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Release Notes for MongoDB 5.1

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Important

MongoDB 5.0 is a rapid release and is only supported for MongoDB Atlas. MongoDB 5.0 is not supported for use on-premises. For more information, see MongoDB Versioning.

Note

MongoDB 5.1 Released November 9, 2021

Patch Releases

5.1.1 - December 9, 2021

Issues fixed:

  • SERVER-61633 Resharding’s RecipientStateMachine doesn’t join thread pool for ReshardingOplogFetcher, leading to server crash at shutdown
  • SERVER-61483 Resharding coordinator fails to recover abort decision on step-up, attempts to commit operation as success, leading to data inconsistency
  • SERVER-61482 Updates to config.reshardingOperations wait for PrimaryOnlyService to be rebuilt while holding oplog slot, stalling replication on config server indefinitely
  • SERVER-61304 Unable to use –use-system-{pcre,boost,…} with r5.1.0
  • SERVER-51329 Unexpected non-retryable error when shutting down a mongos server
  • All JIRA issues closed in 5.1.1
  • 5.1.1 Changelog

Aggregation

New Aggregation Stages

MongoDB 5.1 introduces the following aggregation stages:

Stage Description
$densify Creates new documents in a sequence of documents where values in a specified field are missing.
$documents Returns literal documents from input expressions.

New Aggregation Operators

MongoDB 5.1 introduces the following aggregation operators:

Operator Description
$tsSecond Returns the seconds from a timestamp as a long.
$tsIncrement Returns the incrementing ordinal from a timestamp as a long.

$geoNear accepts let-bound variables

Starting in MongoDB 5.1, the near parameter in the $geoNear aggregation stage supports the let option and bound let option.

$lookup and $graphLookup with sharded collections

Starting in MongoDB 5.1, the $lookup and $graphLookup aggregation stages support sharded collections in the from parameter.

In previous versions of MongoDB, $lookup and $graphLookup only allowed for unsharded from collections.

General Aggregation Improvements

Starting in MongoDB 5.1, $dateSubtract and $dateAdd report an error when an overflow is detected for amount values.

Time Series Collections

Geo Indexing for Time Series collections

Starting in MongoDB 5.1, you can use geo indexes on the metaField of time series collections.

Updates and Deletes

Starting in MongoDB 5.1, time series collections support update and delete operations with limitations.

Sharded Time Series Collections

MongoDB 5.1 provides support for sharded time series collections.

See:

Slot-Based Query Execution Engine

Starting in MongoDB 5.1, a new query execution engine is used for eligible queries. If the slot based query execution engine is used, new fields are included in the query explain plan output.

  • The new query execution engine is used if possible.
  • If the new query execution engine is used, new fields are included in the query explain plan output.

Set Slot-Based Query Execution Plan Cache Size

You can set the size of the plan cache for the new query engine with the planCacheSize parameter.

Increasing the plan cache size adds more cached query shapes for the query planner. This can improve query performance, but increases memory usage.

Change Streams

Starting in MongoDB 5.1, change streams are optimized, providing more efficient resource utilization and faster execution of some aggregation pipeline stages.

Indexes

Starting in MongoDB 5.1, you can use the collMod database command to add the expireAfterSeconds option to an existing single-field non-TTL index.

Platform Support

Starting in MongoDB 5.1.2 the following platforms are no longer supported.

Community Edition

  • RHEL-72-s390x

General Improvements

Metadata Option Added for validate Command

Starting in MongoDB 5.1 (and MongoDB 5.0.4), the validate command and the db.collection.validate() helper method have a new metadata option that quickly validates collections and detects invalid index options.

serverStatus Output Change

serverStatus includes the following new fields in its output:

Aggregation Metrics
Opcode Counters
Resharding Statistics

Extra Logging Fields for Slow Queries with getMore Command

Starting in MongoDB 5.1, when a getMore command is logged as a slow query, the queryHash and planCacheKey fields are added to the slow query log message and the profiler log message.

Schema Validation Errors Contain Description Field

Starting in MongoDB 5.1, when a document fails schema validation, MongoDB includes the validation title and description in the error response. You can use these fields to provide a clearer explanation of the validation when the rules are not immediately clear, such as when using regular expressions.

For an example, see Use Title and Description Fields to Clarify Validation Rules.

Replica Sets

Assert Cluster Wide Write Concern is Set When Starting or Adding Shard

Starting in MongoDB 5.1, when starting, restarting or adding a shard server with sh.addShard() the Cluster Wide Write Concern (CWWC) must be set.

If the CWWC is not set and the shard is configured such that the default write concern is { w : 1 } the shard server will fail to start or be added and returns an error.

See default write concern calculations for details on how the default write concern is calculated.

rs.reconfig Cluster Wide Write Concern Validation

Starting in MongoDB 5.1, you must set the Cluster Wide Write Concern (CWWC) prior to issuing any reconfigs that would otherwise change the default write concern of the new replica set member.

storeFindAndModifyImagesInSideCollection Parameter

Starting in MongoDB 5.1, the storeFindAndModifyImagesInSideCollection parameter determines whether the temporary documents required for retryable findAndModify commands are stored in the side collection (config.image_collection).

Sharded Clusters

mongod Sharded Cluster And Config Server Changes

Starting in MongoDB 5.1, when starting a mongod with the --configsvr option you must also specify a --replSet.

Starting in MongoDB 5.1, when starting a mongod with the --shardsvr option you must also specify a --replSet.

Security

SELinux

Starting in MongoDB 5.1, there is a new SE Linux Policy for:

running on Red Hat Linux. The SELinux policy is for use with default installations using rpm installer packages.

Report an Issue

To report an issue, see https://github.com/mongodb/mongo/wiki/Submit-Bug-Reports for instructions on how to file a JIRA ticket for the MongoDB server or one of the related projects.

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