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Read Concern "linearizable"

New in version 3.4.

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.

Causally Consistent Sessions

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

Aggregation Restriction

Starting in MongoDB 4.2, the $out stage cannot be used in conjunction with read concern "linearizable". That is, if you specify "linearizable" read concern for db.collection.aggregate(), you cannot include the $out stage in the pipeline.

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.

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.

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.