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<div class="section" id="transactions-and-concurrency">
<h1>Transactions and Concurrency<a class="headerlink" href="#transactions-and-concurrency" title="Permalink to this headline"></a></h1>
<div class="section" id="transactions">
<h2>Transactions<a class="headerlink" href="#transactions" title="Permalink to this headline"></a></h2>
<p>As per the <a class="reference external" href="https://docs.mongodb.com/manual/core/write-operations-atomicity/#atomicity-and-transactions">documentation</a>, MongoDB
write operations are &quot;atomic on the level of a single document&quot;.</p>
<p>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.</p>
<p>As stated in the <a class="reference external" href="https://docs.mongodb.com/manual/faq/fundamentals/#does-mongodb-support-transactions">FAQ</a>,
&quot;MongoDB does not support multi-document transactions&quot; and neither does Doctrine MongoDB ODM.</p>
<div class="section" id="limitation">
<h3>Limitation<a class="headerlink" href="#limitation" title="Permalink to this headline"></a></h3>
<p>At the moment, Doctrine MongoDB ODM does not provide any native strategy to emulate multi-document transactions.</p>
</div>
<div class="section" id="workaround">
<h3>Workaround<a class="headerlink" href="#workaround" title="Permalink to this headline"></a></h3>
<p>To work around this limitation, one can utilize <a class="reference external" href="https://docs.mongodb.com/manual/tutorial/perform-two-phase-commits/">two phase commits</a>.</p>
</div>
</div>
<div class="section" id="concurrency">
<h2>Concurrency<a class="headerlink" href="#concurrency" title="Permalink to this headline"></a></h2>
<p>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.</p>
<div class="section" id="optimistic-locking">
<span id="transactions-and-concurrency-optimistic-locking"></span><h3>Optimistic Locking<a class="headerlink" href="#optimistic-locking" title="Permalink to this headline"></a></h3>
<div class="section" id="approach">
<h4>Approach<a class="headerlink" href="#approach" title="Permalink to this headline"></a></h4>
<p>Doctrine has integrated support for automatic optimistic locking
via a <code class="docutils literal"><span class="pre">version</span></code> field. Any document that should be
protected against concurrent modifications during long-running
business transactions gets a <code class="docutils literal"><span class="pre">version</span></code> field that is either a simple
number (mapping type: <code class="docutils literal"><span class="pre">int</span></code>) or a date (mapping type: <code class="docutils literal"><span class="pre">date</span></code>).
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 <code class="docutils literal"><span class="pre">LockException</span></code> is thrown, which indicates that the document was already modified by another query.</p>
<div class="admonition note">
<p class="first admonition-title">Note</p>
<div class="last line-block">
<div class="line">Versioning can only be used on <em>root</em> (top-level) documents.</div>
</div>
</div>
</div>
<div class="section" id="document-configuration">
<h4>Document Configuration<a class="headerlink" href="#document-configuration" title="Permalink to this headline"></a></h4>
<p>The following example designates a version field using the <code class="docutils literal"><span class="pre">int</span></code> type:</p>
<div class="configuration-block">
<ul class="simple">
<li><em>PHP</em><div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="sd">/** @Version @Field(type=&quot;int&quot;) */</span>
<span class="k">private</span> <span class="nv">$version</span><span class="p">;</span>
</pre></div>
</div>
</li>
<li><em>XML</em><div class="highlight-xml"><div class="highlight"><pre><span class="nt">&lt;field</span> <span class="na">fieldName=</span><span class="s">&quot;version&quot;</span> <span class="na">version=</span><span class="s">&quot;true&quot;</span> <span class="na">type=</span><span class="s">&quot;int&quot;</span> <span class="nt">/&gt;</span>
</pre></div>
</div>
</li>
<li><em>YAML</em><div class="highlight-yaml"><div class="highlight"><pre><span class="l l-Scalar l-Scalar-Plain">version</span><span class="p p-Indicator">:</span>
<span class="l l-Scalar l-Scalar-Plain">type</span><span class="p p-Indicator">:</span> <span class="l l-Scalar l-Scalar-Plain">int</span>
<span class="l l-Scalar l-Scalar-Plain">version</span><span class="p p-Indicator">:</span> <span class="l l-Scalar l-Scalar-Plain">true</span>
</pre></div>
</div>
</li>
</ul>
</div>
<p>Alternatively, the <code class="docutils literal"><span class="pre">date</span></code> type may be used:</p>
<div class="configuration-block">
<ul class="simple">
<li><em>PHP</em><div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="sd">/** @Version @Field(type=&quot;date&quot;) */</span>
<span class="k">private</span> <span class="nv">$version</span><span class="p">;</span>
</pre></div>
</div>
</li>
<li><em>XML</em><div class="highlight-xml"><div class="highlight"><pre><span class="nt">&lt;field</span> <span class="na">fieldName=</span><span class="s">&quot;version&quot;</span> <span class="na">version=</span><span class="s">&quot;true&quot;</span> <span class="na">type=</span><span class="s">&quot;date&quot;</span> <span class="nt">/&gt;</span>
</pre></div>
</div>
</li>
<li><em>YAML</em><div class="highlight-yaml"><div class="highlight"><pre><span class="l l-Scalar l-Scalar-Plain">version</span><span class="p p-Indicator">:</span>
<span class="l l-Scalar l-Scalar-Plain">type</span><span class="p p-Indicator">:</span> <span class="l l-Scalar l-Scalar-Plain">date</span>
<span class="l l-Scalar l-Scalar-Plain">version</span><span class="p p-Indicator">:</span> <span class="l l-Scalar l-Scalar-Plain">true</span>
</pre></div>
</div>
</li>
</ul>
</div>
<div class="section" id="choosing-the-field-type">
<h5>Choosing the Field Type<a class="headerlink" href="#choosing-the-field-type" title="Permalink to this headline"></a></h5>
<p>When using the <code class="docutils literal"><span class="pre">date</span></code> 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 <code class="docutils literal"><span class="pre">int</span></code> type.</p>
</div>
<div class="section" id="usage">
<h5>Usage<a class="headerlink" href="#usage" title="Permalink to this headline"></a></h5>
<p>When a version conflict is encountered during
<code class="docutils literal"><span class="pre">DocumentManager#flush()</span></code>, a <code class="docutils literal"><span class="pre">LockException</span></code> is thrown.
This exception can be caught and handled. Potential responses to a
<code class="docutils literal"><span class="pre">LockException</span></code> are to present the conflict to the user or
to refresh or reload objects and then retry the update.</p>
<p>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 <code class="docutils literal"><span class="pre">LockException</span></code> when retrieving the document instead of executing the update.</p>
<p>You can specify the expected version of a document during a query with <code class="docutils literal"><span class="pre">DocumentManager#find()</span></code>:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">use</span> <span class="nx">Doctrine\ODM\MongoDB\LockMode</span><span class="p">;</span>
<span class="k">use</span> <span class="nx">Doctrine\ODM\MongoDB\LockException</span><span class="p">;</span>
<span class="k">use</span> <span class="nx">Doctrine\ODM\MongoDB\DocumentManager</span><span class="p">;</span>
<span class="nv">$theDocumentId</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
<span class="nv">$expectedVersion</span> <span class="o">=</span> <span class="mi">184</span><span class="p">;</span>
<span class="cm">/* @var $dm DocumentManager */</span>
<span class="k">try</span> <span class="p">{</span>
<span class="nv">$document</span> <span class="o">=</span> <span class="nv">$dm</span><span class="o">-&gt;</span><span class="na">find</span><span class="p">(</span><span class="s1">&#39;User&#39;</span><span class="p">,</span> <span class="nv">$theDocumentId</span><span class="p">,</span> <span class="nx">LockMode</span><span class="o">::</span><span class="na">OPTIMISTIC</span><span class="p">,</span> <span class="nv">$expectedVersion</span><span class="p">);</span>
<span class="c1">// do the work</span>
<span class="nv">$dm</span><span class="o">-&gt;</span><span class="na">flush</span><span class="p">();</span>
<span class="p">}</span> <span class="k">catch</span><span class="p">(</span><span class="nx">LockException</span> <span class="nv">$e</span><span class="p">)</span> <span class="p">{</span>
<span class="k">echo</span> <span class="s2">&quot;Sorry, but someone else has already changed this document. Please apply the changes again!&quot;</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Alternatively, an expected version may be specified for an existing document with <code class="docutils literal"><span class="pre">DocumentManager#lock()</span></code>:</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">use</span> <span class="nx">Doctrine\ODM\MongoDB\LockMode</span><span class="p">;</span>
<span class="k">use</span> <span class="nx">Doctrine\ODM\MongoDB\LockException</span><span class="p">;</span>
<span class="k">use</span> <span class="nx">Doctrine\ODM\MongoDB\DocumentManager</span><span class="p">;</span>
<span class="nv">$theDocumentId</span> <span class="o">=</span> <span class="mi">1</span><span class="p">;</span>
<span class="nv">$expectedVersion</span> <span class="o">=</span> <span class="mi">184</span><span class="p">;</span>
<span class="cm">/* @var $dm DocumentManager */</span>
<span class="nv">$document</span> <span class="o">=</span> <span class="nv">$dm</span><span class="o">-&gt;</span><span class="na">find</span><span class="p">(</span><span class="s1">&#39;User&#39;</span><span class="p">,</span> <span class="nv">$theDocumentId</span><span class="p">);</span>
<span class="k">try</span> <span class="p">{</span>
<span class="c1">// assert version</span>
<span class="nv">$dm</span><span class="o">-&gt;</span><span class="na">lock</span><span class="p">(</span><span class="nv">$document</span><span class="p">,</span> <span class="nx">LockMode</span><span class="o">::</span><span class="na">OPTIMISTIC</span><span class="p">,</span> <span class="nv">$expectedVersion</span><span class="p">);</span>
<span class="p">}</span> <span class="k">catch</span><span class="p">(</span><span class="nx">LockException</span> <span class="nv">$e</span><span class="p">)</span> <span class="p">{</span>
<span class="k">echo</span> <span class="s2">&quot;Sorry, but someone else has already changed this document. Please apply the changes again!&quot;</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="important-implementation-notes">
<h4>Important Implementation Notes<a class="headerlink" href="#important-implementation-notes" title="Permalink to this headline"></a></h4>
<p>You can easily get the optimistic locking workflow wrong if you
compare the wrong versions.</p>
<div class="section" id="workflow">
<h5>Workflow<a class="headerlink" href="#workflow" title="Permalink to this headline"></a></h5>
<p>Say you have Alice and Bob editing a
hypothetical blog post:</p>
<ul class="simple">
<li>Alice reads the headline of the blog post being &quot;Foo&quot;, at
optimistic lock version 1 (GET Request)</li>
<li>Bob reads the headline of the blog post being &quot;Foo&quot;, at
optimistic lock version 1 (GET Request)</li>
<li>Bob updates the headline to &quot;Bar&quot;, upgrading the optimistic lock
version to 2 (POST Request of a Form)</li>
<li>Alice updates the headline to &quot;Baz&quot;, ... (POST Request of a
Form)</li>
</ul>
<p>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).</p>
<p>In order to correctly utilize optimistic locking, you <em>must</em> 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.</p>
</div>
<div class="section" id="example-code">
<h5>Example Code<a class="headerlink" href="#example-code" title="Permalink to this headline"></a></h5>
<p>The form (GET Request):</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">use</span> <span class="nx">Doctrine\ODM\MongoDB\DocumentManager</span><span class="p">;</span>
<span class="cm">/* @var $dm DocumentManager */</span>
<span class="nv">$post</span> <span class="o">=</span> <span class="nv">$dm</span><span class="o">-&gt;</span><span class="na">find</span><span class="p">(</span><span class="s1">&#39;BlogPost&#39;</span><span class="p">,</span> <span class="mi">123456</span><span class="p">);</span>
<span class="k">echo</span> <span class="s1">&#39;&lt;input type=&quot;hidden&quot; name=&quot;id&quot; value=&quot;&#39;</span> <span class="o">.</span> <span class="nv">$post</span><span class="o">-&gt;</span><span class="na">getId</span><span class="p">()</span> <span class="o">.</span> <span class="s1">&#39;&quot; /&gt;&#39;</span><span class="p">;</span>
<span class="k">echo</span> <span class="s1">&#39;&lt;input type=&quot;hidden&quot; name=&quot;version&quot; value=&quot;&#39;</span> <span class="o">.</span> <span class="nv">$post</span><span class="o">-&gt;</span><span class="na">getCurrentVersion</span><span class="p">()</span> <span class="o">.</span> <span class="s1">&#39;&quot; /&gt;&#39;</span><span class="p">;</span>
</pre></div>
</div>
<p>And the change headline action (POST Request):</p>
<div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="k">use</span> <span class="nx">Doctrine\ODM\MongoDB\DocumentManager</span><span class="p">;</span>
<span class="k">use</span> <span class="nx">Doctrine\ODM\MongoDB\LockMode</span><span class="p">;</span>
<span class="cm">/* @var $dm DocumentManager */</span>
<span class="nv">$postId</span> <span class="o">=</span> <span class="p">(</span><span class="nx">int</span><span class="p">)</span><span class="nv">$_POST</span><span class="p">[</span><span class="s1">&#39;id&#39;</span><span class="p">];</span>
<span class="nv">$postVersion</span> <span class="o">=</span> <span class="p">(</span><span class="nx">int</span><span class="p">)</span><span class="nv">$_POST</span><span class="p">[</span><span class="s1">&#39;version&#39;</span><span class="p">];</span>
<span class="nv">$post</span> <span class="o">=</span> <span class="nv">$dm</span><span class="o">-&gt;</span><span class="na">find</span><span class="p">(</span><span class="s1">&#39;BlogPost&#39;</span><span class="p">,</span> <span class="nv">$postId</span><span class="p">,</span> <span class="nx">LockMode</span><span class="o">::</span><span class="na">OPTIMISTIC</span><span class="p">,</span> <span class="nv">$postVersion</span><span class="p">);</span>
</pre></div>
</div>
</div>
</div>
</div>
<div class="section" id="pessimistic-locking">
<span id="transactions-and-concurrency-pessimistic-locking"></span><h3>Pessimistic Locking<a class="headerlink" href="#pessimistic-locking" title="Permalink to this headline"></a></h3>
<p>Doctrine MongoDB ODM also supports pessimistic locking via a configurable <code class="docutils literal"><span class="pre">lock</span></code> field.
This functionality is implemented entirely by Doctrine; MongoDB has no native support for pessimistic locking.</p>
<div class="section" id="id1">
<h4>Document Configuration<a class="headerlink" href="#id1" title="Permalink to this headline"></a></h4>
<p>Pessimistic locking requires a document to designate a lock field using the <code class="docutils literal"><span class="pre">int</span></code> type:</p>
<div class="configuration-block">
<ul class="simple">
<li><em>PHP</em><div class="highlight-php"><div class="highlight"><pre><span class="cp">&lt;?php</span>
<span class="sd">/** @Lock @Field(type=&quot;int&quot;) */</span>
<span class="k">private</span> <span class="nv">$lock</span><span class="p">;</span>
</pre></div>
</div>
</li>
<li><em>XML</em><div class="highlight-xml"><div class="highlight"><pre><span class="nt">&lt;field</span> <span class="na">fieldName=</span><span class="s">&quot;lock&quot;</span> <span class="na">lock=</span><span class="s">&quot;true&quot;</span> <span class="na">type=</span><span class="s">&quot;int&quot;</span> <span class="nt">/&gt;</span>
</pre></div>
</div>
</li>
<li><em>YAML</em><div class="highlight-yaml"><div class="highlight"><pre><span class="l l-Scalar l-Scalar-Plain">lock</span><span class="p p-Indicator">:</span>
<span class="l l-Scalar l-Scalar-Plain">type</span><span class="p p-Indicator">:</span> <span class="l l-Scalar l-Scalar-Plain">int</span>
<span class="l l-Scalar l-Scalar-Plain">lock</span><span class="p p-Indicator">:</span> <span class="l l-Scalar l-Scalar-Plain">true</span>
</pre></div>
</div>
</li>
</ul>
</div>
</div>
<div class="section" id="lock-modes">
<h4>Lock Modes<a class="headerlink" href="#lock-modes" title="Permalink to this headline"></a></h4>
<p>Doctrine MongoDB ODM currently supports two pessimistic lock modes:</p>
<ul class="simple">
<li>Pessimistic Write
(<code class="docutils literal"><span class="pre">\Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_WRITE</span></code>): locks the
underlying document for concurrent read and write operations.</li>
<li>Pessimistic Read (<code class="docutils literal"><span class="pre">\Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_READ</span></code>):
locks other concurrent requests that attempt to update or lock documents
in write mode.</li>
</ul>
</div>
<div class="section" id="id2">
<h4>Usage<a class="headerlink" href="#id2" title="Permalink to this headline"></a></h4>
<p>You can use pessimistic locks in two different scenarios:</p>
<ol class="arabic simple">
<li>Using
<code class="docutils literal"><span class="pre">DocumentManager#find($className,</span> <span class="pre">$id,</span> <span class="pre">\Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_WRITE)</span></code>
or
<code class="docutils literal"><span class="pre">DocumentManager#find($className,</span> <span class="pre">$id,</span> <span class="pre">\Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_READ)</span></code></li>
<li>Using
<code class="docutils literal"><span class="pre">DocumentManager#lock($document,</span> <span class="pre">\Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_WRITE)</span></code>
or
<code class="docutils literal"><span class="pre">DocumentManager#lock($document,</span> <span class="pre">\Doctrine\ODM\MongoDB\LockMode::PESSIMISTIC_READ)</span></code></li>
</ol>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<div class="last line-block">
<div class="line">A few things could go wrong:</div>
<div class="line"><br /></div>
<div class="line">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.</div>
<div class="line"><br /></div>
<div class="line"><a class="reference external" href="https://en.wikipedia.org/wiki/Deadlock">Deadlock</a> 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.</div>
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<h3><a href="../index.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">Transactions and Concurrency</a><ul>
<li><a class="reference internal" href="#transactions">Transactions</a><ul>
<li><a class="reference internal" href="#limitation">Limitation</a></li>
<li><a class="reference internal" href="#workaround">Workaround</a></li>
</ul>
</li>
<li><a class="reference internal" href="#concurrency">Concurrency</a><ul>
<li><a class="reference internal" href="#optimistic-locking">Optimistic Locking</a><ul>
<li><a class="reference internal" href="#approach">Approach</a></li>
<li><a class="reference internal" href="#document-configuration">Document Configuration</a><ul>
<li><a class="reference internal" href="#choosing-the-field-type">Choosing the Field Type</a></li>
<li><a class="reference internal" href="#usage">Usage</a></li>
</ul>
</li>
<li><a class="reference internal" href="#important-implementation-notes">Important Implementation Notes</a><ul>
<li><a class="reference internal" href="#workflow">Workflow</a></li>
<li><a class="reference internal" href="#example-code">Example Code</a></li>
</ul>
</li>
</ul>
</li>
<li><a class="reference internal" href="#pessimistic-locking">Pessimistic Locking</a><ul>
<li><a class="reference internal" href="#id1">Document Configuration</a></li>
<li><a class="reference internal" href="#lock-modes">Lock Modes</a></li>
<li><a class="reference internal" href="#id2">Usage</a></li>
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