, it is still possible to create multiple documents with the same version and cause a conflict. This can be avoided by using the int or decimal128 type.

Usage

When a version conflict is encountered during DocumentManager#flush(), a LockException is thrown. This exception can be caught and handled. Potential responses to a LockException are to present the conflict to the user or to refresh or reload objects and then retry the update.

With PHP promoting a share-nothing architecture, the worst case scenario for a delay between rendering an update form (with existing document data) and modifying the document after a form submission may be your application's session timeout. If the document is changed within that time frame by some other request, it may be preferable to encounter a LockException when retrieving the document instead of executing the update.

You can specify the expected version of a document during a query with DocumentManager#find():

1 <?php
use Doctrine\ODM\MongoDB\LockMode;
use Doctrine\ODM\MongoDB\LockException;
use Doctrine\ODM\MongoDB\DocumentManager;

$theDocumentId = 1;
$expectedVersion = 184;

/* @var $dm DocumentManager */

try {
    $document = $dm->find(User::class, $theDocumentId, LockMode::OPTIMISTIC, $expectedVersion);

    // do the work

    $dm->flush();
} catch(LockException $e) {
    echo "Sorry, but someone else has already changed this document. Please apply the changes again!";
}
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Alternatively, an expected version may be specified for an existing document with DocumentManager#lock():

1 <?php
use Doctrine\ODM\MongoDB\LockMode;
use Doctrine\ODM\MongoDB\LockException;
use Doctrine\ODM\MongoDB\DocumentManager;

$theDocumentId = 1;
$expectedVersion = 184;

/* @var $dm DocumentManager */

$document = $dm->find(User::class, $theDocumentId);

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

} catch(LockException $e) {
    echo "Sorry, but someone else has already changed this document. Please apply the changes again!";
}
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Important Implementation Notes

You can easily get the optimistic locking workflow wrong if you compare the wrong versions.

Workflow

Say you have Alice and Bob editing a hypothetical blog post:

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)

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).

In order to correctly utilize optimistic locking, you must add the version as hidden form field or, for more security, session attribute. Otherwise, you cannot verify that the version at the time of update is the same as what was originally read from the database when Alice performed her original GET request for the blog post. Without correlating the version across form submissions, the application could lose updates.

Example Code

The form (GET Request):

1 <?php
use Doctrine\ODM\MongoDB\DocumentManager;

/* @var $dm DocumentManager */

$post = $dm->find(BlogPost::class, 123456);

echo '<input type="hidden" name="id" value="' . $post->getId() . '" />';
echo '<input type="hidden" name="version" value="' . $post->getCurrentVersion() . '" />';
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And the change headline action (POST Request):

1 <?php
use Doctrine\ODM\MongoDB\DocumentManager;
use Doctrine\ODM\MongoDB\LockMode;

/* @var $dm DocumentManager */

$postId = (int)$_POST['id'];
$postVersion = (int)$_POST['version'];

$post = $dm->find(BlogPost::class, $postId, LockMode::OPTIMISTIC, $postVersion);
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Pessimistic Locking

Doctrine MongoDB ODM also supports pessimistic locking via a configurable lock field. This functionality is implemented entirely by Doctrine; MongoDB has no native support for pessimistic locking.

Document Configuration

Pessimistic locking requires a document to designate a lock field using the int type:

1 <?php
/** @Lock @Field(type="int") */
private $lock;
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1 <field field-name="lock" lock="true" type="int" />

Lock Modes

Doctrine MongoDB ODM currently supports two pessimistic lock modes:

Pessimistic Write (\Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_WRITE): locks the underlying document for concurrent read and write operations.
Pessimistic Read (\Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_READ): locks other concurrent requests that attempt to update or lock documents in write mode.

Usage

You can use pessimistic locks in two different scenarios:

Using DocumentManager#find($className, $id, \Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_WRITE) or DocumentManager#find($className, $id, \Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_READ)
Using DocumentManager#lock($document, \Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_WRITE) or DocumentManager#lock($document, \Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_READ)

| A few things could go wrong: | | If a request fails to complete (e.g. unhandled exception), you may end up with stale locks. Said locks would need to be manually released or you would need to devise a strategy to automatically do so. One way to mitigate stale locks after an application error would be to gracefully catch the exception and ensure that relevant documents are unlocked before the request ends. | | Deadlock situations are also possible. Suppose process P1 needs resource R1 and has locked resource R2 and that another process P2 has locked resource R1 but also needs resource R2. If both processes continue waiting for the respective resources, the application will be stuck. When loading a document, Doctrine can immediately throw an exception if it is already locked. A deadlock could be created by endlessly retrying attempts to acquire the lock. One can avoid a possible deadlock by designating a maximum number of retry attempts and automatically releasing any active locks with the request ends, thereby allowing a process to end gracefully while another completes its task.

1	$config = new Configuration(); $config->setUseTransactionalFlush(true); // Other configuration $dm = DocumentManager::create(null, $config);
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1	// To explicitly enable transaction for this write $dm->flush(['withTransaction' => true]); // To disable transaction usage for a write, regardless of the ``useTransactionalFlush`` config: $dm->flush(['withTransaction' => false]);
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1	<?php /** @Version @Field(type="int") */ private $version;
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1	<?php /** @Version @Field(type="decimal128") */ private $version;
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1	<?php /** @Version @Field(type="date") */ private $version;
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1	<?php /** @Version @Field(type="date_immutable") */ private $version;
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1	<?php use Doctrine\ODM\MongoDB\LockMode; use Doctrine\ODM\MongoDB\LockException; use Doctrine\ODM\MongoDB\DocumentManager; $theDocumentId = 1; $expectedVersion = 184; /* @var $dm DocumentManager */ try { $document = $dm->find(User::class, $theDocumentId, LockMode::OPTIMISTIC, $expectedVersion); // do the work $dm->flush(); } catch(LockException $e) { echo "Sorry, but someone else has already changed this document. Please apply the changes again!"; }
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1	<?php use Doctrine\ODM\MongoDB\DocumentManager; /* @var $dm DocumentManager */ $post = $dm->find(BlogPost::class, 123456); echo '<input type="hidden" name="id" value="' . $post->getId() . '" />'; echo '<input type="hidden" name="version" value="' . $post->getCurrentVersion() . '" />';
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