Release Notes for MongoDB 2.0

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Although the major version number has changed, MongoDB 2.0 is a standard, incremental production release and works as a drop-in replacement for MongoDB 1.8.


Read through all release notes before upgrading, and ensure that no changes will affect your deployment.

If you create new indexes in 2.0, then downgrading to 1.8 is possible but you must reindex the new collections.

mongoimport and mongoexport now correctly adhere to the CSV spec for handling CSV input/output. This may break existing import/export workflows that relied on the previous behavior. For more information see SERVER-1097 .

Journaling is enabled by default in 2.0 for 64-bit builds. If you still prefer to run without journaling, start mongod with the --nojournal run-time option. Otherwise, MongoDB creates journal files during startup. The first time you start mongod with journaling, you will see a delay as mongod creates new files. In addition, you may see reduced write throughput.

2.0 mongod instances are interoperable with 1.8 mongod instances; however, for best results, upgrade your deployments using the following procedures:

Upgrading a Standalone mongod

  1. Download the v2.0.x binaries from the MongoDB Download Page .
  2. Shutdown your mongod instance. Replace the existing binary with the 2.0.x mongod binary and restart MongoDB.

Upgrading a Replica Set

  1. Upgrade the secondary members of the set one at a time by shutting down the mongod and replacing the 1.8 binary with the 2.0.x binary from the MongoDB Download Page .

  2. To avoid losing the last few updates on failover you can temporarily halt your application (failover should take less than 10 seconds), or you can set write concern in your application code to confirm that each update reaches multiple servers.

  3. Use the rs.stepDown() to step down the primary to allow the normal failover procedure.

    rs.stepDown() and replSetStepDown provide for shorter and more consistent failover procedures than simply shutting down the primary directly.

    When the primary has stepped down, shut down its instance and upgrade by replacing the mongod binary with the 2.0.x binary.

Upgrading a Sharded Cluster

  1. Upgrade all config server instances first, in any order. Since config servers use two-phase commit, shard configuration metadata updates will halt until all are up and running.
  2. Upgrade mongos routers in any order.


Compact Command

A compact command is now available for compacting a single collection and its indexes. Previously, the only way to compact was to repair the entire database.

Concurrency Improvements

When going to disk, the server will yield the write lock when writing data that is not likely to be in memory. The initial implementation of this feature now exists:

See SERVER-2563 for more information.

The specific operations yield in 2.0 are:

  • Updates by _id
  • Removes
  • Long cursor iterations

Default Stack Size

MongoDB 2.0 reduces the default stack size. This change can reduce total memory usage when there are many (e.g., 1000+) client connections, as there is a thread per connection. While portions of a thread’s stack can be swapped out if unused, some operating systems do this slowly enough that it might be an issue. The default stack size is lesser of the system setting or 1MB.

Index Performance Enhancements

v2.0 includes significant improvements to the index . Indexes are often 25% smaller and 25% faster (depends on the use case). When upgrading from previous versions, the benefits of the new index type are realized only if you create a new index or re-index an old one.

Dates are now signed, and the max index key size has increased slightly from 819 to 1024 bytes.

All operations that create a new index will result in a 2.0 index by default. For example:

  • Reindexing results on an older-version index results in a 2.0 index. However, reindexing on a secondary does not work in versions prior to 2.0. Do not reindex on a secondary. For a workaround, see SERVER-3866 .
  • The repairDatabase command converts indexes to a 2.0 indexes.

To convert all indexes for a given collection to the 2.0 type, invoke the compact command.

Once you create new indexes, downgrading to 1.8.x will require a re-index of any indexes created using 2.0. See /tutorial/roll-back-to-v1.8-index .

Sharding Authentication

Applications can now use authentication with sharded clusters.

Replica Sets

Hidden Nodes in Sharded Clusters

In 2.0, mongos instances can now determine when a member of a replica set becomes “hidden” without requiring a restart. In 1.8, mongos if you reconfigured a member as hidden, you had to restart mongos to prevent queries from reaching the hidden member.


Each replica set member can now have a priority value consisting of a floating-point from 0 to 1000, inclusive. Priorities let you control which member of the set you prefer to have as primary the member with the highest priority that can see a majority of the set will be elected primary.

For example, suppose you have a replica set with three members, A, B, and C, and suppose that their priorities are set as follows:

  • A’s priority is 2.
  • B’s priority is 3.
  • C’s priority is 1.

During normal operation, the set will always chose B as primary. If B becomes unavailable, the set will elect A as primary.

For more information, see the priority documentation.

Data-Center Awareness

You can now “tag” replica set members to indicate their location. You can use these tags to design custom write rules across data centers, racks, specific servers, or any other architecture choice.

For example, an administrator can define rules such as “very important write” or customerData or “audit-trail” to replicate to certain servers, racks, data centers, etc. Then in the application code, the developer would say:, {w : "very important write"})

which would succeed if it fulfilled the conditions the DBA defined for “very important write”.

For more information, see Data Center Awareness.

Drivers may also support tag-aware reads. Instead of specifying slaveOk, you specify slaveOk with tags indicating which data-centers to read from. For details, see the Drivers documentation.

w : majority

You can also set w to majority to ensure that the write propagates to a majority of nodes, effectively committing it. The value for “majority” will automatically adjust as you add or remove nodes from the set.

For more information, see Write Concern.

Reconfiguration with a Minority Up

If the majority of servers in a set has been permanently lost, you can now force a reconfiguration of the set to bring it back online.

For more information see Reconfigure a Replica Set with Unavailable Members.

Primary Checks for a Caught up Secondary before Stepping Down

To minimize time without a primary, the rs.stepDown() method will now fail if the primary does not see a secondary within 10 seconds of its latest optime. You can force the primary to step down anyway, but by default it will return an error message.

See also Force a Member to Become Primary.

Extended Shutdown on the Primary to Minimize Interruption

When you call the shutdown command, the primary will refuse to shut down unless there is a secondary whose optime is within 10 seconds of the primary. If such a secondary isn’t available, the primary will step down and wait up to a minute for the secondary to be fully caught up before shutting down.

Note that to get this behavior, you must issue the shutdown command explicitly; sending a signal to the process will not trigger this behavior.

You can also force the primary to shut down, even without an up-to-date secondary available.

Maintenance Mode

When repair or compact runs on a secondary, the secondary will automatically drop into “recovering” mode until the operation finishes. This prevents clients from trying to read from it while it’s busy.

Geospatial Features

Multi-Location Documents

Indexing is now supported on documents which have multiple location objects, embedded either inline or in embedded documents. Additional command options are also supported, allowing results to return with not only distance but the location used to generate the distance.

For more information, see Multi-location Documents for 2d Indexes.

Polygon searches

Polygonal $within queries are also now supported for simple polygon shapes. For details, see the $within operator documentation.