# The MongoDB\Driver\ReadConcern class

(mongodb >=1.1.0)

## Introduction

MongoDB\Driver\ReadConcern controls the level of isolation for read operations for replica sets and replica set shards. This option requires MongoDB 3.2 or later.

## Class synopsis

final MongoDB\Driver\ReadConcern implements MongoDB\BSON\Serializable , Serializable {
/* Constants */
const string AVAILABLE = "available" ;
const string LINEARIZABLE = "linearizable" ;
const string LOCAL = "local" ;
const string MAJORITY = "majority" ;
/* Methods */
final public bsonSerialize ( void ) : object
final public __construct ([ string `$level` ] )
final public getLevel ( void ) : string|null
final public isDefault ( void ) : bool
final public serialize ( void ) : string
final public unserialize ( string `$serialized` ) : void
}

## Predefined Constants

`MongoDB\Driver\ReadConcern::AVAILABLE`

  Default for reads against secondaries when afterClusterTimeand level are unspecified.

  The query returns the instance's most recent data. Provides no guarantee that the data has been written to a majority of the replica set members (i.e. may be rolled back).

  For unsharded collections (including collections in a standalone deployment or a replica set deployment), "local" and "available" read concerns behave identically.

  For a sharded cluster, "available" read concern provides greater tolerance for partitions since it does not wait to ensure consistency guarantees. However, a query with "available" read concern may return orphan documents if the shard is undergoing chunk migrations since the "available" read concern, unlike "local" read concern, does not contact the shard's primary nor the config servers for updated metadata.

`MongoDB\Driver\ReadConcern::LINEARIZABLE`

  The query returns data that reflects all successful writes issued with a write concern of "majority" and acknowledged prior to the start of the read operation. For replica sets that run with writeConcernMajorityJournalDefault set to `TRUE`, linearizable read concern returns data that will never be rolled back.

  With writeConcernMajorityJournalDefault set to `FALSE`, MongoDB will not wait for w: "majority" writes to be durable before acknowledging the writes. As such, "majority" write operations could possibly roll back in the event of a loss of a replica set member.

  You can specify linearizable read concern for read operations on the primary only.

  Linearizable read concern guarantees only apply if read operations specify a query filter that uniquely identifies a single document.

  Tip

  Always use maxTimeMS with linearizable read concern in case a majority of data bearing members are unavailable. maxTimeMS ensures that the operation does not block indefinitely and instead ensures that the operation returns an error if the read concern cannot be fulfilled.

  Linearizable read concern requires MongoDB 3.4.

`MongoDB\Driver\ReadConcern::LOCAL`

  Default for reads against primary if level is unspecified and for reads against secondaries if level is unspecified but afterClusterTime is specified.

  The query returns the instance's most recent data. Provides no guarantee that the data has been written to a majority of the replica set members (i.e. may be rolled back).

`MongoDB\Driver\ReadConcern::MAJORITY`

  The query returns the instance's most recent data acknowledged as having been written to a majority of members in the replica set.

  To use read concern level of "majority", replica sets must use WiredTiger storage engine and election protocol version 1.

## Changelog

Version | Description
--- | ---
1.7.0 | Implements Serializable.
1.4.0 | Added the `MongoDB\Driver\ReadConcern::AVAILABLE` constant.
1.2.0 | Added the `MongoDB\Driver\ReadConcern::LINEARIZABLE` constant. Implements MongoDB\BSON\Serializable.

## See Also

- » Read Concern reference

## Table of Contents

- MongoDB\Driver\ReadConcern::bsonSerialize — Returns an object for BSON serialization
- MongoDB\Driver\ReadConcern::__construct — Create a new ReadConcern
- MongoDB\Driver\ReadConcern::getLevel — Returns the ReadConcern's "level" option
- MongoDB\Driver\ReadConcern::isDefault — Checks if this is the default read concern
- MongoDB\Driver\ReadConcern::serialize — Serialize a ReadConcern
- MongoDB\Driver\ReadConcern::unserialize — Unserialize a ReadConcern