Compatibility Changes in MongoDB 4.0

Removal of authSchemaUpgrade Command

MongoDB 4.0 removes the authSchemaUpgrade command . The command, available in MongoDB 3.0 through MongoDB 3.6, supports the upgrade process for systems with MONGODB-CR users to SCRAM users.

If your deployment has user credentials stored in MONGODB-CR schema, you must upgrade to Salted Challenge Response Authentication Mechanism (SCRAM) before you upgrade to version 4.0.

Remove MONGODB-CR Support from db.copyDatabase() and copydb

The method db.copyDatabase() cannot copy from a mongod instance that enforces MONGODB-CR authentication.

The command copydb cannot copy from a mongod instance that enforces MONGODB-CR authentication. In conjunction with this change, MongoDB 4.0 removes the copydbgetnonce command.

Remove pv0 for Replica Sets

Starting in version 4.0, MongoDB removes the deprecated replica set protocol version 0 pv0.

Before upgrading to MongoDB 4.0, you must upgrade to pv1.

To upgrade to pv1, connect a mongo shell to the replica set primary and perform the following sequence of operations:

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

To reduce the likelihood of w:1 rollbacks, you can also reconfigure the replica set to a higher settings.catchUpTimeoutMillis setting.

For more information on pv1, see Replica Set Protocol Version.

Remove Master-Slave Replication

MongoDB 4.0 removes support for the deprecated master-slave replication. Before you can upgrade to MongoDB 4.0, if your deployment uses master-slave replication, you must upgrade to a replica set.

To convert from master-slave replication to a replica set, see Convert a Master-Slave Deployment to a Replica Set.

Journaling and Replica Sets

Starting in MongoDB 4.0, you cannot specify --nojournal option or storage.journal.enabled: false for replica set members that use the WiredTiger storage engine.

Index Builds and Replica Sets

You cannot specify --noIndexBuildRetry or storage.indexBuildRetry with --replSet or replication.replSetName. That is, you cannot use --noIndexBuildRetry or storage.indexBuildRetry for a mongod instance that is part of a replica set.

Rollback Limit

MongoDB 4.0 removes the limit on the amount of data that can be rolled back. In previous versions, a mongod instance will not roll back more than 300 megabytes of data and requires manual intervention if more than 300 megabytes of data need to be rolled back.

Starting in MongoDB 4.0, the rollback time limit defaults to 1 day and is configurable using the new parameter rollbackTimeLimitSecs. In earlier versions, the rollback time limit is not configurable and is set to 30 minutes.

cursor.min() and cursor.max()

If you use max() with min() to specify a range, the bound specified by max() must be greater than the bound specified by min().

In previous versions, the bounds could be equal but would scan no index entries, always resulting in an empty result set.

show collections

The mongosh method show collections is similar to:

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

• For users with the required access, show collections lists the non-system collections for the database.

• For users without the required access, show collections lists only the collections for which the users has privileges.

When a version 4.0 mongo shell is connected to an earlier version MongoDB deployment that does not support authorizedCollections and nameOnly options,

• A user must have the required access to run listCollections.

• If a user does not have required access and runs show collections, MongoDB uses the authenticatedUserPrivileges field returned by connectionStatus to return an approximate list of collections for the user.

db.getCollectionNames()

The db.getCollectionNames() is equivalent to:

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

• For users with the required access (privileges that grant listCollections action on the database), the method lists the names of all collections for the database.

• For users without the required access, the method lists only the collections for which the users has privileges. For example, if a user has find on a specific collection in a database, the method would return just that collection.

mongoperf

MongoDB 4.0 removes the mongoperf binary.

copydb and clone Commands

MongoDB 4.0 deprecates the copydb and the clone commands and their mongo shell helpers db.copyDatabase() and db.cloneDatabase().

As alternatives, users can use mongodump and mongorestore (with the mongorestore options --nsFrom and --nsTo) or write a script using the drivers.

For example, to copy the test database from a local instance running on the default port 27017 to the examples database on the same instance, you can:

1. Use mongodump to dump the test database to an archive mongodump-test-db:

   mongodump --archive="mongodump-test-db" --db=test

2. Use mongorestore with --nsFrom and --nsTo to restore (with database name change) from the archive:

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

Alternatively, instead of using an archive file, you can mongodump the test database to the standard output stream and pipe into mongorestore:

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

Parameters

• MongoDB removes the obsolete logUserIds parameter. Use Auditing instead.

$isolated Operator

MongoDB drops support for the $isolated operator. If you have an existing partial index that includes the $isolated operator or a view that includes a $isolated operator, recreate the index or view without the operator in the definition before upgrading.

Instead of the $isolated operator, use transactions instead.

geoNear Command

MongoDB 4.0 deprecates the geoNear command. Use one of the following operations instead.

• $geoNear aggregation stage.

• $near query operator.

• $nearSphere query operator.

maxScan Option

MongoDB deprecates the option maxScan for the find command and the mongo shell helper cursor.maxScan(). Instead, use maxTimeMS option or the helper cursor.maxTimeMS().

Output Field Changes

• The following fields returned from replSetGetStatus are deprecated:

  • replSetGetStatus.syncingTo

  • replSetGetStatus.members[n].syncingTo

  Use replSetGetStatus.syncSourceHost and replSetGetStatus.members[n].syncSourceHost instead.

• The $currentOp aggregation stage, the currentOp command, and db.currentOp() helper no longer return the threadId field in their outputs.

• The serverStatus command now always returns 0 for the asserts.warning field.