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.
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.
Requirements
Linearizable read concern guarantees only apply if read operations specify a query filter that uniquely identifies a single document. Additionally if none of the following criteria are met, linearizable read concern might not read from a consistent snapshot, resulting in a document matching the filter not being returned:
The query uses an immutable field as the search key of the query. For example, searching on the
_idfield or using$natural.No concurrent updates mutate the search key of the query.
The search key has a unique index and the query uses that index.
If any of the preceding criteria are met, the query reads from a consistent snapshot to return the single matching document.
Causally Consistent Sessions
Read concern "linearizable" is unavailable for use with causally consistent sessions.
Aggregation Restriction
You cannot use the $out or the $merge stage in conjunction with read concern "linearizable". That is, if you specify "linearizable" read concern for db.collection.aggregate(), you cannot include either stages 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
You can use causally consistent sessions to read your own writes, if the writes request acknowledgment.
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. |