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Write Concern

Write concern describes the level of acknowledgement requested from MongoDB for write operations to a standalone mongod or to replica sets or to sharded clusters. In sharded clusters, mongos instances will pass the write concern on to the shards.

Changed in version 2.6: A new protocol for write operations integrates write concerns with the write operations and eliminates the need to call the getLastError command. Previous versions required a getLastError command immediately after a write operation to specify the write concern.

Write Concern Specification

Write concern can include the following fields:

{ w: <value>, j: <boolean>, wtimeout: <number> }
  • the w option to request acknowledgment that the write operation has propagated to a specified number of mongod instances or to mongod instances with specified tags.
  • the j option to request acknowledgement that the write operation has been written to the journal, and
  • wtimeout option to specify a time limit to prevent write operations from blocking indefinitely.

w Option

The w option requests acknowledgement that the write operation has propagated to a specified number of mongod instances or to mongod instances with specified tags.

Using the w option, the following w: <value> write concerns are available:

Value Description
<number>

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

w: 1
Requests acknowledgement 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 acknowledgement from the standalone mongod or the primary of a replica set.

Numbers greater than 1 are valid only for replica sets to request acknowledgement from specified number of members, including the primary.

"majority"

Requests acknowledgment that write operations have propagated to the majority of voting nodes [1], including the primary.

Changed in version 3.2.6: For replica sets using protocolVersion: 1, w: "majority" may imply j: true if journaling is enabled. See writeConcernMajorityJournalDefault.

In earlier 3.2 versions, w: "majority" implies j: true if journaling is enabled. With earlier versions of MongoDB, w: "majority" does not imply journaling.

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

<tag set>
Requests acknowledgement that the write operations have propagated to a replica set member with the specified tag.

j Option

The j option requests acknowledgement from MongoDB that the write operation has been written to the journal.

j

Requests acknowledgement that the mongod instances, as specified in the w: <value>, have written to the on-disk journal. j: true does not by itself guarantee that the write will not be rolled back due to replica set primary failover.

Changed in version 3.2: With j: true, MongoDB returns only after the requested number of members, including the primary, have written to the journal. Previously j: true write concern in a replica set only requires the primary to write to the journal, regardless of the w: <value> write concern.

Changed in version 2.6: Specifying a write concern that includes j: true to a mongod or mongos running with --nojournal option produces an error. Previous versions would ignore the j: true.

wtimeout

This option specifies a time limit, in milliseconds, for the write concern. wtimeout is only applicable for w values greater than 1.

wtimeout causes write operations to return with an error after the specified limit, even if the required write concern will eventually succeed. When these write operations return, MongoDB does not undo successful data modifications performed before the write concern exceeded the wtimeout time limit.

If you do not specify the wtimeout option and the level of write concern is unachievable, the write operation will block indefinitely. Specifying a wtimeout value of 0 is equivalent to a write concern without the wtimeout option.

Acknowlegement Behavior

  • Standalone mongod instances acknowledge write operations after applying the write in memory unless j:true.

  • Changed in version 3.2.6: For replica sets using protocolVersion: 1, w: "majority" may imply j: true if journaling is enabled. The writeConcernMajorityJournalDefault replica set configuration setting determines the behavior.

    In 3.2-versions earlier than 3.2.6, w: "majority" implies j: true if journaling is enabled. With earlier versions of MongoDB, w: "majority" does not imply journaling.

[1]

Changed in version 3.0: Prior to MongoDB 3.0, w: "majority" refers to the majority of the replica set’s members.

Changed in version 2.6: In Master/Slave deployments, MongoDB treats w: "majority" as equivalent to w: 1. In earlier versions of MongoDB, w: "majority" produces an error in master/slave deployments.