MongoDB Documentation, Release 3.0.4 Journaled With a journaled write concern, the MongoDB acknowledges the write operation only after committing the data to the journal . This write concern ensures that MongoDB can recover the data following a shutdown or power interruption. You must have journaling enabled to use this write concern. With a journaled write concern, write operations must wait for the next journal commit. To reduce latency for these operations, MongoDB also increases the frequency that it commits operations to the journal. See storage.mmapv1.journal.commitIntervalMs for more information. Note: 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. Replica Acknowledged Replica sets present additional considerations with regards to write concern. The default write concern only requires acknowledgement from the primary. With replica acknowledged write concern, you can guarantee that the write operation propagates to additional members of the replica set. See Write Concern for Replica Sets (page 586) for more information. Note: 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. Write Concern Reference (page 130) In MongoDB, a write operation is atomic on the level of a single document, even if the operation modifies multiple embedded documents within a single document. Chapter 3. MongoDB CRUD Operations 80 See also: Atomicity and Transactions