Release Notes for MongoDB 8.1

Aggregation Accumulators

Starting in MongoDB 8.1, the following aggregation accumulators are available:

• $concatArrays

• $setUnion

Aggregation Stages

Starting in MongoDB 8.1, the following aggregation stages are available:

• $listClusterCatalog

$geoNear Improvements

Starting in MongoDB 8.1, some previously failing $geoNear queries with hidden 2d or 2dsphere indexes will now succeed. In previous releases, some $geoNear queries used to unnecessarily fail with an IndexNotFound error when they included a hidden index due to confusion over which index to use.

$merge Missing Fields

Starting in MongoDB 8.1, if the $merge aggregation stage's supporting index is not sparse, the fields specified for the on option can be missing or contain a null value.

Query Statistics

Starting in MongoDB 8.1, query statistics are collected and reported for count and distinct commands. For details, see count Command Query Shape and distinct Command Query Shape.

Improved Accuracy for Operation Duration Metric

Starting in MongoDB 8.1, the durationMillis metric reported in slow query logs accounts for time spent processing authorization and parsing the command. As a result, durationMillis more closely reflects the full command duration.

serverStatus Metrics

Starting in MongoDB 8.1 (and 8.0.4 and 7.0.14), the indexStats section of the serverStatus command output tracks indexes in prepareUnique state.

MongoDB 8.1 adds the following server status metrics:

• maxTimestampEligibleForTruncate in changeStreamPreImages.purgingJob

• metrics.ttl.invalidTTLIndexSkips

• metrics.repl.timestamps

• metrics.repl.timestamps.oldestTimestamp

• query.planning.fastPath.express

• query.planning.fastPath.idHack

• metrics.abortExpiredTransactions.successfulKills

• metrics.abortExpiredTransactions.timedOutKills

• wiredTiger.version

Add Comments to Query Settings

Starting in MongoDB 8.1 (and 8.0.4), you can use setQuerySettings to add comments to query settings. For example, add a comment that indicates why you added query settings.

Slow Query Metrics for Disk Spilling

Starting in MongoDB 8.1, log messages for slow queries contain new metrics if the query execution writes temporary files to disk. These metrics are prefixed by the query execution stage that caused the query to exceed the memory limit. For example, sortSpills indicates the number of times that the sort stage of query execution wrote temporary files to disk.

For more information on writing temporary files to disk, see allowDiskUse().

Server Parameters

Starting in MongoDB 8.1, the following server parameters are available:

• enableAutoCompaction

• upsertMaxRetryAttemptsOnDuplicateKeyError

• AbortExpiredTransactionsSessionCheckoutTimeout

• JWKSMinimumQuiescePeriodSecs

• catalogCacheCollectionMaxEntries

• catalogCacheDatabaseMaxEntries

• catalogCacheIndexMaxEntries

Inconsistency Types

Starting in MongoDB 8.1, the following inconsistency types are available:

• CollectionAuxiliaryMetadataMismatch

connectionStatus Connection UUID

Starting in MongoDB 8.1, you can access the currently connected client's UUID through connectionStatus.authInfo.UUID.

Validate Command Index Specifications Output

Starting in MongoDB 8.1, the validate command includes index specifications in the validate.indexDetails output field.

Validation Action for Handling Invalid Documents

Starting in MongoDB 8.1, you can set your schema's validationAction option to errorAndLog, in which MongoDB rejects any insert or update that violates the validation criteria and logs the error to the mongod log file.

For more information, see Choose How to Handle Invalid Documents.

Disk Space

Starting in MongoDB 8.1 (and 8.0.5), if disk space is running low, MongoDB will fail queries that are spilling to disk.