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Compatibility Changes in MongoDB 6.0

On this page

  • Aggregation
  • Change Streams
  • Indexes
  • Legacy mongo Shell Removed
  • Platform Support
  • Regular Expressions
  • Removed Operators
  • Removed Options
  • Removed Parameters
  • Renamed Parameters
  • TTL expireAfterSeconds Behavior When Set to NaN
  • Replica Sets
  • Security
  • Time Series Collections
  • General Changes
  • Downgrade Considerations

This page describes changes introduced in MongoDB 6.0 that can affect compatibility with older versions of MongoDB.

MongoDB 6.0 is a Major Release, which means that it is supported for both MongoDB Atlas and on-premises deployments. MongoDB 6.0 includes changes introduced in MongoDB Rapid Releases 5.1, 5.2, and 5.3. This page describes compatibility changes introduced in those Rapid Releases and MongoDB 6.0.

To learn more about the differences between Major and Rapid releases, see MongoDB Versioning.

Starting in MongoDB 6.0, pipeline stages that require more than 100 megabytes of memory to execute write temporary files to disk by default. These temporary files last for the duration of the pipeline execution and can influence storage space on your instance. In earlier versions of MongoDB, you must pass { allowDiskUse: true } to individual find and aggregate commands to enable this behavior.

Individual find and aggregate commands can override the allowDiskUseByDefault parameter by either:

  • Using { allowDiskUse: true } to allow writing temporary files out to disk when allowDiskUseByDefault is set to false

  • Using { allowDiskUse: false } to prohibit writing temporary files out to disk when allowDiskUseByDefault is set to true

Starting in MongoDB 6.0, the Atlas Search $$SEARCH_META aggregation variable can be used anywhere after a $search stage in any pipeline, but it cannot be used after the $lookup or $unionWith stage in any pipeline. The $$SEARCH_META aggregation variable cannot be used in any subsequent stage after a $searchMeta stage.

Starting in MongoDB 5.3, during range migration, change stream events are not generated for updates to orphaned documents.

Starting in MongoDB 6.0, whenever possible, match filters are applied to change streams earlier than in prior releases. This improves performance. However, when a filter is narrowly defined, an earlier match may cause an operation that succeeds in prior versions to fail in 6.0.

Starting in MongoDB 6.0, passing "*" to dropIndexes or db.collection.dropIndexes() drops all indexes except for the _id index and the last remaining shard key index, if one exists. Attempts to explicitly drop the last remaining shard key index raise an error.

Starting in MongoDB 5.2, you can use dropIndexes or db.collection.dropIndexes() to drop existing indexes on the same collection even if there is an index build in progress. In earlier versions, attempting to drop a different index during an in-progress index build results in a BackgroundOperationInProgressForNamespace error.

To prevent out of memory errors, indexMaxNumGeneratedKeysPerDocument limits the maximum number of 2dsphere index keys generated for a single document.

See indexMaxNumGeneratedKeysPerDocument.

Starting in MongoDB 6.0, a change to the unique index key format was introduced. If you create a unique index in MongoDB 6.0, the index won't work with MongoDB versions earlier than 5.3.2 or 5.0.7.

The mongo shell is removed from MongoDB 6.0. The replacement is mongosh.

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

  • RHEL-72-s390x

Starting in MongoDB 5.1, invalid $regex options options are no longer ignored. This change makes $regex options more consistent with the use of $regex in the aggregate command and projection queries.

Starting in MongoDB 5.1, if a collection has schema validation rules that contain invalid $regex options the server:

  • Prevents all insert and update operations until the schema validation rules containing the invalid regex pattern are modified with the collMod command.

  • Writes a warning error to the mongod log file.

Starting in MongoDB 5.1, these legacy query operators are removed:

Removed Operator

MongoDB 6.0 removes the --cpu mongod option.

MongoDB 6.0 removes the following server parameters:

Removed Parameter

This option is removed from the MongoDB Community Edition. It is available in MongoDB Enterprise edition.

FIPS was not a supported feature in MongoDB Community Edition. If your installation made use of FIPS anyway, you will need to reconfigure your TLS/SSL connections before upgrading.

Starting in MongoDB 6.0, the following parameters have been renamed:

Setting TTL expireAfterSeconds to NaN experiences a behavior change from MongoDB 4.4 to MongoDB 6.0 that affects initial sync from MongoDB 4.4 and earlier and mongorestore from MongoDB 4.4 and earlier. Performing any of those actions causes an expireAfterSeconds of NaN to be treated as an expireAfterSeconds of 0. Immediate document expiration may occur as a result.

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.

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.

Starting in MongoDB 5.3, SCRAM-SHA-1 cannot be used for intra-cluster authentication. Only SCRAM-SHA-256 is supported.

In previous MongoDB versions, SCRAM-SHA-1 and SCRAM-SHA-256 can both be used for intra-cluster authentication, even if SCRAM is not explicitly enabled.

Starting in MongoDB 5.1, instances running in FIPS mode have the SCRAM-SHA-1 authentication mechanism disabled by default. You can enable the SCRAM-SHA-1 authentication mechanism with the setParameter.authenticationMechanisms command.

This change will not affect drivers which target MongoDB setFeatureCompatibilityVersion 4.0+.

Starting in MongoDB 6.0, if ocspEnabled is set to true during initial sync, all nodes must be able to reach the OCSP responder.

If a member fails in the STARTUP2 state, set tlsOCSPVerifyTimeoutSecs to a value that is less than 5.


If you create a sharded time series collection in MongoDB 5.1 or greater, downgrading to a version older than MongoDB 5.0.4 will result in data loss.

Before downgrading to a version older than 5.0.4, drop all sharded time series collections.

If there are secondary indexes on time series collections and you need to downgrade the feature compatibility version (fCV), you must first drop any secondary indexes that are incompatible with the downgraded fCV. See setFeatureCompatibilityVersion.

The db.collection.reIndex() method is deprecated in MongoDB v6.0.
The reIndex command is deprecated in MongoDB v6.0.
Simple Network Management Protocol (SNMP)

Starting in MongoDB 6.0, SNMP is deprecated and will be removed in the next release. To monitor your deployment, use MongoDB Ops Manager.

Starting in MongoDB 5.1 (and 5.0.4), the $mod operator returns an error if the divisor or remainder values evaluate to certain values. See $mod behavior.

MongoDB 6.0 removes support for the following legacy opcodes and database commands:


Upgrade Drivers

To avoid disruption due to the removal of these opcodes, upgrade your driver to the latest version.

If you attempt to connect to a MongoDB 3.4 or older mongod instance with a MongoDB 5.1 or newer mongo shell, you will receive an error message like the following:

Connection handshake failed. Is your mongod 3.4 or older?
:: caused by :: network error while attempting to run command
'isMaster' on host ''

Since MongoDB 3.6, MongoDB drivers have used OP_MSG instead of OP_QUERY and the other legacy opcodes and commands.

Starting in MongoDB 6.0:



The OP_QUERY RPC protocol may be used with the following commands:

  • _isSelf

  • authenticate

  • buildinfo

  • buildInfo

  • hello

  • ismaster

  • isMaster

  • saslContinue

  • saslStart

All other commands will be rejected if issued as OP_QUERY.

MongoDB 6.0 upgrades the internal JavaScript engine used for server-side JavaScript, $accumulator, $function, and $where expressions and from MozJS-60 to MozJS-91. Several deprecated, non-standard array and string functions that existed in MozJS-60 are removed in MozJS-91.

For the complete list of removed array and string functions, see the next sections on this page.


Only Static Functions are Removed

Only static JavaScript functions are removed. Prototype function equivalents of the removed functions can still be used.

For example:

  • Array.concat(<array1>, <array2>) is a static function and no longer works in MongoDB 6.0.

  • <array1>.concat(<array2>) is a prototype function and still works in MongoDB 6.0.

This behavior applies to both removed array and removed string functions.

Starting in MongoDB 6.0, the following array functions are removed and cannot be used in server-side JavaScript with $accumulator, $function, and $where expressions:

  • Array.concat

  • Array.every

  • Array.filter

  • Array.forEach

  • Array.indexOf

  • Array.join

  • Array.lastIndexOf


  • Array.pop

  • Array.push

  • Array.reduce

  • Array.reduceRight

  • Array.reverse

  • Array.shift

  • Array.slice

  • Array.some

  • Array.sort

  • Array.splice

  • Array.unshift

Starting in MongoDB 6.0, the following array functions are removed and cannot be used in server-side JavaScript with $accumulator, $function, and $where expressions:

  • String.charAt

  • String.charCodeAt

  • String.concat

  • String.contains

  • String.endsWith

  • String.includes

  • String.indexOf

  • String.lastIndexOf

  • String.localeCompare

  • String.match

  • String.normalize

  • String.replace


  • String.slice

  • String.split

  • String.startsWith

  • String.substr

  • String.substring

  • String.toLocaleLowerCase

  • String.toLocaleUpperCase

  • String.toLowerCase

  • String.toUpperCase

  • String.trim

  • String.trimLeft

  • String.trimRight

Starting in MongoDB 6.0, the dbStats command and the db.stats() method only report free space assigned to collections if the freeStorage parameter is set to 1.

Starting in MongoDB 6.0, an index filter uses the collation previously set using the planCacheSetFilter command.

Starting in MongoDB 6.0, the distinct command returns the same results for collections and views when using arrays.

See Arrays in Collections and Views.

The following sections provide information for removing backward-incompatible features from your deployment. If you are downgrading from MongoDB 6.0 to an earlier version, review the following sections to ensure that your deployment runs successfully after downgrading.

Starting in MongoDB 5.3, if you are using clustered collections, you must drop those collections before you can downgrade to an earlier MongoDB version.

Starting in MongoDB 6.0, if you need to downgrade the feature compatibility version, ensure you disable cluster-to-cluster replication and user write blocking.

See Cluster-to-Cluster Sync and User Write Blocking.

You must drop time series collections before downgrading:

  • MongoDB 6.0 or later to MongoDB 5.0.7 or earlier.

  • MongoDB 5.3 to MongoDB 5.0.5 or earlier.

See Time Series Collections.

Starting in MongoDB 6.0, ensure that all setClusterParameter operations have completed. fCV downgrade cannot occur successfully if there are any ongoing setClusterParameter operations on sharded clusters.

Starting in MongoDB 5.1, you must run the following command from the directory into which the SELinux policy was previously cloned before you can downgrade to an earlier MongoDB version:

sudo make uninstall

Starting in MongoDB 5.3 Enterprise, if you are using the following KMIP settings, you must remove them from the configuration file before you can downgrade to an earlier MongoDB version:

Starting in MongoDB 6.0, if you are using changeStreamOptions.preAndPostImages.expireAfterSeconds to control time-based retention of change streams pre- and post-image collections, you must ensure there are no active setClusterParameter operations when downgrading.

Starting in MongoDB 6.0 Enterprise, if you are using audit log encryption, you must remove the following settings from the configuration file before you can downgrade to an earlier MongoDB version:

Existing encrypted audit logs remain encrypted, and you can keep any procedures you have developed for storage and processing of encrypted logs.

See Audit Log.

Starting in MongoDB 6.0, if you are using document pre- and post-images for change streams, you must disable changeStreamPreAndPostImages for each collection using the collMod command before you can downgrade to an earlier MongoDB version.

If your application uses change streams, ensure that it does not require the showExpandedEvents option, which will not be available after downgrade.

If your cluster's configuration is using the new "srv:" or "srv_raw:" URL types in its LDAP configuration, it will be unable to restart after a downgrade. Remove the new URL types from your cluster's configuration before or downgrading.

You must drop collections that use encrypted fields before you can complete the fCV downgrade. The downgrade will not complete if there are collections using encryptedFields.


Release Notes for MongoDB 6.0