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<div class="section" id="module-pymongo.read_preferences">
<span id="read-preferences-utilities-for-choosing-which-member-of-a-replica-set-to-read-from"></span><h1><code class="xref py py-mod docutils literal notranslate"><span class="pre">read_preferences</span></code> – Utilities for choosing which member of a replica set to read from.<a class="headerlink" href="#module-pymongo.read_preferences" title="Permalink to this headline">¶</a></h1>
<p>Utilities for choosing which member of a replica set to read from.</p>
<dl class="class">
<dt id="pymongo.read_preferences.Primary">
<em class="property">class </em><code class="descclassname">pymongo.read_preferences.</code><code class="descname">Primary</code><a class="headerlink" href="#pymongo.read_preferences.Primary" title="Permalink to this definition">¶</a></dt>
<dd><p>Primary read preference.</p>
<ul class="simple">
<li>When directly connected to one mongod queries are allowed if the server
is standalone or a replica set primary.</li>
<li>When connected to a mongos queries are sent to the primary of a shard.</li>
<li>When connected to a replica set queries are sent to the primary of
the replica set.</li>
</ul>
<dl class="attribute">
<dt id="pymongo.read_preferences.Primary.document">
<code class="descname">document</code><a class="headerlink" href="#pymongo.read_preferences.Primary.document" title="Permalink to this definition">¶</a></dt>
<dd><p>Read preference as a document.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Primary.mode">
<code class="descname">mode</code><a class="headerlink" href="#pymongo.read_preferences.Primary.mode" title="Permalink to this definition">¶</a></dt>
<dd><p>The mode of this read preference instance.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Primary.name">
<code class="descname">name</code><a class="headerlink" href="#pymongo.read_preferences.Primary.name" title="Permalink to this definition">¶</a></dt>
<dd><p>The name of this read preference.</p>
</dd></dl>
</dd></dl>
<dl class="class">
<dt id="pymongo.read_preferences.PrimaryPreferred">
<em class="property">class </em><code class="descclassname">pymongo.read_preferences.</code><code class="descname">PrimaryPreferred</code><span class="sig-paren">(</span><em>tag_sets=None</em>, <em>max_staleness=-1</em><span class="sig-paren">)</span><a class="headerlink" href="#pymongo.read_preferences.PrimaryPreferred" title="Permalink to this definition">¶</a></dt>
<dd><p>PrimaryPreferred read preference.</p>
<ul class="simple">
<li>When directly connected to one mongod queries are allowed to standalone
servers, to a replica set primary, or to replica set secondaries.</li>
<li>When connected to a mongos queries are sent to the primary of a shard if
available, otherwise a shard secondary.</li>
<li>When connected to a replica set queries are sent to the primary if
available, otherwise a secondary.</li>
</ul>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><cite>tag_sets</cite>: The <a class="reference internal" href="#pymongo.read_preferences.PrimaryPreferred.tag_sets" title="pymongo.read_preferences.PrimaryPreferred.tag_sets"><code class="xref py py-attr docutils literal notranslate"><span class="pre">tag_sets</span></code></a> to use if the primary is not
available.</li>
<li><cite>max_staleness</cite>: (integer, in seconds) The maximum estimated
length of time a replica set secondary can fall behind the primary in
replication before it will no longer be selected for operations.
Default -1, meaning no maximum. If it is set, it must be at least
90 seconds.</li>
</ul>
</td>
</tr>
</tbody>
</table>
<dl class="attribute">
<dt id="pymongo.read_preferences.PrimaryPreferred.document">
<code class="descname">document</code><a class="headerlink" href="#pymongo.read_preferences.PrimaryPreferred.document" title="Permalink to this definition">¶</a></dt>
<dd><p>Read preference as a document.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.PrimaryPreferred.max_staleness">
<code class="descname">max_staleness</code><a class="headerlink" href="#pymongo.read_preferences.PrimaryPreferred.max_staleness" title="Permalink to this definition">¶</a></dt>
<dd><p>The maximum estimated length of time (in seconds) a replica set
secondary can fall behind the primary in replication before it will
no longer be selected for operations, or -1 for no maximum.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.PrimaryPreferred.min_wire_version">
<code class="descname">min_wire_version</code><a class="headerlink" href="#pymongo.read_preferences.PrimaryPreferred.min_wire_version" title="Permalink to this definition">¶</a></dt>
<dd><p>The wire protocol version the server must support.</p>
<p>Some read preferences impose version requirements on all servers (e.g.
maxStalenessSeconds requires MongoDB 3.4 / maxWireVersion 5).</p>
<p>All servers’ maxWireVersion must be at least this read preference’s
<cite>min_wire_version</cite>, or the driver raises
<a class="reference internal" href="errors.html#pymongo.errors.ConfigurationError" title="pymongo.errors.ConfigurationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ConfigurationError</span></code></a>.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.PrimaryPreferred.mode">
<code class="descname">mode</code><a class="headerlink" href="#pymongo.read_preferences.PrimaryPreferred.mode" title="Permalink to this definition">¶</a></dt>
<dd><p>The mode of this read preference instance.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.PrimaryPreferred.mongos_mode">
<code class="descname">mongos_mode</code><a class="headerlink" href="#pymongo.read_preferences.PrimaryPreferred.mongos_mode" title="Permalink to this definition">¶</a></dt>
<dd><p>The mongos mode of this read preference.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.PrimaryPreferred.name">
<code class="descname">name</code><a class="headerlink" href="#pymongo.read_preferences.PrimaryPreferred.name" title="Permalink to this definition">¶</a></dt>
<dd><p>The name of this read preference.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.PrimaryPreferred.tag_sets">
<code class="descname">tag_sets</code><a class="headerlink" href="#pymongo.read_preferences.PrimaryPreferred.tag_sets" title="Permalink to this definition">¶</a></dt>
<dd><p>Set <code class="docutils literal notranslate"><span class="pre">tag_sets</span></code> to a list of dictionaries like [{‘dc’: ‘ny’}] to
read only from members whose <code class="docutils literal notranslate"><span class="pre">dc</span></code> tag has the value <code class="docutils literal notranslate"><span class="pre">"ny"</span></code>.
To specify a priority-order for tag sets, provide a list of
tag sets: <code class="docutils literal notranslate"><span class="pre">[{'dc':</span> <span class="pre">'ny'},</span> <span class="pre">{'dc':</span> <span class="pre">'la'},</span> <span class="pre">{}]</span></code>. A final, empty tag
set, <code class="docutils literal notranslate"><span class="pre">{}</span></code>, means “read from any member that matches the mode,
ignoring tags.” MongoReplicaSetClient tries each set of tags in turn
until it finds a set of tags with at least one matching member.</p>
<blockquote>
<div><div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="http://www.mongodb.org/display/DOCS/Data+Center+Awareness">Data-Center Awareness</a></p>
</div>
</div></blockquote>
</dd></dl>
</dd></dl>
<dl class="class">
<dt id="pymongo.read_preferences.Secondary">
<em class="property">class </em><code class="descclassname">pymongo.read_preferences.</code><code class="descname">Secondary</code><span class="sig-paren">(</span><em>tag_sets=None</em>, <em>max_staleness=-1</em><span class="sig-paren">)</span><a class="headerlink" href="#pymongo.read_preferences.Secondary" title="Permalink to this definition">¶</a></dt>
<dd><p>Secondary read preference.</p>
<ul class="simple">
<li>When directly connected to one mongod queries are allowed to standalone
servers, to a replica set primary, or to replica set secondaries.</li>
<li>When connected to a mongos queries are distributed among shard
secondaries. An error is raised if no secondaries are available.</li>
<li>When connected to a replica set queries are distributed among
secondaries. An error is raised if no secondaries are available.</li>
</ul>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><cite>tag_sets</cite>: The <a class="reference internal" href="#pymongo.read_preferences.Secondary.tag_sets" title="pymongo.read_preferences.Secondary.tag_sets"><code class="xref py py-attr docutils literal notranslate"><span class="pre">tag_sets</span></code></a> for this read preference.</li>
<li><cite>max_staleness</cite>: (integer, in seconds) The maximum estimated
length of time a replica set secondary can fall behind the primary in
replication before it will no longer be selected for operations.
Default -1, meaning no maximum. If it is set, it must be at least
90 seconds.</li>
</ul>
</td>
</tr>
</tbody>
</table>
<dl class="attribute">
<dt id="pymongo.read_preferences.Secondary.document">
<code class="descname">document</code><a class="headerlink" href="#pymongo.read_preferences.Secondary.document" title="Permalink to this definition">¶</a></dt>
<dd><p>Read preference as a document.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Secondary.max_staleness">
<code class="descname">max_staleness</code><a class="headerlink" href="#pymongo.read_preferences.Secondary.max_staleness" title="Permalink to this definition">¶</a></dt>
<dd><p>The maximum estimated length of time (in seconds) a replica set
secondary can fall behind the primary in replication before it will
no longer be selected for operations, or -1 for no maximum.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Secondary.min_wire_version">
<code class="descname">min_wire_version</code><a class="headerlink" href="#pymongo.read_preferences.Secondary.min_wire_version" title="Permalink to this definition">¶</a></dt>
<dd><p>The wire protocol version the server must support.</p>
<p>Some read preferences impose version requirements on all servers (e.g.
maxStalenessSeconds requires MongoDB 3.4 / maxWireVersion 5).</p>
<p>All servers’ maxWireVersion must be at least this read preference’s
<cite>min_wire_version</cite>, or the driver raises
<a class="reference internal" href="errors.html#pymongo.errors.ConfigurationError" title="pymongo.errors.ConfigurationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ConfigurationError</span></code></a>.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Secondary.mode">
<code class="descname">mode</code><a class="headerlink" href="#pymongo.read_preferences.Secondary.mode" title="Permalink to this definition">¶</a></dt>
<dd><p>The mode of this read preference instance.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Secondary.mongos_mode">
<code class="descname">mongos_mode</code><a class="headerlink" href="#pymongo.read_preferences.Secondary.mongos_mode" title="Permalink to this definition">¶</a></dt>
<dd><p>The mongos mode of this read preference.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Secondary.name">
<code class="descname">name</code><a class="headerlink" href="#pymongo.read_preferences.Secondary.name" title="Permalink to this definition">¶</a></dt>
<dd><p>The name of this read preference.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Secondary.tag_sets">
<code class="descname">tag_sets</code><a class="headerlink" href="#pymongo.read_preferences.Secondary.tag_sets" title="Permalink to this definition">¶</a></dt>
<dd><p>Set <code class="docutils literal notranslate"><span class="pre">tag_sets</span></code> to a list of dictionaries like [{‘dc’: ‘ny’}] to
read only from members whose <code class="docutils literal notranslate"><span class="pre">dc</span></code> tag has the value <code class="docutils literal notranslate"><span class="pre">"ny"</span></code>.
To specify a priority-order for tag sets, provide a list of
tag sets: <code class="docutils literal notranslate"><span class="pre">[{'dc':</span> <span class="pre">'ny'},</span> <span class="pre">{'dc':</span> <span class="pre">'la'},</span> <span class="pre">{}]</span></code>. A final, empty tag
set, <code class="docutils literal notranslate"><span class="pre">{}</span></code>, means “read from any member that matches the mode,
ignoring tags.” MongoReplicaSetClient tries each set of tags in turn
until it finds a set of tags with at least one matching member.</p>
<blockquote>
<div><div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="http://www.mongodb.org/display/DOCS/Data+Center+Awareness">Data-Center Awareness</a></p>
</div>
</div></blockquote>
</dd></dl>
</dd></dl>
<dl class="class">
<dt id="pymongo.read_preferences.SecondaryPreferred">
<em class="property">class </em><code class="descclassname">pymongo.read_preferences.</code><code class="descname">SecondaryPreferred</code><span class="sig-paren">(</span><em>tag_sets=None</em>, <em>max_staleness=-1</em><span class="sig-paren">)</span><a class="headerlink" href="#pymongo.read_preferences.SecondaryPreferred" title="Permalink to this definition">¶</a></dt>
<dd><p>SecondaryPreferred read preference.</p>
<ul class="simple">
<li>When directly connected to one mongod queries are allowed to standalone
servers, to a replica set primary, or to replica set secondaries.</li>
<li>When connected to a mongos queries are distributed among shard
secondaries, or the shard primary if no secondary is available.</li>
<li>When connected to a replica set queries are distributed among
secondaries, or the primary if no secondary is available.</li>
</ul>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><cite>tag_sets</cite>: The <a class="reference internal" href="#pymongo.read_preferences.SecondaryPreferred.tag_sets" title="pymongo.read_preferences.SecondaryPreferred.tag_sets"><code class="xref py py-attr docutils literal notranslate"><span class="pre">tag_sets</span></code></a> for this read preference.</li>
<li><cite>max_staleness</cite>: (integer, in seconds) The maximum estimated
length of time a replica set secondary can fall behind the primary in
replication before it will no longer be selected for operations.
Default -1, meaning no maximum. If it is set, it must be at least
90 seconds.</li>
</ul>
</td>
</tr>
</tbody>
</table>
<dl class="attribute">
<dt id="pymongo.read_preferences.SecondaryPreferred.document">
<code class="descname">document</code><a class="headerlink" href="#pymongo.read_preferences.SecondaryPreferred.document" title="Permalink to this definition">¶</a></dt>
<dd><p>Read preference as a document.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.SecondaryPreferred.max_staleness">
<code class="descname">max_staleness</code><a class="headerlink" href="#pymongo.read_preferences.SecondaryPreferred.max_staleness" title="Permalink to this definition">¶</a></dt>
<dd><p>The maximum estimated length of time (in seconds) a replica set
secondary can fall behind the primary in replication before it will
no longer be selected for operations, or -1 for no maximum.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.SecondaryPreferred.min_wire_version">
<code class="descname">min_wire_version</code><a class="headerlink" href="#pymongo.read_preferences.SecondaryPreferred.min_wire_version" title="Permalink to this definition">¶</a></dt>
<dd><p>The wire protocol version the server must support.</p>
<p>Some read preferences impose version requirements on all servers (e.g.
maxStalenessSeconds requires MongoDB 3.4 / maxWireVersion 5).</p>
<p>All servers’ maxWireVersion must be at least this read preference’s
<cite>min_wire_version</cite>, or the driver raises
<a class="reference internal" href="errors.html#pymongo.errors.ConfigurationError" title="pymongo.errors.ConfigurationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ConfigurationError</span></code></a>.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.SecondaryPreferred.mode">
<code class="descname">mode</code><a class="headerlink" href="#pymongo.read_preferences.SecondaryPreferred.mode" title="Permalink to this definition">¶</a></dt>
<dd><p>The mode of this read preference instance.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.SecondaryPreferred.mongos_mode">
<code class="descname">mongos_mode</code><a class="headerlink" href="#pymongo.read_preferences.SecondaryPreferred.mongos_mode" title="Permalink to this definition">¶</a></dt>
<dd><p>The mongos mode of this read preference.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.SecondaryPreferred.name">
<code class="descname">name</code><a class="headerlink" href="#pymongo.read_preferences.SecondaryPreferred.name" title="Permalink to this definition">¶</a></dt>
<dd><p>The name of this read preference.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.SecondaryPreferred.tag_sets">
<code class="descname">tag_sets</code><a class="headerlink" href="#pymongo.read_preferences.SecondaryPreferred.tag_sets" title="Permalink to this definition">¶</a></dt>
<dd><p>Set <code class="docutils literal notranslate"><span class="pre">tag_sets</span></code> to a list of dictionaries like [{‘dc’: ‘ny’}] to
read only from members whose <code class="docutils literal notranslate"><span class="pre">dc</span></code> tag has the value <code class="docutils literal notranslate"><span class="pre">"ny"</span></code>.
To specify a priority-order for tag sets, provide a list of
tag sets: <code class="docutils literal notranslate"><span class="pre">[{'dc':</span> <span class="pre">'ny'},</span> <span class="pre">{'dc':</span> <span class="pre">'la'},</span> <span class="pre">{}]</span></code>. A final, empty tag
set, <code class="docutils literal notranslate"><span class="pre">{}</span></code>, means “read from any member that matches the mode,
ignoring tags.” MongoReplicaSetClient tries each set of tags in turn
until it finds a set of tags with at least one matching member.</p>
<blockquote>
<div><div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="http://www.mongodb.org/display/DOCS/Data+Center+Awareness">Data-Center Awareness</a></p>
</div>
</div></blockquote>
</dd></dl>
</dd></dl>
<dl class="class">
<dt id="pymongo.read_preferences.Nearest">
<em class="property">class </em><code class="descclassname">pymongo.read_preferences.</code><code class="descname">Nearest</code><span class="sig-paren">(</span><em>tag_sets=None</em>, <em>max_staleness=-1</em><span class="sig-paren">)</span><a class="headerlink" href="#pymongo.read_preferences.Nearest" title="Permalink to this definition">¶</a></dt>
<dd><p>Nearest read preference.</p>
<ul class="simple">
<li>When directly connected to one mongod queries are allowed to standalone
servers, to a replica set primary, or to replica set secondaries.</li>
<li>When connected to a mongos queries are distributed among all members of
a shard.</li>
<li>When connected to a replica set queries are distributed among all
members.</li>
</ul>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Parameters:</th><td class="field-body"><ul class="first last simple">
<li><cite>tag_sets</cite>: The <a class="reference internal" href="#pymongo.read_preferences.Nearest.tag_sets" title="pymongo.read_preferences.Nearest.tag_sets"><code class="xref py py-attr docutils literal notranslate"><span class="pre">tag_sets</span></code></a> for this read preference.</li>
<li><cite>max_staleness</cite>: (integer, in seconds) The maximum estimated
length of time a replica set secondary can fall behind the primary in
replication before it will no longer be selected for operations.
Default -1, meaning no maximum. If it is set, it must be at least
90 seconds.</li>
</ul>
</td>
</tr>
</tbody>
</table>
<dl class="attribute">
<dt id="pymongo.read_preferences.Nearest.document">
<code class="descname">document</code><a class="headerlink" href="#pymongo.read_preferences.Nearest.document" title="Permalink to this definition">¶</a></dt>
<dd><p>Read preference as a document.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Nearest.max_staleness">
<code class="descname">max_staleness</code><a class="headerlink" href="#pymongo.read_preferences.Nearest.max_staleness" title="Permalink to this definition">¶</a></dt>
<dd><p>The maximum estimated length of time (in seconds) a replica set
secondary can fall behind the primary in replication before it will
no longer be selected for operations, or -1 for no maximum.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Nearest.min_wire_version">
<code class="descname">min_wire_version</code><a class="headerlink" href="#pymongo.read_preferences.Nearest.min_wire_version" title="Permalink to this definition">¶</a></dt>
<dd><p>The wire protocol version the server must support.</p>
<p>Some read preferences impose version requirements on all servers (e.g.
maxStalenessSeconds requires MongoDB 3.4 / maxWireVersion 5).</p>
<p>All servers’ maxWireVersion must be at least this read preference’s
<cite>min_wire_version</cite>, or the driver raises
<a class="reference internal" href="errors.html#pymongo.errors.ConfigurationError" title="pymongo.errors.ConfigurationError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ConfigurationError</span></code></a>.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Nearest.mode">
<code class="descname">mode</code><a class="headerlink" href="#pymongo.read_preferences.Nearest.mode" title="Permalink to this definition">¶</a></dt>
<dd><p>The mode of this read preference instance.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Nearest.mongos_mode">
<code class="descname">mongos_mode</code><a class="headerlink" href="#pymongo.read_preferences.Nearest.mongos_mode" title="Permalink to this definition">¶</a></dt>
<dd><p>The mongos mode of this read preference.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Nearest.name">
<code class="descname">name</code><a class="headerlink" href="#pymongo.read_preferences.Nearest.name" title="Permalink to this definition">¶</a></dt>
<dd><p>The name of this read preference.</p>
</dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.Nearest.tag_sets">
<code class="descname">tag_sets</code><a class="headerlink" href="#pymongo.read_preferences.Nearest.tag_sets" title="Permalink to this definition">¶</a></dt>
<dd><p>Set <code class="docutils literal notranslate"><span class="pre">tag_sets</span></code> to a list of dictionaries like [{‘dc’: ‘ny’}] to
read only from members whose <code class="docutils literal notranslate"><span class="pre">dc</span></code> tag has the value <code class="docutils literal notranslate"><span class="pre">"ny"</span></code>.
To specify a priority-order for tag sets, provide a list of
tag sets: <code class="docutils literal notranslate"><span class="pre">[{'dc':</span> <span class="pre">'ny'},</span> <span class="pre">{'dc':</span> <span class="pre">'la'},</span> <span class="pre">{}]</span></code>. A final, empty tag
set, <code class="docutils literal notranslate"><span class="pre">{}</span></code>, means “read from any member that matches the mode,
ignoring tags.” MongoReplicaSetClient tries each set of tags in turn
until it finds a set of tags with at least one matching member.</p>
<blockquote>
<div><div class="admonition seealso">
<p class="first admonition-title">See also</p>
<p class="last"><a class="reference external" href="http://www.mongodb.org/display/DOCS/Data+Center+Awareness">Data-Center Awareness</a></p>
</div>
</div></blockquote>
</dd></dl>
</dd></dl>
<dl class="class">
<dt id="pymongo.read_preferences.ReadPreference">
<em class="property">class </em><code class="descclassname">pymongo.read_preferences.</code><code class="descname">ReadPreference</code><a class="headerlink" href="#pymongo.read_preferences.ReadPreference" title="Permalink to this definition">¶</a></dt>
<dd><p>An enum that defines the read preference modes supported by PyMongo.</p>
<p>See <a class="reference internal" href="../../examples/high_availability.html"><span class="doc">High Availability and PyMongo</span></a> for code examples.</p>
<p>A read preference is used in three cases:</p>
<p><a class="reference internal" href="mongo_client.html#pymongo.mongo_client.MongoClient" title="pymongo.mongo_client.MongoClient"><code class="xref py py-class docutils literal notranslate"><span class="pre">MongoClient</span></code></a> connected to a single mongod:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">PRIMARY</span></code>: Queries are allowed if the server is standalone or a replica
set primary.</li>
<li>All other modes allow queries to standalone servers, to a replica set
primary, or to replica set secondaries.</li>
</ul>
<p><a class="reference internal" href="mongo_client.html#pymongo.mongo_client.MongoClient" title="pymongo.mongo_client.MongoClient"><code class="xref py py-class docutils literal notranslate"><span class="pre">MongoClient</span></code></a> initialized with the
<code class="docutils literal notranslate"><span class="pre">replicaSet</span></code> option:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">PRIMARY</span></code>: Read from the primary. This is the default, and provides the
strongest consistency. If no primary is available, raise
<a class="reference internal" href="errors.html#pymongo.errors.AutoReconnect" title="pymongo.errors.AutoReconnect"><code class="xref py py-class docutils literal notranslate"><span class="pre">AutoReconnect</span></code></a>.</li>
<li><code class="docutils literal notranslate"><span class="pre">PRIMARY_PREFERRED</span></code>: Read from the primary if available, or if there is
none, read from a secondary.</li>
<li><code class="docutils literal notranslate"><span class="pre">SECONDARY</span></code>: Read from a secondary. If no secondary is available,
raise <a class="reference internal" href="errors.html#pymongo.errors.AutoReconnect" title="pymongo.errors.AutoReconnect"><code class="xref py py-class docutils literal notranslate"><span class="pre">AutoReconnect</span></code></a>.</li>
<li><code class="docutils literal notranslate"><span class="pre">SECONDARY_PREFERRED</span></code>: Read from a secondary if available, otherwise
from the primary.</li>
<li><code class="docutils literal notranslate"><span class="pre">NEAREST</span></code>: Read from any member.</li>
</ul>
<p><a class="reference internal" href="mongo_client.html#pymongo.mongo_client.MongoClient" title="pymongo.mongo_client.MongoClient"><code class="xref py py-class docutils literal notranslate"><span class="pre">MongoClient</span></code></a> connected to a mongos, with a
sharded cluster of replica sets:</p>
<ul class="simple">
<li><code class="docutils literal notranslate"><span class="pre">PRIMARY</span></code>: Read from the primary of the shard, or raise
<a class="reference internal" href="errors.html#pymongo.errors.OperationFailure" title="pymongo.errors.OperationFailure"><code class="xref py py-class docutils literal notranslate"><span class="pre">OperationFailure</span></code></a> if there is none.
This is the default.</li>
<li><code class="docutils literal notranslate"><span class="pre">PRIMARY_PREFERRED</span></code>: Read from the primary of the shard, or if there is
none, read from a secondary of the shard.</li>
<li><code class="docutils literal notranslate"><span class="pre">SECONDARY</span></code>: Read from a secondary of the shard, or raise
<a class="reference internal" href="errors.html#pymongo.errors.OperationFailure" title="pymongo.errors.OperationFailure"><code class="xref py py-class docutils literal notranslate"><span class="pre">OperationFailure</span></code></a> if there is none.</li>
<li><code class="docutils literal notranslate"><span class="pre">SECONDARY_PREFERRED</span></code>: Read from a secondary of the shard if available,
otherwise from the shard primary.</li>
<li><code class="docutils literal notranslate"><span class="pre">NEAREST</span></code>: Read from any shard member.</li>
</ul>
<dl class="attribute">
<dt id="pymongo.read_preferences.ReadPreference.PRIMARY">
<code class="descname">PRIMARY</code><em class="property"> = Primary()</em><a class="headerlink" href="#pymongo.read_preferences.ReadPreference.PRIMARY" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.ReadPreference.PRIMARY_PREFERRED">
<code class="descname">PRIMARY_PREFERRED</code><em class="property"> = PrimaryPreferred(tag_sets=None, max_staleness=-1)</em><a class="headerlink" href="#pymongo.read_preferences.ReadPreference.PRIMARY_PREFERRED" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.ReadPreference.SECONDARY">
<code class="descname">SECONDARY</code><em class="property"> = Secondary(tag_sets=None, max_staleness=-1)</em><a class="headerlink" href="#pymongo.read_preferences.ReadPreference.SECONDARY" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.ReadPreference.SECONDARY_PREFERRED">
<code class="descname">SECONDARY_PREFERRED</code><em class="property"> = SecondaryPreferred(tag_sets=None, max_staleness=-1)</em><a class="headerlink" href="#pymongo.read_preferences.ReadPreference.SECONDARY_PREFERRED" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
<dl class="attribute">
<dt id="pymongo.read_preferences.ReadPreference.NEAREST">
<code class="descname">NEAREST</code><em class="property"> = Nearest(tag_sets=None, max_staleness=-1)</em><a class="headerlink" href="#pymongo.read_preferences.ReadPreference.NEAREST" title="Permalink to this definition">¶</a></dt>
<dd></dd></dl>
</dd></dl>
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