# The MongoDB\Driver\ReadPreference class

(mongodb >=1.0.0)

## はじめに

## クラス概要

final MongoDB\Driver\ReadPreference implements MongoDB\BSON\Serializable {
/* Constants */
const integer RP_PRIMARY = 1 ;
const integer RP_PRIMARY_PREFERRED = 5 ;
const integer RP_SECONDARY = 2 ;
const integer RP_SECONDARY_PREFERRED = 6 ;
const integer RP_NEAREST = 10 ;
const integer NO_MAX_STALENESS = -1 ;
const integer SMALLEST_MAX_STALENESS_SECONDS = 90 ;
/* メソッド */
final public bsonSerialize ( void ) : object
final public __construct ( string|integer `$mode` [, array `$tagSets` = `NULL` [, array `$options` = array() ]] )
final public getMaxStalenessSeconds ( void ) : int
final public getMode ( void ) : int
final public getTagSets ( void ) : array
}

## 定義済み定数

`MongoDB\Driver\ReadPreference::RP_PRIMARY`

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

`MongoDB\Driver\ReadPreference::RP_PRIMARY_PREFERRED`

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

`MongoDB\Driver\ReadPreference::RP_SECONDARY`

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

`MongoDB\Driver\ReadPreference::RP_SECONDARY_PREFERRED`

  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`

  Operations read from member of the replica set with the least network latency, irrespective of the member's type.

`MongoDB\Driver\ReadPreference::NO_MAX_STALENESS`

  The default value for the "maxStalenessSeconds" option is to specify no limit on maximum staleness, which means that the driver will not consider a secondary's lag when choosing where to direct a read operation.

`MongoDB\Driver\ReadPreference::SMALLEST_MAX_STALENESS_SECONDS`

  The minimum value for the "maxStalenessSeconds" option is 90 seconds. The driver estimates secondaries' staleness by periodically checking the latest write date of each replica set member. Since these checks are infrequent, the staleness estimate is coarse. Thus, the driver cannot enforce a max staleness value of less than 90 seconds.

## 変更履歴

バージョン | 説明
--- | ---
1.2.0 | Added the `MongoDB\Driver\ReadPreference::NO_MAX_STALENESS` and `MongoDB\Driver\ReadPreference::SMALLEST_MAX_STALENESS_SECONDS` constants. Implements MongoDB\BSON\Serializable.

## 目次

- MongoDB\Driver\ReadPreference::bsonSerialize — Returns an object for BSON serialization
- MongoDB\Driver\ReadPreference::__construct — Create a new ReadPreference
- MongoDB\Driver\ReadPreference::getMaxStalenessSeconds — Returns the ReadPreference's "maxStalenessSeconds" option
- MongoDB\Driver\ReadPreference::getMode — Returns the ReadPreference's "mode" option
- MongoDB\Driver\ReadPreference::getTagSets — Returns the ReadPreference's "tagSets" option