# The MongoDB\Driver\ReadConcern class

(mongodb >=1.1.0)

## 简介

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.

## 类摘要

final class MongoDB\Driver\ReadConcern implements MongoDB\BSON\Serializable, Serializable {
/* 常量 */
const string AVAILABLE = "available";
const string LINEARIZABLE = "linearizable";
const string LOCAL = "local";
const string MAJORITY = "majority";
const string SNAPSHOT = "snapshot";
/* 方法 */
final public bsonSerialize(): stdClass
final public __construct(?string `$level` = `null`)
final public getLevel(): ?string
final public isDefault(): bool
}

## 预定义常量

`MongoDB\Driver\ReadConcern::AVAILABLE`

  Default for reads against secondaries when `afterClusterTime`and `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.

  小技巧

  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.

`MongoDB\Driver\ReadConcern::SNAPSHOT`

  Read concern `"snapshot"` is available for multi-document transactions, and starting in MongoDB 5.0, certain read operations outside of multi-document transactions.

  If the transaction is not part of a causally consistent session, upon transaction commit with write concern `"majority"`, the transaction operations are guaranteed to have read from a snapshot of majority-committed data.

  If the transaction is part of a causally consistent session, upon transaction commit with write concern `"majority"`, the transaction operations are guaranteed to have read from a snapshot of majority-committed data that provides causal consistency with the operation immediately preceding the transaction start.

  Outside of multi-document transactions, read concern `"snapshot"` is available on primaries and secondaries for the following read operations: `find`, `aggregate`, and `distinct` (on unsharded collections). All other read commands prohibit `"snapshot"`.

## 更新日志

版本 | 说明
--- | ---
PECL mongodb 1.11.0 | Added the `MongoDB\Driver\ReadConcern::SNAPSHOT` constant.
PECL mongodb 1.7.0 | Implements Serializable.
PECL mongodb 1.4.0 | Added the `MongoDB\Driver\ReadConcern::AVAILABLE` constant.
PECL mongodb 1.2.0 | Added the `MongoDB\Driver\ReadConcern::LINEARIZABLE` constant. Implements MongoDB\BSON\Serializable.

## 参见

- » Read Concern reference

## 目录

- 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

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