Release Notes for MongoDB 6.1

PCRE Library Upgrade

Perl Compatible Regular Expressions (PCRE) is the library used by MongoDB to implement regular expression pattern matching. Starting in version 6.1, MongoDB upgrades the PCRE library to PCRE2. PCRE2 is the current PCRE library and is actively maintained and updated.

To learn how to perform regex matches in MongoDB, see the following pages:

• $regex (query operator)

• $regexFind (aggregation operator)

• $regexFindAll (aggregation operator)

• $regexMatch (aggregation operator)

*UCP Regex Option to Match Non-ASCII Characters

Starting in MongoDB 6.1, you can use the *UCP option for regex queries. The *UCP option matches non-ASCII characters (specifically, the option can match UTF-8 characters). However, the *UCP option results in a slower query than one without the option specified.

For an example that uses the *UCP option, see Extend Regex Options to Match Characters Outside of ASCII.

Retryable Writes Error Handling

Starting in MongoDB 6.1, if both the first and second attempt of a retryable write fail without a single write being performed, MongoDB returns an error with the NoWritesPerformed label.

The NoWritesPerformed label differentiates the results of batch operations like insertMany(). In an insertMany operation, one of the following outcomes can occur:

Applications can use the NoWritesPerformed label to definitively determine that no documents were inserted. This error reporting lets the application maintain an accurate state of the database when handling retryable writes.

In previous versions of MongoDB, an error is returned when both the first and second attempts of a retryable write fail. However, there is no distinction made to indicate that no writes were performed.

Balancing Policy Changes

Starting in MongoDB 6.1, data in sharded clusters is distributed based on data size rather than number of chunks. As a result, you should be aware of the following significant changes in sharded cluster data distribution behavior:

• The balancer distributes ranges of data rather than chunks. The balancing policy looks for evenness of data distribution rather than chunk distribution.

• Chunks are not subject to auto-splitting. Instead, chunks are split only when moved across shards.

• A chunk is now referred to as a range.

• moveRange has replaced moveChunk.

Changes to Journaling

Starting in MongoDB 6.1, journaling is always enabled. As a result, MongoDB removes the storage.journal.enabled option and the corresponding --journal and --nojournal command-line options.

Improved Efficiency for Multiple Document Deletions

Starting in MongoDB 6.1:

• To improve efficiency, MongoDB may batch multiple document deletions together.

• The explain command results contain a new BATCHED_DELETE stage for batched document deletions.

Resharding

Starting in MongoDB 6.1, there are new metrics available resharding. The output of the following commands has changed:

• currentOp

• db.currentOp()

• serverStatus

• db.serverStatus()

For details, see the release notes for currentOp and serverStatus.

currentOp Metrics

Starting in MongoDB 6.1, the currentOp command and the db.currentOp() method have expanded output for resharding.

Resharding operations can involve multiple MongoDB instances, and MongoDB instances can play different roles in the resharding operation. The particular operation and the role the host instance plays in the resharding process determine when each metric updates.

serverStatus Metrics

Starting in MongoDB 6.1, the serverStatus command and the db.serverStatus() method have these output changes:

Storage Engine

In rare circumstances, a write can fail due to cache pressure. When this happens MongoDB issues a TemporarilyUnavailable error and increments the temporarilyUnavailableErrors counter in two places: the slow query log and the Full Time Diagnostic Data Collection (FTDC).

Individual operations within multi-document transactions never return TemporarilyUnavailable errors.

Adjust the write retry properties by modifying the temporarilyUnavailableBackoffBaseMs and temporarilyUnavailableMaxRetries parameters.

$addFields and $set Allow Empty Objects as Expressions

Starting in MongoDB 6.1, the aggregation stages $addFields and $set allow you to set paths to empty objects without using the $literal expression.

Auditing

MongoDB 6.1 adds the following cluster audit events:

• getClusterParameter

• setClusterParameter

• updateCachedClusterServerParameter

Starting in MongoDB 6.1, the startup audit event has this structure:

{
    originalClusterServerParameter: <original parameter value>,
    updatedClusterServerParameter": <new parameter value>
}

For additional details, see Audit Event Actions, Details, and Results.