mgo release r2014.07.21, now at

This is a special release of mgo, the Go driver for MongoDB. Besides the several new features, this release marks the change of the Go package import path to, after years using the current one based on a static file that lives at Note that the package API is still not changing in any backwards incompatible way, though, so it is safe to replace in-use import paths right away. Instead, the change is being done for a few other reasons:

  • is more reliable, offering a geographically distributed replicated deployment with automatic failover
  • has a nice landing page with pointers to source code, API documentation, versions available, etc.
  • is backed by git and, which is one of the most requested changes over the years it has been hosted with Bazaar at Launchpad

So, from now on the import path to use when using and retrieving the package should be:

Starting with this release, the source code is also maintained and updated only on GitHub at:

The old repository and import path will remain up for the time being, to ensure existing applications continue to work, but it’s already holding out-of-date code.

In terms of changes, the r2014.07.21 release brings the following news:

Socket pool size limit may be changed

A new Session.SetPoolLimit was added for changing the number of sockets that may be in use for the desired server before the session will block waiting for an available one. The value defaults to 4096 if unset.

Note that the driver actually had the ability to limit concurrent sockets for a long time, and the hardcoded default was already 4096 sockets per server. The reason why this wasn’t exposed via the API is because people commonly delegate to the database driver the management of concurrency for the whole application, and this is a bad design. Instead, this limit must be set to cover any expected workload of the application, and application concurrency should be controlled “at the door”, by properly restricting used resources (goroutines, etc) before they are even created.

This feature was requested by a number of people, with the most notable threads being with Travis Reader, from IronIO, and the feature is finally being added after Tyler Bunnel, from, asked for the ability to increase that limit. Thanks to both, and everybody else who chimed in as well.

TCP name resolution times out after 10 seconds

Previous releases had no driver-enforced timeout.

Reported by Cailin Nelson, MongoDB.

json.Number is marshalled as a number

The encoding/json package in the standard library allows numbers to be held as a string with the json.Number type so that code can take into account their visual representation for differentiating between integers and floats (a non-standard behavior). The mgo/bson package will now recognize those values properly, and marshal them as BSON int64s or float64s based on the number representation.

Patch by Min-Young Wu, Facebook.

New GridFile.Abort method for canceling uploads

When called, GridFile.Abort cancels an upload in progress so that closing the file being written to will drop all uploaded chunks rather than atomically making the file available.

Feature requested by Roger Peppe, Canonical.

GridFile.Close drops chunks on write errors

Previously, a write error would prevent the GridFile file from being created, but would leave already written chunks in the database. Now those chunks are dropped when the file is closed.

Support for PLAIN (LDAP) authentication

The driver can now authenticate against MongoDB servers set up using the PLAIN mechanism, which enables authentication against LDAP servers as documented.

Feature requested by Cailin Nelson, MongoDB.

Preliminary support for Bulk API

The ability to execute certain operations in bulk mode is being added to the 2.6 release of the MongoDB server. This allows queuing up inserts, updates, and removals to be sent in batches rather than one by one.

This release includes an experimental API to support that, including compatibility with previous version of the MongoDB server for the added features. The functionality is exposed via the Collection.Bulk method, which returns a value with type *mgo.Bulk that contains the following methods at the moment:

  • Bulk.Insert – enqueues one or more insert operations
  • Bulk.Run – runs all operations in the queue
  • Bulk.Unordered – use unordered mode, so latter operations may proceed when prior ones failed

Besides preparing for the complete bulk API with more methods, this preliminary change adds support for the ContinueOnError wire protocol flag of the insert operation in a way that will remain compatible with the upcoming bulk operation commands, via the unordered mode.

The latter feature was requested by Chandra Sekar, and justifies the early release of the API.

Various compatibility improvements in user handling for 2.6

The MongoDB 2.6 release includes a number of changes in user handling functionality. The existing methods have been internally changed to preserve compatibility to the extent possible. Certain features, such as the handling of the User.UserSource field, cannot be preserved and will cause an error to be reported when used against 2.6.

Wake up nonce waiters on socket death

Problem reported and diagnosed by John Morales, MongoDB.

Don’t burn CPU if no masters are found and FailFast is set

Problem also reported by John Morales, MongoDB.

Stop Iter.Next if failure happens at get-more issuing time

Problem reported by Daniel Gottlieb, MongoDB.

Various innocuous race detector reports fixed

Running the test suite with the race detector enabled would raise various issues due to global variable modifications that are only done and only accessible to the test suite itself. These were fixed and the race detector now runs cleanly over the test suite.

Thanks to everybody that contributed to this release.

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