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312 lines
11 KiB
XML
312 lines
11 KiB
XML
<?xml version="1.0" encoding="utf-8"?>
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<!-- $Revision$ -->
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<chapter xml:id="mongo.readpreferences" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
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<title>Read Preferences</title>
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<para>
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MongoDB 2.2 and version 1.3.0 of the driver add support for
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<link xlink:href="&url.mongodb.docs.readpreferences;">read preferences</link>,
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which allow control over how queries are directed to mongod instances in a
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replica set environment. Read preferences may be specified on either a
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per-connection, per-database, or per-collection basis. Preferences defined
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at a higher level
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will be inherited by default (e.g. <classname>MongoCollection</classname> will
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inherit read perferences defined on the corresponding
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<classname>MongoDB</classname> instance).
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</para>
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<para>
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Read preferences are specified with a combination of modes and tag sets. Modes
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determine how mongod instances are prioritized, while
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<link xlink:href="&url.mongodb.docs.tagsets;">tag sets</link> specify criteria
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for eligible mongod instances. Mongod instances are only eligible if they
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are within a 15ms difference in ping time from the nearest mongod instance.
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</para>
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<simplesect xml:id="mongo.readpreferences.modes">
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<title>Read Preference Modes</title>
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<warning>
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<para>
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All read preference modes except <literal>MongoClient::RP_PRIMARY</literal>
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may return stale data as secondaries replicate operations from the primary
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with some delay. Ensure that your application can tolerate stale data if you
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choose to use a mode other than <literal>MongoClient::RP_PRIMARY</literal>.
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</para>
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</warning>
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<itemizedlist>
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<listitem>
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<para>
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<literal>MongoClient::RP_PRIMARY</literal>
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</para>
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<para>
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All read operations use only the current replica set primary. This is the
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default. If the primary is unavailable, read operations will produce an
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exception.
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</para>
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<para>
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This mode is incompatible with use of tag sets. Specifying a tag set with
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<literal>MongoClient::RP_PRIMARY</literal> will result in an exception.
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>MongoClient::RP_PRIMARY_PREFERRED</literal>
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</para>
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<para>
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In most situations, operations read from the primary member of the set.
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However, if the primary is unavailable, as is the case during failover
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situations, operations read from secondary members.
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</para>
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<para>
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When the read preference includes a tag set, the client reads first from
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the primary, if available, and then from secondaries that match the
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specified tags. If no secondaries have matching tags, the read operation
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will produce an exception.
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</para>
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<warning>
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<para>
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Version 2.2 of mongos added full support for read preferences. When
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connecting to older mongos instances,
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<literal>MongoClient::RP_PRIMARY_PREFERRED</literal> will send queries to
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secondaries.
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</para>
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</warning>
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</listitem>
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<listitem>
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<para>
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<literal>MongoClient::RP_SECONDARY</literal>
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</para>
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<para>
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Operations read only from the secondary members of the set. If no
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secondaries are available, read operations will produce an exception.
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</para>
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<para>
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Most sets have at least one secondary, but there are situations where
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there may be no available secondary. For example, a set with a primary, a
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secondary, and an arbiter may not have any secondaries if a member is in
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recovering state or unavailable.
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</para>
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<para>
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When the read preference includes a tag set, the client attempts to find
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secondary members that match the specified tag set and directs reads to a
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random secondary from among the nearest group. If no secondaries have
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matching tags, the read operation will produce an exception.
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>MongoClient::RP_SECONDARY_PREFERRED</literal>
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</para>
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<para>
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In most situations, operations read from secondary members, but in
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situations where the set consists of a single primary with no other
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members, the read operation will use the set's primary.
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</para>
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<para>
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When the read preference includes a tag set, the client attempts to find a
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secondary member that matches the specified tag set and directs reads to a
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random secondary from among the nearest group. If no secondaries have
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matching tags, the read operation will produce an exception.
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>MongoClient::RP_NEAREST</literal>
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</para>
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<para>
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The driver reads from a <emphasis>random</emphasis> member of the set
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that has a ping time that is less than 15ms slower than the member with
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the lowest ping time. Reads in the
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<literal>MongoClient::RP_NEAREST</literal> mode do not consider the
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member's type and may read from both primaries and secondaries.
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</para>
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<para>
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Set this mode to minimize the effect of network latency on read operations
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without preference for current or stale data.
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</para>
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<para>
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If you specify a tag set, the client attempts to find a member
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that matches the specified tag set and directs reads to a random node
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from among the nearest group.
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</para>
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<note>
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<para>
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All operations read from the nearest member of the replica set that
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matches the specified read preference mode. The
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<literal>MongoClient::RP_NEAREST</literal> mode prefers low latency reads
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over a member's primary or secondary status.
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</para>
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</note>
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</listitem>
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</itemizedlist>
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</simplesect>
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<simplesect xml:id="mongo.readpreferences.tagsets">
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<title>Tag Sets</title>
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<para>
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<link xlink:href="&url.mongodb.docs.tagsets;">Tag sets</link> allow you to
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specify criteria so that your application can target read operations to
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specific members, based on custom parameters. Tag sets make it possible to
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ensure that read operations target members in a particular data center or
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target mongod instances designated for a particular class of operations, such
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as reporting or analytics.
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</para>
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<para>
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You can specify tag sets with the following read preference modes:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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<literal>MongoClient::RP_PRIMARY_PREFERRED</literal>
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>MongoClient::RP_SECONDARY</literal>
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>MongoClient::RP_SECONDARY_PREFERRED</literal>
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>MongoClient::RP_NEAREST</literal>
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</para>
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</listitem>
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</itemizedlist>
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<para>
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You cannot specify tag sets with the
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<literal>MongoClient::RP_PRIMARY</literal> read preference mode. Tags apply
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only when selecting a secondary member of a set, except for the when in the
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nearest mode.
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</para>
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</simplesect>
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<simplesect xml:id="mongo.readpreference.usage">
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<title>Specifying Read Preferences</title>
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<para>
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Read preferences may be specified in either the connection URI provided to
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<function>MongoClient::__construct</function>, which uses a query string
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syntax, or via setter methods on the core classes, which use an array syntax
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for tag sets.
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</para>
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<para>
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When specifying read preference modes in a query string, the tag sets for
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the <literal>readPreferenceTags</literal> value should be a comma-delimited
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sequence of colon-delimited key/value pairs.
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</para>
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<note>
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<para>
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Each tag set defined in the query string will use the
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<literal>readPreferenceTags</literal> name. Unlike how PHP might handle URL
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query strings, successive values for <literal>readPreferenceTags</literal>
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will not overwrite each other. The driver will collect tag sets in the order
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they appear in the query string.
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</para>
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</note>
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<warning>
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<para>
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If the driver cannot find a matching tag set the read will fail! It is
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your responsibility of providing suitable fallback, such as an empty
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<literal>readPreferenceTags</literal> value to fallback to "no tag set
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preference".
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</para>
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</warning>
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<para>
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<example>
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<title>Connection URI read preferences with query string syntax</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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// Prefer the nearest server with no tag preference
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$uri = 'mongodb://rs1.example.com,rs2.example.com/';
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$uri .= '?readPreference=nearest';
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$m = new MongoClient($uri, array('replicaSet' => 'rs'));
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// Pick the nearest server in the "east" data center
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$uri = 'mongodb://rs1.example.com,rs2.example.com/';
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$uri .= '?readPreference=nearest';
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$uri .= '&readPreferenceTags=dc:east';
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$m = new MongoClient($uri, array('replicaSet' => 'rs'));
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// Prefer the nearest server in the "east" data center also used for reporting,
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// but fall back to a server in the "west" data center
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$uri = 'mongodb://rs1.example.com,rs2.example.com/';
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$uri .= '?readPreference=nearest';
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$uri .= '&readPreferenceTags=dc:east,use:reporting';
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$uri .= '&readPreferenceTags=dc:west';
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$m = new MongoClient($uri, array('replicaSet' => 'rs'));
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// Prefer the nearest server in the "east" data center, then a server in the
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// "west" data center, and finally fall back to no tag set preference
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$uri = 'mongodb://rs1.example.com,rs2.example.com/';
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$uri .= '?readPreference=nearest';
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$uri .= '&readPreferenceTags=dc:east';
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$uri .= '&readPreferenceTags=dc:west';
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$uri .= '&readPreferenceTags=';
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$m = new MongoClient($uri, array('replicaSet' => 'rs'));
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?>
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]]>
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</programlisting>
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</example>
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</para>
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<para>
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<example>
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<title>Setting read preferences with array syntax for tag sets</title>
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<programlisting role="php">
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<![CDATA[
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<?php
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$m = new MongoClient('mongodb://rs1.example.com,rs2.example.com', array(
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'replicaSet' => 'rs',
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));
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// Prefer the nearest server with no tag preference
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$m->setReadPreference(MongoClient::RP_NEAREST, array());
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// Pick the nearest server in the "east" data center
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$m->setReadPreference(MongoClient::RP_NEAREST, array(
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array('dc' => 'east'),
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));
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// Prefer the nearest server in the "east" data center also used for reporting,
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// but fall back to a server in the "west" data center
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$m->setReadPreference(MongoClient::RP_NEAREST, array(
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array('dc' => 'east', 'use' => 'reporting'),
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array('dc' => 'west'),
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));
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// Prefer the nearest server in the "east" data center, then a server in the
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// "west" data center, and finally fall back to no tag set preference
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$m->setReadPreference(MongoClient::RP_NEAREST, array(
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array('dc' => 'east'),
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array('dc' => 'west'),
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array(),
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));
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?>
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]]>
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</programlisting>
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</example>
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</para>
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</simplesect>
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</chapter>
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<!-- Keep this comment at the end of the file
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Local variables:
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sgml-exposed-tags:nil
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sgml-local-ecat-files:nil
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End:
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vim600: syn=xml fen fdm=syntax fdl=2 si
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vim: et tw=78 syn=sgml
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vi: ts=1 sw=1
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-->
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