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<title>Create a new ReadPreference</title>
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<h1 class="refname">MongoDB\Driver\ReadPreference::__construct</h1>
<p class="verinfo">(mongodb >=1.0.0)</p><p class="refpurpose"><span class="refname">MongoDB\Driver\ReadPreference::__construct</span> — <span class="dc-title">Create a new ReadPreference</span></p>
</div>
<div class="refsect1 description" id="refsect1-mongodb-driver-readpreference.construct-description">
<h3 class="title">Description</h3>
<div class="methodsynopsis dc-description">
<span class="modifier">final</span> <span class="modifier">public</span> <span class="methodname"><strong>MongoDB\Driver\ReadPreference::__construct</strong></span>
( <span class="methodparam"><span class="type"><span class="type string|integer">string|integer</span></span> <code class="parameter">$mode</code></span>
[, <span class="methodparam"><span class="type">array</span> <code class="parameter">$tagSets</code><span class="initializer"> = <strong><code>NULL</code></strong></span></span>
[, <span class="methodparam"><span class="type">array</span> <code class="parameter">$options</code><span class="initializer"> = array()</span></span>
]] )</div>
<p class="para rdfs-comment">
Constructs a new <a href="class.mongodb-driver-readpreference.html" class="classname">MongoDB\Driver\ReadPreference</a>, which
is an immutable value object.
</p>
</div>
<div class="refsect1 parameters" id="refsect1-mongodb-driver-readpreference.construct-parameters">
<h3 class="title">Parameters</h3>
<dl>
<dt>
<code class="parameter">mode</code></dt>
<dd>
<p class="para">
<table class="doctable table">
<caption><strong>Read preference mode</strong></caption>
<thead>
<tr>
<th>Value</th>
<th>Description</th>
</tr>
</thead>
<tbody class="tbody">
<tr>
<td><strong><code>MongoDB\Driver\ReadPreference::RP_PRIMARY</code></strong> or <em>"primary"</em></td>
<td>
<p class="para">
All operations read from the current replica set primary. This is
the default read preference for MongoDB.
</p>
</td>
</tr>
<tr>
<td><strong><code>MongoDB\Driver\ReadPreference::RP_PRIMARY_PREFERRED</code></strong> or <em>"primaryPreferred"</em></td>
<td>
<p class="para">
In most situations, operations read from the primary but if it is
unavailable, operations read from secondary members.
</p>
</td>
</tr>
<tr>
<td><strong><code>MongoDB\Driver\ReadPreference::RP_SECONDARY</code></strong> or <em>"secondary"</em></td>
<td>
<p class="para">
All operations read from the secondary members of the replica set.
</p>
</td>
</tr>
<tr>
<td><strong><code>MongoDB\Driver\ReadPreference::RP_SECONDARY_PREFERRED</code></strong> or <em>"secondaryPreferred"</em></td>
<td>
<p class="para">
In most situations, operations read from secondary members but if no
secondary members are available, operations read from the primary.
</p>
</td>
</tr>
<tr>
<td><strong><code>MongoDB\Driver\ReadPreference::RP_NEAREST</code></strong> or <em>"nearest"</em></td>
<td>
<p class="para">
Operations read from member of the replica set with the least
network latency, irrespective of the member's type.
</p>
</td>
</tr>
</tbody>
</table>
</p>
</dd>
<dt>
<code class="parameter">tagSets</code></dt>
<dd>
<p class="para">
Tag sets allow you to target read operations to specific members of a
replica set. This parameter should be an array of associative arrays, each
of which contain zero or more key/value pairs. When selecting a server for
a read operation, the driver attempt to select a node having all tags in a
set (i.e. the associative array of key/value pairs). If selection fails,
the driver will attempt subsequent sets. An empty tag set
(<em>array()</em>) will match any node and may be used as a
fallback.
</p>
<p class="para">
Tags are not compatible with the
<strong><code>MongoDB\Driver\ReadPreference::RP_PRIMARY</code></strong> mode and,
in general, only apply when selecting a secondary member of a set for a
read operation. However, the
<strong><code>MongoDB\Driver\ReadPreference::RP_NEAREST</code></strong> mode, when
combined with a tag set, selects the matching member with the lowest
network latency. This member may be a primary or secondary.
</p>
</dd>
<dt>
<code class="parameter">options</code></dt>
<dd>
<p class="para">
<table class="doctable table">
<caption><strong>options</strong></caption>
<thead>
<tr>
<th>Option</th>
<th>Type</th>
<th>Description</th>
</tr>
</thead>
<tbody class="tbody">
<tr>
<td>maxStalenessSeconds</td>
<td><span class="type"><a href="language.types.integer.html" class="type integer">integer</a></span></td>
<td>
<p class="para">
Specifies a maximum replication lag, or "staleness", for reads from
secondaries. When a secondary's estimated staleness exceeds
this value, the driver stops using it for read operations.
</p>
<p class="para">
If specified, the max staleness must be a signed 32-bit integer
greater than or equal to
<strong><code>MongoDB\Driver\ReadPreference::SMALLEST_MAX_STALENESS_SECONDS</code></strong>.
</p>
<p class="para">
Defaults to
<strong><code>MongoDB\Driver\ReadPreference::NO_MAX_STALENESS</code></strong>,
which means that the driver will not consider a secondary's lag
when choosing where to direct a read operation.
</p>
<p class="para">
This option is not compatible with the
<strong><code>MongoDB\Driver\ReadPreference::RP_PRIMARY</code></strong> mode.
Specifying a max staleness also requires all MongoDB instances in
the deployment to be using MongoDB 3.4+. An exception will be thrown
at execution time if any MongoDB instances in the deployment are of
an older server version.
</p>
</td>
</tr>
</tbody>
</table>
</p>
</dd>
</dl>
</div>
<div class="refsect1 errors" id="refsect1-mongodb-driver-readpreference.construct-errors">
<h3 class="title">Errors/Exceptions</h3>
<ul class="simplelist">
<li class="member">Throws <a href="class.mongodb-driver-exception-invalidargumentexception.html" class="classname">MongoDB\Driver\Exception\InvalidArgumentException</a> on argument parsing errors.</li>
<li class="member">Throws <a href="class.mongodb-driver-exception-invalidargumentexception.html" class="classname">MongoDB\Driver\Exception\InvalidArgumentException</a> if <code class="parameter">mode</code> is invalid.</li>
<li class="member">Throws <a href="class.mongodb-driver-exception-invalidargumentexception.html" class="classname">MongoDB\Driver\Exception\InvalidArgumentException</a> if <code class="parameter">tagSets</code> is provided for a primary read preference or is malformed (i.e. not an array of zero or more documents).</li>
<li class="member">Throws <a href="class.mongodb-driver-exception-invalidargumentexception.html" class="classname">MongoDB\Driver\Exception\InvalidArgumentException</a> if the <em>"maxStalenessSeconds"</em> option is provided for a primary read preference or is out of range.</li>
</ul>
</div>
<div class="refsect1 changelog" id="refsect1-mongodb-driver-readpreference.construct-changelog">
<h3 class="title">Changelog</h3>
<p class="para">
<table class="doctable informaltable">
<thead>
<tr>
<th>Version</th>
<th>Description</th>
</tr>
</thead>
<tbody class="tbody">
<tr>
<td>1.3.0</td>
<td>
<p class="para">
The <code class="parameter">mode</code> argument now accepts a string value,
which is consistent with the <em>"readPreference"</em> URI
option for <span class="function"><a href="mongodb-driver-manager.construct.html" class="function">MongoDB\Driver\Manager::__construct()</a></span>.
</p>
</td>
</tr>
<tr>
<td>1.2.0</td>
<td>
<p class="para">
Added a third <code class="parameter">options</code> argument, which supports
the <em>"maxStalenessSeconds"</em> option.
</p>
</td>
</tr>
</tbody>
</table>
</p>
</div>
<div class="refsect1 examples" id="refsect1-mongodb-driver-readpreference.construct-examples">
<h3 class="title">Examples</h3>
<div class="example" id="example-1713">
<p><strong>Example #1 <span class="function"><strong>MongoDB\Driver\ReadPreference::__construct()</strong></span> example</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /><br /></span><span style="color: #FF8000">/* Prefer a secondary node but fall back to a primary. */<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(new </span><span style="color: #0000BB">MongoDB</span><span style="color: #007700">\</span><span style="color: #0000BB">Driver</span><span style="color: #007700">\</span><span style="color: #0000BB">ReadPreference</span><span style="color: #007700">(</span><span style="color: #0000BB">MongoDB</span><span style="color: #007700">\</span><span style="color: #0000BB">Driver</span><span style="color: #007700">\</span><span style="color: #0000BB">ReadPreference</span><span style="color: #007700">::</span><span style="color: #0000BB">RP_SECONDARY_PREFERRED</span><span style="color: #007700">));<br /><br /></span><span style="color: #FF8000">/* Prefer a node in the New York data center with lowest latency. */<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(new </span><span style="color: #0000BB">MongoDB</span><span style="color: #007700">\</span><span style="color: #0000BB">Driver</span><span style="color: #007700">\</span><span style="color: #0000BB">ReadPreference</span><span style="color: #007700">(</span><span style="color: #0000BB">MongoDB</span><span style="color: #007700">\</span><span style="color: #0000BB">Driver</span><span style="color: #007700">\</span><span style="color: #0000BB">ReadPreference</span><span style="color: #007700">::</span><span style="color: #0000BB">RP_NEAREST</span><span style="color: #007700">, [[</span><span style="color: #DD0000">'dc' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'ny'</span><span style="color: #007700">]]));<br /><br /></span><span style="color: #FF8000">/* Require a secondary node whose replication lag is within two minutes of the primary */<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(new </span><span style="color: #0000BB">MongoDB</span><span style="color: #007700">\</span><span style="color: #0000BB">Driver</span><span style="color: #007700">\</span><span style="color: #0000BB">ReadPreference</span><span style="color: #007700">(</span><span style="color: #0000BB">MongoDB</span><span style="color: #007700">\</span><span style="color: #0000BB">Driver</span><span style="color: #007700">\</span><span style="color: #0000BB">ReadPreference</span><span style="color: #007700">::</span><span style="color: #0000BB">RP_SECONDARY</span><span style="color: #007700">, </span><span style="color: #0000BB">null</span><span style="color: #007700">, [</span><span style="color: #DD0000">'maxStalenessSeconds' </span><span style="color: #007700">=> </span><span style="color: #0000BB">120</span><span style="color: #007700">]));<br /><br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen">
<div class="cdata"><pre>
object(MongoDB\Driver\ReadPreference)#1 (1) {
["mode"]=>
string(18) "secondaryPreferred"
}
object(MongoDB\Driver\ReadPreference)#1 (2) {
["mode"]=>
string(7) "nearest"
["tags"]=>
array(1) {
[0]=>
object(stdClass)#2 (1) {
["dc"]=>
string(2) "ny"
}
}
}
object(MongoDB\Driver\ReadPreference)#1 (2) {
["mode"]=>
string(9) "secondary"
["maxStalenessSeconds"]=>
int(120)
}
</pre></div>
</div>
</div>
</div>
<div class="refsect1 seealso" id="refsect1-mongodb-driver-readpreference.construct-seealso">
<h3 class="title">See Also</h3>
<ul class="simplelist">
<li class="member"><a href="https://docs.mongodb.com/manual/core/read-preference/" class="link external">» Read Preference reference</a></li>
</ul>
</div>
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