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

预定义常量

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.

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

参见

• » Read Concern reference