You can migrate data from a user-created MongoDB database to an ApsaraDB for MongoDB instance. Please pay close attention to the limits of ApsaraDB for MongoDB.

Operation Limit
Deploy a replica set instance The database version must match the storage engine. For more information, see MongoDB versions and storage engines.
Build replica set nodes
Restart a replica set instance You must restart the instance in the ApsaraDB for MongoDB console or by calling the RestartDBInstance operation.
Migrate data from a replica set instance You can use the built-in commands of MongoDB or Data Transmission Service (DTS) to migrate data. For more information, see Migrate the replica set of a user-created MongoDB database to ApsaraDB for MongoDB by using DTS or Migrate user-created MongoDB databases to Alibaba Cloud by using the built-in commands of MongoDB.
Back up the data of a replica set instance Configure automatic backup for an ApsaraDB for MongoDB instance: Only physical backup is supported. Manually back up an ApsaraDB for MongoDB instance: Both physical backup and logical backup are supported.
Note If the database version of the instance is 3.2 or 3.4, the number of collections and indexes in the instance cannot exceed 10,000. Otherwise, physical backup may fail. If your business may exceed this limit, we recommend that you Upgrade the database version of an ApsaraDB for MongoDB instanceto 4.0 or 4.2, you can also select the database version 4.0 or 4.2 when creating the instance.
Restore the data of a replica set instance
Modify parameters of a replica set instance For security and stability, you are not allowed to modify certain parameters of a replica set instance. For more information, see Configure database parameters for an ApsaraDB for MongoDB instance.