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- cursor.readConcern()
cursor.readConcern()¶
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Definition¶
- cursor.readConcern(level)¶
3.2 新版功能.
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 values are:
- "local". This is the default read concern level.
- "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",
- you must start the mongod instances with the --enableMajorityReadConcern command line option (or the replication.enableMajorityReadConcern set to true if using a configuration file).
- replica sets must use WiredTiger storage engine and election protocol version 1.
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.
Linearizable Read Concern Performance¶
When specifying linearizable read concern, always use maxTimeMS() in case a majority of data bearing members are unavailable.
db.restaurants.find( { _id: 5 } ).readConcern("linearizable").maxTimeMS(10000)
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