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.

As stated in the FAQ, “MongoDB does not support multi-document transactions” and neither does Doctrine MongoDB ODM.

Limitation

At the moment, Doctrine MongoDB ODM does not provide any native strategy to emulate multi-document transactions.

Workaround

To work around this limitation, one can utilize two phase commits.

Concurrency

Doctrine MongoDB ODM offers native support for pessimistic and optimistic locking strategies. This allows for very 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 that is either a simple number (mapping type: int) or a date (mapping type: date). 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.

Note

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

Document Configuration

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

  • PHP
    <?php
    /** @Version @Field(type="int") */
    private $version;
    
  • XML
    <field fieldName="version" version="true" type="int" />
    
  • YAML
    version:
      type: int
      version: true
    

Alternatively, the date type may be used:

  • PHP
    <?php
    /** @Version @Field(type="date") */
    private $version;
    
  • XML
    <field fieldName="version" version="true" type="date" />
    
  • YAML
    version:
      type: date
      version: true
    
Choosing the Field Type

When using the date 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 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():

<?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', $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!";
}

Alternatively, an expected version may be specified for an existing document with DocumentManager#lock():

<?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', $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!";
}

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):

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

/* @var $dm DocumentManager */

$post = $dm->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
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', $postId, LockMode::OPTIMISTIC, $postVersion);

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:

  • PHP
    <?php
    /** @Lock @Field(type="int") */
    private $lock;
    
  • XML
    <field fieldName="lock" lock="true" type="int" />
    
  • YAML
    lock:
      type: int
      lock: true
    

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:

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

Warning

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.
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