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Read Concern

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The readConcern option allows you to control the recency, consistency, and isolation properties of the data read from replica sets and replica set shards.

Through the effective use of write concerns and read concerns, you can adjust the level of consistency and availability guarantees as appropriate, such as waiting for stronger consistency guarantees, or loosening consistency requirements to provide higher availability.

MongoDB drivers updated for MongoDB 3.2 or later support specifying read concern.

Read Concern Levels

The following read concern levels are available:

level Description
"local"

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

Default for:

Read concern local is available for use with causally consistent sessions.

For more information, see the "local" reference page.
"available"

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

Default for reads against secondaries if the reads are not associated with causally consistent sessions.

For sharded collections, "available" read concern provides the lowest latency reads possible among the various read concerns but at the expense of consistency as "available" read concern can return orphan documents.

Read concern available is unavailable for use with causally consistent sessions.

For more information, see the "available" reference page.

New in version 3.6.
"majority"

The query returns data that has been acknowledged by a majority of the replica set members. The documents returned by the read operation are durable, even in the event of failure.

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

Starting in MongoDB 3.6, support for read concern "majority" is enabled by default. For MongoDB 3.6.1 - 3.6.x, you can disable read concern "majority". For more information, see Disable Read Concern Majority.

Read concern majority is available for use with causally consistent sessions.

For more information, see the "majority" reference page.
"linearizable"

The query returns data that reflects all successful majority-acknowledged writes that completed prior to the start of the read operation. The query may wait for concurrently executing writes to propagate to a majority of replica set members before returning results.

If a majority of your replica set members crash and restart after the read operation, documents returned by the read operation are durable if writeConcernMajorityJournalDefault is set to the default state of true.

With writeConcernMajorityJournalDefault set to false, MongoDB does not wait for w: "majority" writes to be written to the on-disk journal before acknowledging the writes. As such, majority write operations could possibly roll back in the event of a transient loss (e.g. crash and restart) of a majority of nodes in a given replica set.

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

Read concern linearizable is unavailable for use with causally consistent sessions.

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.

For more information, see the "linearizable" reference page.

Regardless of the read concern level, the most recent data on a node may not reflect the most recent version of the data in the system.

For more information on each read concern level, see:

readConcern Syntax

You can specify a readConcern level [1] as an option:

readConcern: { level: <"majority"|"local"|"linearizable"|"available"> }

Causally Consistent Sessions and Available Read Concerns

For operations in a causally consistent session, "local" and "majority" levels are available. However, to guarantee causal consistency, you must use "majority". For details, see Causal Consistency.

Operations That Support Read Concern

The following operations support the readConcern option:

To specify the read concern level for the mongo shell method db.collection.find(), use the cursor.readConcern() method:

db.collection.find().readConcern(<"majority"|"local"|"linearizable"|"available">)
[1] For causally consistent sessions, MongoDB drivers automatically specifies the afterClusterTime value in the read concern.

Considerations

Read Your Own Writes

Changed in version 3.6.

Starting in MongoDB 3.6, you can use causally consistent sessions to read your own writes, if the writes request acknowledgement.

Prior to MongoDB 3.6, you must have issued your write operation with { w: "majority" } write concern and then use either "majority" or "linearizable" read concern for the read operations to ensure that a single thread can read its own writes.

Real Time Order

Combined with "majority" write concern, "linearizable" read concern enables multiple threads to perform reads and writes on a single document as if a single thread performed these operations in real time; that is, the corresponding schedule for these reads and writes is considered linearizable.

Performance Comparisons

Unlike "majority", "linearizable" read concern confirms with secondary members that the read operation is reading from a primary that is capable of confirming writes with { w: "majority" } write concern. [2] As such, reads with linearizable read concern may be significantly slower than reads with "majority" or "local" read concerns.

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.

For example:

db.restaurants.find( { _id: 5 } ).readConcern("linearizable").maxTimeMS(10000)

db.runCommand( {
     find: "restaurants",
     filter: { _id: 5 },
     readConcern: { level: "linearizable" },
     maxTimeMS: 10000
} )
[2] In some circumstances, two nodes in a replica set may transiently believe that they are the primary, but at most, one of them will be able to complete writes with { w: "majority" } write concern. The node that can complete { w: "majority" } writes is the current primary, and the other node is a former primary that has not yet recognized its demotion, typically due to a network partition. When this occurs, clients that connect to the former primary may observe stale data despite having requested read preference primary, and new writes to the former primary will eventually roll back.

Read Operations and Causally Consistent Sessions

New in version 3.6.

MongoDB 3.6 introduces support for causally consistent sessions. For read operations associated with causally consistent session, MongoDB 3.6 introduces the afterClusterTime read concern option to be set automatically by the drivers for operations associated with causally consistent sessions.

The afterClusterTime read concern option is available for "local" and "majority" read concern levels:

Important

Do not manually set afterClusterTime. MongoDB drivers set this value automatically for operations associated with causally consistent sessions.

readConcern: { level: <"majority"|"local"> , afterClusterTime: <Timestamp> }

To satisfy a read request with an afterClusterTime value of T, a mongod must perform the request after its oplog reaches time T. If its oplog has not reached time T, the mongod must wait to service the request.

Read operations with a specified afterClusterTime return data that meet both the read concern level requirement and the specified afterClusterTime requirement.

For read operations not associated with causally consistent sessions, afterClusterTime is unset.