Write Concern Reference¶
Write concern describes the guarantee that MongoDB provides when reporting on the success of a write operation.
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.
Read Isolation Behavior¶
MongoDB allows clients to read documents inserted or modified before it commits these modifications to disk, regardless of write concern level or journaling configuration. As a result, applications may observe two classes of behaviors:
- For systems with multiple concurrent readers and writers, MongoDB will allow clients to read the results of a write operation before the write operation returns.
- If the mongod terminates before the journal commits, even if a write returns successfully, queries may have read data that will not exist after the mongod restarts.
Other database systems refer to these isolation semantics as read uncommitted. For all inserts and updates, MongoDB modifies each document in isolation: clients never see documents in intermediate states. For multi-document operations, MongoDB does not provide any multi-document transactions or isolation.
For replica sets, write operations are durable only after a write replicates and commits to the journal of a majority of the members of the set. MongoDB regularly commits data to the journal regardless of journaled write concern: use the commitIntervalMs to control how often a mongod commits the journal.
Available Write Concern¶
Write concern can include the w option to specify the required number of acknowledgments before returning, the j option to require writes to the journal before returning, and wtimeout option to specify a time limit to prevent write operations from blocking indefinitely.
In sharded clusters, mongos instances will pass the write concern on to the shard.
The w option provides the ability to disable write concern entirely as well as specify the write concern for replica sets.
MongoDB uses w: 1 as the default write concern. w: 1 provides basic receipt acknowledgment.
The w option accepts the following values:
This is the default write concern for MongoDB.
Disables basic acknowledgment of write operations, but returns information about socket exceptions and networking errors to the application.
If you disable basic write operation acknowledgment but require journal commit acknowledgment, the journal commit prevails, and the server will require that mongod acknowledge the write operation.
|<Number greater than 1>||
Guarantees that write operations have propagated successfully to the specified number of replica set members including the primary.
For example, w: 2 indicates acknowledgements from the primary and at least one secondary.
If you set w to a number that is greater than the number of set members that hold data, MongoDB waits for the non-existent members to become available, which means MongoDB blocks indefinitely.
Confirms that write operations have propagated to the majority of configured replica set: a majority of the set’s configured members must acknowledge the write operation before it succeeds. This allows you to avoid hard coding assumptions about the size of your replica set into your application.
|<tag set>||By specifying a tag set, you can have fine-grained control over which replica set members must acknowledge a write operation to satisfy the required level of write concern.|
Requiring journaled write concern in a replica set only requires a journal commit of the write operation to the primary of the set regardless of the level of replica acknowledged write concern.
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.