MongoDB Documentation, Release 3.0.4 Changed in version 2.6: Specifying a write concern that includes j: true to a mongod or mongos running with --nojournal option now errors. Previous versions would ignore the j: true . 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. 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. Write Concern Introduction (page 78) and Write Concern for Replica Sets (page 80). See also: SQL to MongoDB Mapping Chart In addition to the charts that follow, you might want to consider the Frequently Asked Questions (page 743) section for a selection of common questions about MongoDB. The following table presents the various SQL terminology and concepts and the corresponding MongoDB terminology and concepts. Terminology and Concepts Executables SQL Terms/Concepts MongoDB Terms/Concepts database database table collection row document or BSON document column field index index table joins embedded documents and linking primary key Specify any unique column or column combination as primary key. primary key In MongoDB, the primary key is automatically set to the _id field. aggregation (e.g. group by) aggregation pipeline See the SQL to Aggregation Mapping Chart (page 476). MongoDB MySQL Oracle Informix DB2 Database Server mongod mysqld oracle IDS DB2 Server Database Client mongo mysql sqlplus DB-Access DB2 Client The following table presents some database executables and the corresponding MongoDB executables. This table is not meant to be exhaustive. 3.4. MongoDB CRUD Reference 131