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Read Concern

New in version 3.2.

The readConcern query option for replica sets and replica set shards determines which data to return from a query.

Read Concern Levels

Changed in version 3.4: Adds support for "linearizable" read concern.

The following read concern levels are available:

level Description
"local"
Default. 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).
"majority"

The query returns data that has been acknowledged by a majority of the replica set members.

To use read concern level of "majority",

"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.

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.

Lineariable read concern is available for both MMAPv1 and WiredTiger. See Storage Engine and Drivers Support.

New in version 3.4.

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.

Storage Engine and Drivers Support

Read Concern WiredTiger MMAPv1
"local"
"majority"  
"linearizable"

Tip

The serverStatus command returns the storageEngine.supportsCommittedReads field which indicates whether the storage engine supports "majority" read concern.

MongoDB drivers updated for 3.2 and later versions support specifying a read concern option.

readConcern Option

Use the readConcern option to specify the read concern level.

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

The readConcern option is available for the following operations:

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

Considerations

Read Your Own Writes

If using "majority" or "linearizable" read concern for read operations, use { w: "majority" } write concern for write operations on the primary 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. [1] 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
} )
[1]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.