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<title>Read Preferences</title>
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</div><hr /><div id="mongo.readpreferences" class="chapter">
<h1>Read Preferences</h1>
<p class="para">
MongoDB 2.2 and version 1.3.0 of the driver add support for
<a href="http://docs.mongodb.org/manual/core/read-preference/" class="link external">» read preferences</a>,
which allow control over how queries are directed to mongod instances in a
replica set environment. Read preferences may be specified on either a
per-connection, per-database, or per-collection basis. Preferences defined
at a higher level
will be inherited by default (e.g. <a href="class.mongocollection.html" class="classname">MongoCollection</a> will
inherit read perferences defined on the corresponding
<a href="class.mongodb.html" class="classname">MongoDB</a> instance).
</p>
<p class="para">
Read preferences are specified with a combination of modes and tag sets. Modes
determine how mongod instances are prioritized, while
<a href="http://docs.mongodb.org/manual/tutorial/configure-replica-set-tag-sets/" class="link external">» tag sets</a> specify criteria
for eligible mongod instances.
</p>
<div class="simplesect">
<h3 class="title">Read Preference Modes</h3>
<div class="warning"><strong class="warning">Warning</strong>
<p class="para">
All read preference modes except <em>MongoClient::RP_PRIMARY</em>
may return stale data as secondaries replicate operations from the primary
with some delay. Ensure that your application can tolerate stale data if you
choose to use a mode other than <em>MongoClient::RP_PRIMARY</em>.
</p>
</div>
<ul class="itemizedlist">
<li class="listitem">
<p class="para">
<em>MongoClient::RP_PRIMARY</em>
</p>
<p class="para">
All read operations use only the current replica set primary. This is the
default. If the primary is unavailable, read operations will produce an
exception.
</p>
<p class="para">
This mode is incompatible with use of tag sets. Specifying a tag set with
<em>MongoClient::RP_PRIMARY</em> will result in an exception.
</p>
</li>
<li class="listitem">
<p class="para">
<em>MongoClient::RP_PRIMARY_PREFERRED</em>
</p>
<p class="para">
In most situations, operations read from the primary member of the set.
However, if the primary is unavailable, as is the case during failover
situations, operations read from secondary members.
</p>
<p class="para">
When the read preference includes a tag set, the client reads first from
the primary, if available, and then from secondaries that match the
specified tags. If no secondaries have matching tags, the read operation
will produce an exception.
</p>
<div class="warning"><strong class="warning">Warning</strong>
<p class="para">
Version 2.2 of mongos added full support for read preferences. When
connecting to older mongos instances,
<em>MongoClient::RP_PRIMARY_PREFERRED</em> will send queries to
secondaries.
</p>
</div>
</li>
<li class="listitem">
<p class="para">
<em>MongoClient::RP_SECONDARY</em>
</p>
<p class="para">
Operations read only from the secondary members of the set. If no
secondaries are available, read operations will produce an exception.
</p>
<p class="para">
Most sets have at least one secondary, but there are situations where
there may be no available secondary. For example, a set with a primary, a
secondary, and an arbiter may not have any secondaries if a member is in
recovering state or unavailable.
</p>
<p class="para">
When the read preference includes a tag set, the client attempts to find
secondary members that match the specified tag set and directs reads to a
random secondary from among the nearest group. If no secondaries have
matching tags, the read operation will produce an exception.
</p>
</li>
<li class="listitem">
<p class="para">
<em>MongoClient::RP_SECONDARY_PREFERRED</em>
</p>
<p class="para">
In most situations, operations read from secondary members, but in
situations where the set consists of a single primary with no other
members, the read operation will use the set's primary.
</p>
<p class="para">
When the read preference includes a tag set, the client attempts to find a
secondary member that matches the specified tag set and directs reads to a
random secondary from among the nearest group. If no secondaries have
matching tags, the read operation will produce an exception.
</p>
</li>
<li class="listitem">
<p class="para">
<em>MongoClient::RP_NEAREST</em>
</p>
<p class="para">
The driver reads from the nearest member of the set according to the member
selection process. Reads in the <em>MongoClient::RP_NEAREST</em>
mode do not consider the member's type and may read from both primaries and
secondaries.
</p>
<p class="para">
Set this mode to minimize the effect of network latency on read operations
without preference for current or stale data.
</p>
<p class="para">
If you specify a tag set, the client attempts to find a member
that matches the specified tag set and directs reads to a random node
from among the nearest group.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
All operations read from the nearest member of the replica set that
matches the specified read preference mode. The
<em>MongoClient::RP_NEAREST</em> mode prefers low latency reads
over a member's primary or secondary status.
</p>
</p></blockquote>
</li>
</ul>
</div>
<div class="simplesect">
<h3 class="title">Tag Sets</h3>
<p class="para">
<a href="http://docs.mongodb.org/manual/tutorial/configure-replica-set-tag-sets/" class="link external">» Tag sets</a> allow you to
specify criteria so that your application can target read operations to
specific members, based on custom parameters. Tag sets make it possible to
ensure that read operations target members in a particular data center or
target mongod instances designated for a particular class of operations, such
as reporting or analytics.
</p>
<p class="para">
You can specify tag sets with the following read preference modes:
</p>
<ul class="itemizedlist">
<li class="listitem">
<p class="para">
<em>MongoClient::RP_PRIMARY_PREFERRED</em>
</p>
</li>
<li class="listitem">
<p class="para">
<em>MongoClient::RP_SECONDARY</em>
</p>
</li>
<li class="listitem">
<p class="para">
<em>MongoClient::RP_SECONDARY_PREFERRED</em>
</p>
</li>
<li class="listitem">
<p class="para">
<em>MongoClient::RP_NEAREST</em>
</p>
</li>
</ul>
<p class="para">
You cannot specify tag sets with the
<em>MongoClient::RP_PRIMARY</em> read preference mode. Tags apply
only when selecting a secondary member of a set, except for the when in the
nearest mode.
</p>
</div>
<div class="simplesect">
<h3 class="title">Specifying Read Preferences</h3>
<p class="para">
Read preferences may be specified in either the connection URI provided to
<span class="function"><a href="mongoclient.construct.html" class="function">MongoClient::__construct()</a></span>, which uses a query string
syntax, or via setter methods on the core classes, which use an array syntax
for tag sets.
</p>
<p class="para">
When specifying read preference modes in a query string, the tag sets for
the <em>readPreferenceTags</em> value should be a comma-delimited
sequence of colon-delimited key/value pairs.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Each tag set defined in the query string will use the
<em>readPreferenceTags</em> name. Unlike how PHP might handle URL
query strings, successive values for <em>readPreferenceTags</em>
will not overwrite each other. The driver will collect tag sets in the order
they appear in the query string.
</p>
</p></blockquote>
<div class="warning"><strong class="warning">Warning</strong>
<p class="para">
If the driver cannot find a matching tag set the read will fail! It is
your responsibility of providing suitable fallback, such as an empty
<em>readPreferenceTags</em> value to fallback to "no tag set
preference".
</p>
</div>
<p class="para">
<div class="example" id="example-1368">
<p><strong>Example #1 Connection URI read preferences with query string syntax</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #FF8000">// Prefer the nearest server with no tag preference<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">= </span><span style="color: #DD0000">'mongodb://rs1.example.com,rs2.example.com/'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'?readPreference=nearest'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$m </span><span style="color: #007700">= new </span><span style="color: #0000BB">MongoClient</span><span style="color: #007700">(</span><span style="color: #0000BB">$uri</span><span style="color: #007700">, array(</span><span style="color: #DD0000">'replicaSet' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'rs'</span><span style="color: #007700">));<br /><br /></span><span style="color: #FF8000">// Pick the nearest server in the "east" data center<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">= </span><span style="color: #DD0000">'mongodb://rs1.example.com,rs2.example.com/'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'?readPreference=nearest'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'&readPreferenceTags=dc:east'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$m </span><span style="color: #007700">= new </span><span style="color: #0000BB">MongoClient</span><span style="color: #007700">(</span><span style="color: #0000BB">$uri</span><span style="color: #007700">, array(</span><span style="color: #DD0000">'replicaSet' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'rs'</span><span style="color: #007700">));<br /><br /></span><span style="color: #FF8000">// Prefer the nearest server in the "east" data center also used for reporting,<br />// but fall back to a server in the "west" data center<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">= </span><span style="color: #DD0000">'mongodb://rs1.example.com,rs2.example.com/'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'?readPreference=nearest'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'&readPreferenceTags=dc:east,use:reporting'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'&readPreferenceTags=dc:west'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$m </span><span style="color: #007700">= new </span><span style="color: #0000BB">MongoClient</span><span style="color: #007700">(</span><span style="color: #0000BB">$uri</span><span style="color: #007700">, array(</span><span style="color: #DD0000">'replicaSet' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'rs'</span><span style="color: #007700">));<br /><br /></span><span style="color: #FF8000">// Prefer the nearest server in the "east" data center, then a server in the<br />// "west" data center, and finally fall back to no tag set preference<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">= </span><span style="color: #DD0000">'mongodb://rs1.example.com,rs2.example.com/'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'?readPreference=nearest'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'&readPreferenceTags=dc:east'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'&readPreferenceTags=dc:west'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$uri </span><span style="color: #007700">.= </span><span style="color: #DD0000">'&readPreferenceTags='</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$m </span><span style="color: #007700">= new </span><span style="color: #0000BB">MongoClient</span><span style="color: #007700">(</span><span style="color: #0000BB">$uri</span><span style="color: #007700">, array(</span><span style="color: #DD0000">'replicaSet' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'rs'</span><span style="color: #007700">));<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="para">
<div class="example" id="example-1369">
<p><strong>Example #2 Setting read preferences with array syntax for tag sets</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /><br />$m </span><span style="color: #007700">= new </span><span style="color: #0000BB">MongoClient</span><span style="color: #007700">(</span><span style="color: #DD0000">'mongodb://rs1.example.com,rs2.example.com'</span><span style="color: #007700">, array(<br /> </span><span style="color: #DD0000">'replicaSet' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'rs'</span><span style="color: #007700">,<br />));<br /><br /></span><span style="color: #FF8000">// Prefer the nearest server with no tag preference<br /></span><span style="color: #0000BB">$m</span><span style="color: #007700">-></span><span style="color: #0000BB">setReadPreference</span><span style="color: #007700">(</span><span style="color: #0000BB">MongoClient</span><span style="color: #007700">::</span><span style="color: #0000BB">RP_NEAREST</span><span style="color: #007700">, array());<br /><br /></span><span style="color: #FF8000">// Pick the nearest server in the "east" data center<br /></span><span style="color: #0000BB">$m</span><span style="color: #007700">-></span><span style="color: #0000BB">setReadPreference</span><span style="color: #007700">(</span><span style="color: #0000BB">MongoClient</span><span style="color: #007700">::</span><span style="color: #0000BB">RP_NEAREST</span><span style="color: #007700">, array(<br /> array(</span><span style="color: #DD0000">'dc' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'east'</span><span style="color: #007700">),<br />));<br /><br /></span><span style="color: #FF8000">// Prefer the nearest server in the "east" data center also used for reporting,<br />// but fall back to a server in the "west" data center<br /></span><span style="color: #0000BB">$m</span><span style="color: #007700">-></span><span style="color: #0000BB">setReadPreference</span><span style="color: #007700">(</span><span style="color: #0000BB">MongoClient</span><span style="color: #007700">::</span><span style="color: #0000BB">RP_NEAREST</span><span style="color: #007700">, array(<br /> array(</span><span style="color: #DD0000">'dc' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'east'</span><span style="color: #007700">, </span><span style="color: #DD0000">'use' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'reporting'</span><span style="color: #007700">),<br /> array(</span><span style="color: #DD0000">'dc' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'west'</span><span style="color: #007700">),<br />));<br /><br /></span><span style="color: #FF8000">// Prefer the nearest server in the "east" data center, then a server in the<br />// "west" data center, and finally fall back to no tag set preference<br /></span><span style="color: #0000BB">$m</span><span style="color: #007700">-></span><span style="color: #0000BB">setReadPreference</span><span style="color: #007700">(</span><span style="color: #0000BB">MongoClient</span><span style="color: #007700">::</span><span style="color: #0000BB">RP_NEAREST</span><span style="color: #007700">, array(<br /> array(</span><span style="color: #DD0000">'dc' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'east'</span><span style="color: #007700">),<br /> array(</span><span style="color: #DD0000">'dc' </span><span style="color: #007700">=> </span><span style="color: #DD0000">'west'</span><span style="color: #007700">),<br /> array(),<br />));<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
</div>
</div>
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