Compatibility Changes in MongoDB 8.1

$densify Validation Changes

Starting in MongoDB 8.1, the $densify stage errors if field shares its prefix with any field in the partitionByFields array. For example, the following combinations of field and partitionByFields result in an error:

• field: "timestamp", partitionByFields: ["timestamp"]

• field: "timestamp", partitionByFields: ["timestamp.hours"]

• field: "timestamp.hours", partitionByFields: ["timestamp"]

Validation Action for Handling Invalid Documents

If you use an errorAndLog validation action on a collection, MongoDB cannot downgrade until you drop the collection, or if you change the validation action for the collection to one supported in older versions. To change the validation action on a collection, use the collMod command.

Hedged Reads Removed

Starting in MongoDB 8.1, hedged reads are removed. If you specify hedged read options in a query, MongoDB executes the query but ignores the hedging options and logs a warning.

The following server parameters related to hedged reads no longer have an effect:

• maxTimeMSForHedgedReads

• opportunisticSecondaryTargeting

• readHedgingMode

The serverStatus command no longer returns the hedgingMetrics object.

Driver Incompatibility

Any application that runs the buildInfo command before authenticating is incompatible with MongoDB 8.1. The following driver versions are incompatible with MongoDB 8.1 because of this change:

• JVM drivers v3.x and earlier (includes Java Sync, Java Reactive Streams, Kotlin Coroutine, Kotlin Sync, and Scala drivers)

• .NET/C# driver v2.14.1 and earlier

Maximum Incoming Connections

Starting in MongoDB 8.1, the maximum number of incoming connections that you can set with the net.maxIncomingConnections configuration file option on Linux systems is the value of (RLIMIT_NOFILE / 2) * 0.8. If you specify a larger value, MongoDB uses the default value.

Subtype Conversion

Starting in MongoDB 8.1, $convert returns an error when attempting to convert between different binData subtypes. In MongoDB 8.0, $convert returns the original value and original subtype: no conversion is performed. MongoDB versions before 8.0 don't have binData conversion.

Interpolating Identical Values in Different Partitions

Starting in MongoDB 8.1, $fill can use the linear method to interpolate if there are identical values in different partitions.

Earlier MongoDB versions return an error stating the sort field cannot have repeated values.

For details, see Interpolate Identical Values in Different Partitions.