MongoDB\Driver\ReadPreference::__construct

(mongodb >=1.0.0)

MongoDB\Driver\ReadPreference::__construct — Create a new ReadPreference

Description

final public MongoDB\Driver\ReadPreference::__construct(string|int $mode, ?array $tagSets = null, ?array $options = null)

Constructs a new MongoDB\Driver\ReadPreference, which is an immutable value object.

Parameters

mode

Read preference mode
Value Description
MongoDB\Driver\ReadPreference::RP_PRIMARY or "primary"

All operations read from the current replica set primary. This is the default read preference for MongoDB.

MongoDB\Driver\ReadPreference::RP_PRIMARY_PREFERRED or "primaryPreferred"

In most situations, operations read from the primary but if it is unavailable, operations read from secondary members.

MongoDB\Driver\ReadPreference::RP_SECONDARY or "secondary"

All operations read from the secondary members of the replica set.

MongoDB\Driver\ReadPreference::RP_SECONDARY_PREFERRED or "secondaryPreferred"

In most situations, operations read from secondary members but if no secondary members are available, operations read from the primary.

MongoDB\Driver\ReadPreference::RP_NEAREST or "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 MongoDB\Driver\ReadPreference::RP_PRIMARY mode and, in general, only apply when selecting a secondary member of a set for a read operation. However, the MongoDB\Driver\ReadPreference::RP_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 to MongoDB\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 MongoDB\Driver\ReadPreference::RP_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

Changelog

Version Description
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::RP_SECONDARY_PREFERRED));

/* Prefer a node in the New York data center with lowest latency. */
var_dump(new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::RP_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::RP_SECONDARY, null, ['maxStalenessSeconds' => 120]));

/* Explicitly enable server hedged reads */
var_dump(new MongoDB\Driver\ReadPreference(MongoDB\Driver\ReadPreference::RP_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)
  }
}