It is my pleasure to announce the 1.2.0 release of libbson and libmongoc, the C libraries that compose the MongoDB C Driver. This is the most significant C Driver release of the year. It includes rewritten client code with parallel server discovery, plus many features and fixes.

These notes summarize changes since the previous stable release, 1.1.11, including changes in the 1.2.0 betas and release candidate.


libbson 1.2.0 can be downloaded here:

libbson is a C library for creating, parsing, and manipulating BSON documents. It also serves as a portability base for libmongoc.

Changes since 1.1.11:

  • Add bson_mem_restore_vtable(), inverse of bson_mem_set_vtable()
  • Enable runtime asserts in release build.
  • Fixed compiler warnings and build failures on various platforms.
  • Improvements to the formatting and contents of the documentation.

The libbson documentation is here:


libmongoc 1.2.0 can be downloaded here:

libmongoc is the C Driver for MongoDB, a library for building high-performance applications that communicate with MongoDB in the C language. It can also serve as the base for drivers in higher-level languages.

Internal rewrite

The main feature is Jason Carey and Samantha Ritter's rewrite of the mongoc_client_t internals. They overhauled it to match the Server Discovery And Monitoring Spec and the Server Selection Spec. The payoff is huge:

  • All replica set members or mongos servers are discovered and periodically checked in parallel. The driver's performance is dramatically better and more predictable with multi-server deployments, or with a flaky network, or when some servers are slow or down.
  • Clients from the same mongoc_client_pool_t share a background thread that discovers and monitors all servers in parallel.
  • Unnecessary round trips for server checks and pings are eliminated.
  • Behavior is documented in the specs, and consistent with other drivers, even in complex or unusual scenarios.
  • The URI's "replicaSet" option is enforced: the driver now refuses to connect to a server unless it is a member of a replica set with the right setName.
  • Many race conditions related to changing deployment conditions are fixed.

The worst code in the old driver, the cause of most of the bugfix releases in the 1.1.x series, has been completely replaced with a well-designed architecture.

To conform to the new specs, the client accepts these options in the MongoDB URI; see the mongoc_uri_t documentation for details:

  • heartbeatFrequencyMS
  • serverSelectionTimeoutMS
  • serverSelectionTryOnce
  • socketCheckIntervalMS

Other features:

Other changes:

  • Enable runtime asserts in release build.
  • The libbson submodule's URL now uses the recommended https://, not git://
  • mongoc_client_kill_cursor() is now deprecated and will be removed in 2.0.
  • The write concern "w=-1" is documented as obsolete.


These notable bugs have been fixed since 1.1.11:

  • The driver now uses the server's maxWireVersion to avoid an error and extra round-trip when executing aggregations on MongoDB 2.4 and older.
  • Much improved reporting of network errors, unavailable servers, and authentication failure
  • Off-by-one error in mongoc_gridfs_file_seek with mode SEEK_END
  • The writeConcernErrors field of bulk results is properly formatted.
  • A cursor with a server "hint" sets slaveOkay and / or $readPreference.
  • Destroying an exhaust cursor must close its socket
  • "wtimeoutms" was ignored for write concerns besides "majority".
  • Bulk write operations might fail in mixed-version sharded clusters with some pre-2.6 mongos servers.
  • A variety of bugs and incorrect results in mongoc_bulk_operation_execute.
  • Numerous compiler warnings and build failures on various platforms.
  • Copious refinements to the documentation.

The documentation is here:

Thanks to everyone who contributed to this version of libbson and libmongoc.

  • Jason Carey
  • Samantha Ritter
  • A. Jesse Jiryu Davis
  • Hannes Magnusson
  • Kyle Suarez
  • Jeremy Mikola
  • Remi Collet
  • Jose Sebastian Battig
  • Derick Rethans
  • David Hatch
  • Yuchen Xie
  • Manuel Schoenlaub
  • Sujan Dutta
  • Lloyd Zhou
  • rubicks
  • Pawel Szczurko
  • Yuval Hager

   —A. Jesse Jiryu Davis

Image: Lisa Murray.