MongoDB副本集提高读写速率

<number>¶

Requests acknowledgment that the write operation has propagated to the specified number of mongod instances. For example:

w: 1

Requests acknowledgment that the write operation has propagated to the standalone mongod or the primary in a replica set. w: 1 is the default write concern for MongoDB.

w: 0

Requests no acknowledgment of the write operation. However, w: 0 may return information about socket exceptions and networking errors to the application.

If you specify w: 0 but include j: true, the j: true prevails to request acknowledgment from the standalone mongod or the primary of a replica set.

w greater than 1 requires acknowledgment from the primary and as many additional data-bearing secondaries to meet the specified write concern. For example, consider a 3-member replica set with no arbiters. Specifying w: 2 would require acknowledgment from the primary and one of the secondaries. Specifying w: 3 would require acknowledgment from the primary and both secondaries.

See Acknowledgment Behavior for when mongod instances acknowledge the write.

"majority"¶

Requests acknowledgment that write operations have propagated to the calculated majority of the data-bearing voting members (i.e. primary and secondaries with members[n].votes greater than 0).

For example, consider a replica set with 3 voting members, Primary-Secondary-Secondary (P-S-S). For this replica set, calculated majority is two, and the write must propagate to the primary and one secondary to acknowledge the write concern to the client.

After the write operation returns with a w: "majority" acknowledgment to the client, the client can read the result of that write with a "majority" readConcern.

See Acknowledgment Behavior for when mongod instances acknowledge the write.

<custom write concern name>¶

Requests acknowledgment that the write operations have propagated to tagged members that satisfy the custom write concern defined in settings.getLastErrorModes.

For an example, see Custom Multi-Datacenter Write Concerns.

See Acknowledgment Behavior for when mongod instances acknowledge the write.