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<title>The MongoDB\Driver\ReadConcern class</title>
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<h1 class="title">The MongoDB\Driver\ReadConcern class</h1>
<div class="partintro"><p class="verinfo">(mongodb >=1.1.0)</p>
<div class="section" id="mongodb-driver-readconcern.intro">
<h2 class="title">Introduction</h2>
<p class="para">
<strong class="classname">MongoDB\Driver\ReadConcern</strong> controls the level of
isolation for read operations for replica sets and replica set shards. This
option requires MongoDB 3.2 or later.
</p>
</div>
<div class="section" id="mongodb-driver-readconcern.synopsis">
<h2 class="title">Class synopsis</h2>
<div class="classsynopsis">
<div class="ooclass"></div>
<div class="classsynopsisinfo">
<span class="modifier">final</span>
<span class="ooclass">
<strong class="classname">MongoDB\Driver\ReadConcern</strong>
</span>
<span class="oointerface">implements
<span class="interfacename"><a href="class.mongodb-bson-serializable.html" class="interfacename">MongoDB\BSON\Serializable</a></span>
</span>
{</div>
<div class="classsynopsisinfo classsynopsisinfo_comment">/* Constants */</div>
<div class="fieldsynopsis">
<span class="modifier">const</span>
<span class="type">string</span>
<var class="fieldsynopsis_varname"><a href="class.mongodb-driver-readconcern.html#mongodb-driver-readconcern.constants.linearizable"><var class="varname">AVAILABLE</var></a></var>
<span class="initializer"> = "available"</span>
;</div>
<div class="fieldsynopsis">
<span class="modifier">const</span>
<span class="type">string</span>
<var class="fieldsynopsis_varname"><a href="class.mongodb-driver-readconcern.html#mongodb-driver-readconcern.constants.linearizable"><var class="varname">LINEARIZABLE</var></a></var>
<span class="initializer"> = "linearizable"</span>
;</div>
<div class="fieldsynopsis">
<span class="modifier">const</span>
<span class="type">string</span>
<var class="fieldsynopsis_varname"><a href="class.mongodb-driver-readconcern.html#mongodb-driver-readconcern.constants.local"><var class="varname">LOCAL</var></a></var>
<span class="initializer"> = "local"</span>
;</div>
<div class="fieldsynopsis">
<span class="modifier">const</span>
<span class="type">string</span>
<var class="fieldsynopsis_varname"><a href="class.mongodb-driver-readconcern.html#mongodb-driver-readconcern.constants.majority"><var class="varname">MAJORITY</var></a></var>
<span class="initializer"> = "majority"</span>
;</div>
<div class="classsynopsisinfo classsynopsisinfo_comment">/* Methods */</div>
<div class="methodsynopsis dc-description">
<span class="modifier">final</span> <span class="modifier">public</span> <span class="type">object</span> <span class="methodname"><a href="mongodb-driver-readconcern.bsonserialize.html" class="methodname">bsonSerialize</a></span>
( <span class="methodparam">void</span>
)</div>
<div class="methodsynopsis dc-description">
<span class="modifier">final</span> <span class="modifier">public</span> <span class="methodname"><a href="mongodb-driver-readconcern.construct.html" class="methodname">__construct</a></span>
([ <span class="methodparam"><span class="type">string</span> <code class="parameter">$level</code></span>
] )</div>
<div class="methodsynopsis dc-description">
<span class="modifier">final</span> <span class="modifier">public</span> <span class="type">string|null</span> <span class="methodname"><a href="mongodb-driver-readconcern.getlevel.html" class="methodname">getLevel</a></span>
( <span class="methodparam">void</span>
)</div>
<div class="methodsynopsis dc-description">
<span class="modifier">final</span> <span class="modifier">public</span> <span class="type">bool</span> <span class="methodname"><a href="mongodb-driver-readconcern.isdefault.html" class="methodname">isDefault</a></span>
( <span class="methodparam">void</span>
)</div>
}</div>
</div>
<div class="section" id="mongodb-driver-readconcern.constants">
<h2 class="title">Predefined Constants</h2>
<dl>
<dt id="mongodb-driver-readconcern.constants.available"><strong><code>MongoDB\Driver\ReadConcern::AVAILABLE</code></strong></dt>
<dd>
<p class="para">
Default for reads against secondaries when
<em>afterClusterTime</em>and <em>level</em> are
unspecified.
</p>
<p class="para">
The query returns the instance's most recent data. Provides no
guarantee that the data has been written to a majority of the replica set
members (i.e. may be rolled back).
</p>
<p class="para">
For unsharded collections (including collections in a standalone
deployment or a replica set deployment), <em>"local"</em> and
<em>"available"</em> read concerns behave identically.
</p>
<p class="para">
For a sharded cluster, <em>"available"</em> read concern
provides greater tolerance for partitions since it does not wait to
ensure consistency guarantees. However, a query with
<em>"available"</em> read concern may return orphan documents
if the shard is undergoing chunk migrations since the
<em>"available"</em> read concern, unlike
<em>"local"</em> read concern, does not contact the
shard's primary nor the config servers for updated metadata.
</p>
</dd>
<dt id="mongodb-driver-readconcern.constants.linearizable"><strong><code>MongoDB\Driver\ReadConcern::LINEARIZABLE</code></strong></dt>
<dd>
<p class="para">
The query returns data that reflects all successful writes issued with a
write concern of <em>"majority"</em> <em class="emphasis">and</em>
acknowledged prior to the start of the read operation. For replica sets
that run with <em>writeConcernMajorityJournalDefault</em> set
to <strong><code>TRUE</code></strong>, linearizable read concern returns data that will never be
rolled back.
</p>
<p class="para">
With <em>writeConcernMajorityJournalDefault</em> set to
<strong><code>FALSE</code></strong>, MongoDB will not wait for <em>w: "majority"</em>
writes to be durable before acknowledging the writes. As such,
<em>"majority"</em> write operations could possibly roll back
in the event of a loss of a replica set member.
</p>
<p class="para">
You can specify linearizable read concern for read operations on the
primary only.
</p>
<p class="para">
Linearizable read concern guarantees only apply if read
operations specify a query filter that uniquely identifies a single
document.
</p>
<div class="tip"><strong class="tip">Tip</strong>
<p class="simpara">
Always use <em>maxTimeMS</em> with linearizable read concern
in case a majority of data bearing members are unavailable.
<em>maxTimeMS</em> ensures that the operation does not block
indefinitely and instead ensures that the operation returns an error if
the read concern cannot be fulfilled.
</p>
</div>
<p class="para">
Linearizable read concern requires MongoDB 3.4.
</p>
</dd>
<dt id="mongodb-driver-readconcern.constants.local"><strong><code>MongoDB\Driver\ReadConcern::LOCAL</code></strong></dt>
<dd>
<p class="para">
Default for reads against primary if <em>level</em> is
unspecified and for reads against secondaries if <em>level</em>
is unspecified but <em>afterClusterTime</em> is specified.
</p>
<p class="para">
The query returns the instance's most recent data. Provides no
guarantee that the data has been written to a majority of the replica set
members (i.e. may be rolled back).
</p>
</dd>
<dt id="mongodb-driver-readconcern.constants.majority"><strong><code>MongoDB\Driver\ReadConcern::MAJORITY</code></strong></dt>
<dd>
<p class="para">
The query returns the instance's most recent data acknowledged as
having been written to a majority of members in the replica set.
</p>
<p class="para">
To use read concern level of <em>"majority"</em>, replica sets
must use WiredTiger storage engine and election protocol version 1.
</p>
</dd>
</dl>
</div>
<div class="section">
<h2 class="title">Changelog</h2>
<p class="para">
<table class="doctable informaltable">
<thead>
<tr>
<th>Version</th>
<th>Description</th>
</tr>
</thead>
<tbody class="tbody">
<tr>
<td>1.4.0</td>
<td>
<p class="para">
Added the <strong><code>MongoDB\Driver\ReadConcern::AVAILABLE</code></strong>
constant.
</p>
</td>
</tr>
<tr>
<td>1.2.0</td>
<td>
<p class="para">
Added the
<strong><code>MongoDB\Driver\ReadConcern::LINEARIZABLE</code></strong>
constant.
</p>
<p class="para">
Implements <span class="interfacename"><a href="class.mongodb-bson-serializable.html" class="interfacename">MongoDB\BSON\Serializable</a></span>.
</p>
</td>
</tr>
</tbody>
</table>
</p>
</div>
<div class="section" id="mongodb-driver-readconcern.seealso">
<h2 class="title">See Also</h2>
<ul class="simplelist">
<li class="member"><a href="https://docs.mongodb.com/manual/reference/read-concern/" class="link external">» Read Concern reference</a></li>
</ul>
</div>
</div>
<h2>Table of Contents</h2><ul class="chunklist chunklist_reference"><li><a href="mongodb-driver-readconcern.bsonserialize.html">MongoDB\Driver\ReadConcern::bsonSerialize</a> — Returns an object for BSON serialization</li><li><a href="mongodb-driver-readconcern.construct.html">MongoDB\Driver\ReadConcern::__construct</a> — Create a new ReadConcern</li><li><a href="mongodb-driver-readconcern.getlevel.html">MongoDB\Driver\ReadConcern::getLevel</a> — Returns the ReadConcern's "level" option</li><li><a href="mongodb-driver-readconcern.isdefault.html">MongoDB\Driver\ReadConcern::isDefault</a> — Checks if this is the default read concern</li></ul>
</div>
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