Compatibility Changes in MongoDB 5.1

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.

Arbiters

Arbiters are not supported with quarterly rapid releases releases. If your deployment includes arbiters, only use LTS releases.

$regex Find Queries No Longer Ignore Invalid Regex

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.

$regex Schema Validation Error Behavior

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.

FIPS Mode Defaults SCRAM-SHA-1 Authentication to Off

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+.

$mod Error Behavior

Starting in MongoDB 5.1 (and 5.0.4 and 4.4.10), the $mod operator returns an error if the divisor or remainder values evaluate to certain values. See $mod behavior.

Legacy Opcodes Removed

MongoDB drivers have used OP_MSG instead of OP_QUERY and the other legacy opcodes since MongoDB v3.6.

This release removes support for the following legacy opcodes:

• OP_INSERT

• OP_DELETE

• OP_UPDATE

• OP_KILL_CURSORS

• OP_GET_MORE

• OP_QUERY

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

mongod will close the connection and will not respond to:

• OP_INSERT

• OP_DELETE

• OP_UPDATE

• OP_KILL_CURSORS

mongod will return an error for:

• OP_GET_MORE

• OP_QUERY legacy find

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

• _isSelf

• authenticate

• buildinfo

• buildInfo

• hello

• ismaster

• isMaster

• saslContinue

• saslStart

OP_QUERY will return an error if it is used for a find operation. All other commands will be rejected if issued as OP_QUERY.

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:

// Reformated for clarity. The message appears on a single line.
Connection handshake failed. Is your mongod 3.4 or older?
:: caused by :: network error while attempting to run command
'isMaster' on host '127.0.0.1:27017'

Community Edition

• RHEL-72-s390x