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MongoDB\Driver\ReadPreference::__construct
(mongodb >=1.0.0)
MongoDB\Driver\ReadPreference::__construct — Create a new ReadPreference
Parameters
mode
Read preference mode
| Value | Description |
|---|---|
| "primary" | All operations read from the current replica set primary. This is the default read preference for MongoDB. |
| "primaryPreferred" | In most situations, operations read from the primary but if it is unavailable, operations read from secondary members. |
| "secondary" | All operations read from the secondary members of the replica set. |
| "secondaryPreferred" | In most situations, operations read from secondary members but if no secondary members are available, operations read from the primary. |
| "nearest" | Operations read from member of the replica set with the least network latency, irrespective of the member's type. |
tagSets
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 (array()) will match any node and may be used as a fallback.
Tags are not compatible with the "primary" mode and, in general, only apply when selecting a secondary member of a set for a read operation. However, the "nearest" mode, when combined with a tag set, selects the matching member with the lowest network latency. This member may be a primary or secondary.
options
options
| Option | Type | Description |
|---|---|---|
| hedge | object|array | Specifies whether to use » hedged reads, which are supported by MongoDB 4.4+ for sharded queries. Server hedged reads are available for all non-primary read preferences and are enabled by default when using the "nearest" mode. This option allows explicitly enabling server hedged reads for non-primary read preferences by specifying['enabled' => true], or explicitly disabling server hedged reads for the "nearest" read preference by specifying ['enabled' => false]. |
| maxStalenessSeconds | int | 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. If specified, the max staleness must be a signed 32-bit integer greater than or equal to**MongoDB\Driver\ReadPreference::SMALLEST_MAX_STALENESS_SECONDS. Defaults toMongoDB\Driver\ReadPreference::NO_MAX_STALENESS**, which means that the driver will not consider a secondary's lag when choosing where to direct a read operation. This option is not compatible with the "primary" 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. |
Errors/Exceptions
- Throws MongoDB\Driver\Exception\InvalidArgumentException on argument parsing errors.
- Throws MongoDB\Driver\Exception\InvalidArgumentException if
modeis invalid. - Throws MongoDB\Driver\Exception\InvalidArgumentException if
tagSetsis provided for a primary read preference or is malformed (i.e. not an array of zero or more documents). - Throws MongoDB\Driver\Exception\InvalidArgumentException if the
"maxStalenessSeconds"option is provided for a primary read preference or is out of range.
Changelog
| Version | Description |
|---|---|
| PECL mongodb 2.0.0 | Passing an int for the mode argument is no longer supported. |
| PECL mongodb 1.20.0 | Passing an int for the mode argument is DEPRECATED. |
| PECL mongodb 1.8.0 | Added the "hedge" option. |
| PECL mongodb 1.3.0 | The mode argument now accepts a string value, which is consistent with the "readPreference" URI option for MongoDB\Driver\Manager::__construct(). |
| PECL mongodb 1.2.0 | Added a third options argument, which supports the "maxStalenessSeconds" option. |
Examples
Example #1 MongoDB\Driver\ReadPreference::__construct() example
<?php/* Prefer a secondary node but fall back to a primary. */ var_dump(new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::SECONDARY_PREFERRED));/* Prefer a node in the New York data center with lowest latency. */ var_dump(new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::NEAREST, [['dc' => 'ny']]));/* Require a secondary node whose replication lag is within two minutes of the primary */ var_dump(new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::SECONDARY, null, ['maxStalenessSeconds' => 120]));/* Explicitly enable server hedged reads */ var_dump(new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::SECONDARY, null, ['hedge' => ['enabled' => true]]));?>
The above example will output:
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) } object(MongoDB\Driver\ReadPreference)#1 (2) { ["mode"]=> string(9) "secondary" ["hedge"]=> object(stdClass)#1 (1) { ["enabled"]=> bool(true) } }
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