cursor.readConcern()¶

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Definition
Considerations

Definition¶

cursor.readConcern(level)¶

New in version 3.2.

Specify a read concern for the db.collection.find() method.

The readConcern() method has the following form:

db.collection.find().readConcern(<level>)

The readConcern() method has the following parameter:

Parameter Type Description

level

string

Read concern level.

Possible read concern levels are:

"local". This is the default read concern level for read operations against primary and read operations against secondaries when associated with causally consistent sessions.
"available". This is the default for reads against secondaries when when not associated with causally consistent sessions. The query returns the instance’s most recent data.
"majority". Available for replica sets that use WiredTiger storage engine.
"linearizable". Available for read operations on the primary only.

For more formation on the read concern levels, see Read Concern Levels.

Considerations¶

`"majority"` Read Concern¶

To use read concern level of "majority", replica sets must use WiredTiger storage engine.

For MongoDB 4.0.3+ (and 3.6.1+), you can disable read concern "majority". For more information, see Disable Read Concern Majority.

Read Your Own Writes¶

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.

Linearizable Read Concern Performance¶

When specifying linearizable read concern, always use maxTimeMS() in case a majority of data bearing members are unavailable.

copy

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