Release Notes for MongoDB 8.0 (Release Candidate)

Database Profiler

When you specify a filter for the database profiler, you can log operations based on the new workingMillis metric. You can log operations based on both workingMillis and durationMillis and set each metric to a different threshold.

BinData Conversion

Starting in MongoDB 8.0, you can use the $convert operator to perform the following conversions:

• String values to binData values

• binData values to string values

MongoDB 8.0 also includes a new helper expression, $toUUID, which provides simplified syntax for converting strings to UUID values.

$queryStats Change Stream Improvements

Starting in MongoDB 8.0, $queryStats improves tracking and reporting metrics for change streams. For more information, see $queryStats Change Streams Behavior.

$rank and $denseRank Behavior

Starting in MongoDB 8.0, null and missing field values in $denseRank and $rank sortBy operations are treated the same when calculating rankings. This change makes the behavior of denseRank and rank consistent with $sort.

Unshard Collections

Starting in MongoDB 8.0, you can unshard existing sharded collections with the unshardCollection command or the sh.unshardCollection() method. This operation moves all documents in the collection onto a specified shard or the shard with the least amount of data.

New replSetGetStatus Metrics

Starting in MongoDB 8.0, the following metrics are available when using the replSetGetStatus command:

• replSetGetStatus.electionCandidateMetrics.lastSeenWrittenOpTimeAtElection

• replSetGetStatus.electionParticipantMetrics.lastWrittenOpTimeAtElection

• replSetGetStatus.electionParticipantMetrics.maxWrittenOpTimeInSet

• replSetGetStatus.members[n].optimeWritten

• replSetGetStatus.members[n].optimeWrittenDate

• replSetGetStatus.members[n].lastWrittenWallTime

• replSetGetStatus.optimeWritten

• replSetGetStatus.optimeWrittenDate

• replSetGetStatus.optimes.lastWrittenWallTime

• replSetGetStatus.optimes.writtenOpTime

Oplog Buffers

Starting in MongoDB 8.0, secondaries write and apply oplog entries for each batch in parallel. A writer thread reads new entries from the primary and writes them to the local oplog. An applier thread asynchronously applies these changes to the local database. This changes increases replication throughput for secondaries.

This introduces a breaking change for metrics.repl.buffer, as it now provides metrics on two buffers instead of one.

MongoDB 8.0 deprecates the following server status metrics:

• metrics.repl.buffer.count

• metrics.repl.buffer.maxSizeBytes

• metrics.repl.buffer.sizeBytes

MongoDB 8.0 adds the following server status metrics:

• metrics.repl.buffer.apply

• metrics.repl.buffer.apply.count

• metrics.repl.buffer.apply.maxCount

• metrics.repl.buffer.apply.maxSizeBytes

• metrics.repl.buffer.apply.sizeBytes

• metrics.repl.buffer.write

• metrics.repl.buffer.write.count

• metrics.repl.buffer.write.maxSizeBytes

• metrics.repl.buffer.write.sizeBytes

• metrics.repl.write

• metrics.repl.write.batchSize

• metrics.repl.write.batches

• metrics.repl.write.batches.num

• metrics.repl.write.batches.totalMillis

Moving Collections that have Change Streams

Starting in MongoDB 8.0, movePrimary doesn't invalidate collections that have change streams. The change streams can continue to read events from collections after the collections are moved to a new shard.

For details, see Moving Collections that have Change Streams.

New Database Commands

• abortMoveCollection

• abortUnshardCollection

• moveCollection

• unshardCollection

New mongosh Methods

• sh.abortMoveCollection()

• sh.abortUnshardCollection()

• sh.moveCollection()

• sh.unshardCollection()

Moveable Collections

Starting in MongoDB 8.0, you can move an unsharded collection to a different shard using the moveCollection command.

To learn more, see Moveable Collections.

Shutdown Performance

Starting in MongoDB 8.0, Bulk.insert() and data ingestion workloads may perform better. However, shutting down MongoDB immediately after running these workloads might take longer because of the extra data being flushed to disk.

Store Application Data on Config Shards

Starting in MongoDB 8.0, you can configure a config server to store application data in addition to the usual sharded cluster metadata. The config server is then known as a config shard. For details, see Config Shards.

$lookup Stage in Transactions with Sharded Collections

Starting in MongoDB 8.0, you can use the $lookup stage within a transaction while targeting a sharded collection.

Compaction Improvements

Parameter Filtering

Starting in MongoDB 8.0, the getParameter command accepts a setAt field. You can use this field to filter the allParameters: true return document to show only those parameters that you can set at startup or runtime.

New Bulk Write Command

Starting in MongoDB 8.0, you can use the new bulkWrite command to perform many insert, update, and delete operations on multiple collections in one request. The existing db.collection.bulkWrite() method only allows you to modify one collection in one request.

Read Concern on Capped Collections

Starting in MongoDB 8.0, you can use read concern "snapshot" on capped collections.

DDL Operations

In MongoDB 8.0, if you add or remove a shard while your cluster executes a DDL operation (operation that modifies a collection such as reshardCollection), any operation that adds or removes a shard only executes after the concurrent DDL operation finishes.

Default Timeout for Queries

Starting in MongoDB 8.0, you can use the defaultMaxTimeMS cluster parameter to specify a default time limit for individual read operations to complete.

New Query Shape and Query Settings

MongoDB 8.0 introduces a new query shape. The pre-existing query shape is renamed as the plan cache query shape.

Starting in MongoDB 8.0, you can add query settings for the new query shape. The query optimizer uses the query settings as an additional input during query planning, which affects the plan selected to run a query that has a matching query shape.

Query settings have improved functionality compared to index filters. Index filters are also deprecated starting in MongoDB 8.0, and you should avoid using them.

• To add query settings, use the setQuerySettings command.

• To delete query settings, use the removeQuerySettings command.

• To retrieve query settings, use a $querySettings stage in an aggregation pipeline.

• To block a query shape, see operation rejection filters.

Starting in MongoDB 8.0, queryShapeHash is included in the following output:

• explain command in the explain.queryShapeHash field

• database profiler in the system.profile.queryShapeHash field

• $currentOp aggregation pipeline stage in the $currentOp.queryShapeHash field

• slow query log

Starting in MongoDB 8.0, the pre-existing queryHash field is renamed to planCacheShapeHash. If you're using an earlier MongoDB version, you'll see queryHash instead of planCacheShapeHash.

Sort Support for updateOne

Starting in MongoDB 8.0, the updateOne() method supports a sort option. This controls how MongoDB orders the document before it selects the document to receive the update.

Previous releases use the findAndModify() and findOneAndUpdate() methods to update the first document in a user-specified sort order. Support for retryable writes requires these methods to copy the entire document to a special side collection for each node, which is a more expensive operation than the updateOne() method with the new sort option.

Upgraded TCMalloc

Starting in MongoDB 8.0, MongoDB uses an upgraded version of TCMalloc that uses per-CPU caches, instead of per-thread caches, to reduce memory fragmentation and make your database more resilient to high-stress workloads.

Query Planning and Execution

Batch Multi-Document Insert Operations

Starting in MongoDB 8.0, insert operations for multi-document transactions no longer produce individual oplog entries. Instead, multi-document inserts are batched as a single entry. The change stream insert event for each document has the same clusterTime.

This change increases performance of multi-document insert operations and eliminates possible replication lag caused by multiple oplog writes.

OIDC Identity Providers Can Share an Issuer

Starting in MongoDB 8.0, when multiple identity providers (IDP) are defined, the oidcIdentityProviders parameter accepts duplicate issuer values as long as the audience value is unique for each issuer. This is also available in versions 7.3 and 7.0.

Namespaces in Subpipelines

Starting in MongoDB 8.0, namespaces in subpipelines inside $lookup and $unionWith are validated to ensure the correct use of from and coll fields.

For details, see $lookup subpipelines and $unionWith subpipelines.

Queryable Encryption Range Queries

Starting in MongoDB 8.0, Queryable Encryption supports range queries on encrypted fields using the $lt, $lte, $gt, and $gte operators. For details, see Supported Operations for Queryable Encryption. To enable range queries on encrypted fields, see Create an Encryption Schema.

OCSF Schema for Log Messages

Starting in MongoDB 8.0, you can specify the OCSF schema for audit log messages. The OCSF schema provides logs in a standardized format compatible with log processors.

To set the schema used for log messages, use the auditLog.schema configuration file option.

For example log messages in OCSF format, see OCSF Schema Audit Messages.