:Compatibility changes in MongoDB 4.0

To view the official MongoDB compatibility change documentation, visit Legacy Documentation.

Deprecation of MONGODB-CR

Starting from MongoDB 4.0, MongoDB no longer supports the deprecated MONGODB-CR authentication mode.

Starting from MongoDB 3.0, MongoDB no longer allows you to create an MONGODB-CR user, unless your deployment is upgraded from V2.6 or earlier and does not upgrade the authentication mode for your original MONGODB-CR user.

If your deployment still uses the MONGODB-CR mode to store user credentials, you must upgrade the mode to Salted Challenge Response Authentication Mechanism (SCRAM) before upgrading the deployment to MongoDB 4.0.

Deprecation of MMAPv1

Starting from MongoDB 4.0, MongoDB deprecates the MMAPv1 storage engine. You must migrate your instance to the WiredTiger storage engine.

Limits on X.509 authentication certificates

Starting from MongoDB 4.0, if you specify --sslAllowInvalidCertificates or net.ssl.allowInvalidCertificates: true (renamed to --tlsAllowInvalidateCertificates or net.tls.allowInvalidCertificates: true in MongoDB 4.2) when using x.509 authentication, an invalid certificate is sufficient only to establish a TLS/SSL connection but is insufficient for authentication.

To resolve this issue, update an invalid certificate to a valid certificate (such as one signed by a trusted CA) or use net.ssl.CAFile to specify a custom CA.

Replica set instances

cfg = rs.conf();
cfg.protocolVersion=1;
rs.reconfig(cfg);

Sharded cluster instances

mongos uses the "majority" write concern level in the following operations that affect sharded cluster metadata.

MongoDB 4.0 feature compatibility

Some features in MongoDB 4.0 require V4.0 binary files and featureCompatibilityVersion set to 4.0. The following features are included:

• SCRAM-SHA-256

• New type conversion operators (including $toBool and $toInt) and enhancements

• Multi-document transactions

• $dateToString option changes

• New change stream methods

• Change stream resume token data type changes

Other changes

• Sharded cluster instances support geospatial query operators $near and $nearSphere.

• For the create command and the mongo shell method db.createCollection(), when creating collections in databases other than the local database, you cannot set the autoIndexId option to false.

• If you enable authentication and run the listDatabases command without the listDatabases operation permissions, MongoDB returns a list of all database on which you have the find operation permissions. Prior to MongoDB 4.0, If you run the command without the listDatabases operation permissions, MongoDB returns Unauthorized.

• The default value of taskExecutorPoolSize is changed from 0 to 1. On Linux, to restore the previous behavior of a V4.0 deployment, set taskExecutorPoolSize to 0 and AsyncRequestsSenderUseBaton to false.

• MongoDB 4.0 does not allow you to set transportLayer and net.transportLayer to legacy for mongod and mongos instances. transportLayer is set to asio by default and cannot be changed.

• Starting from MongoDB 4.0, the reIndex command and its db.collection.reIndex() take a global exclusive lock and block other operations until completion.

• If the values of fields other than year, isoYear, and timezone exceed their valid ranges, $dateFromParts subtracts the difference from other date parts to calculate the date. Prior to MongoDB 4.0, values that exceed the valid ranges cause an error.

• The behavior of a killCursors operation is changed. Prior to MongoDB 4.0, if obtaining a cursor ID, you can kill any cursor. Starting from MongoDB 4.0, killCursors allows you to kill any existing cursor. If you do not have permissions to kill a cursor, killCursors returns an error.

• MongoDB 4.0 adds a killAnyCursor operation that grants the permissions to kill any cursor for a specified collection.

• Starting from MongoDB 4.0, when you attempt to connect a mongos instance to another instance with a higher feature compatibility version (fCV), the mongos instance may crash. For example, a mongos instance of MongoDB 4.0 cannot connect to a sharded cluster instance with fCV set to 4.2. However, if the fCV of the sharded cluster instance remains 4.0, the mongos instance can normally connect to another instance.

• Starting from MongoDB 4.0, MongoDB resolves localhost IP addresses based on specified settings, rather than assuming 127.0.0.1.

Disabled TLS 1.0

MongoDB binary files (mongod, mongos, and mongo) disable TLS 1.0 encryption by default for systems that support TLS 1.1+.

To forcibly enable TLS 1.0:

• For mongod instances, specify net.ssl.disabledProtocols: none in the configuration file, or use the command line parameter --sslDisabledProtocols none.

• For mongos instances, use net.ssl.disabledProtocols: none or --sslDisabledProtocols none.

• For the mongo shell, add the --sslDisabledProtocols none parameter.

  The --sslDisabledProtocols parameter can be used in the following mongo shell versions:

  • MongoDB 4.0+

  • MongoDB 3.6.5+

  • MongoDB 3.4.15+

On macOS, if using the mongo shell of MongoDB 3.6.4 or earlier to connect to a sharded cluster instance of MongoDB 4.0+, you must explicitly enable TLS 1.0.

Mongo shell

db.runCommand( { listCollections: 1.0, authorizedCollections: true, nameOnly: true } )

db.runCommand( { listCollections: 1.0, authorizedCollections: true, nameOnly: true } )

Removal of binary files and deprecated fields or commands

mongodump --archive --db=test | mongorestore --archive  --nsFrom='test.*' --nsTo='examples.*'