A database specialist wants to perform online data migration from on-premises MongoDB cluster to Amazon DocumentDB to minimize source database downtime. AWS DMS is being used to perform data migration. According to migration best practices, what should the specialist do before beginning a full load of migration data?

Answer options:

A.Ensure that DMS is using the primary node of the source cluster.
B.Turn off CDC on the source database.
C.Set oplog to 0 on the source database.
D.Create indexes on the target database.

Answer: D
Option A is incorrect because data migration can be performed from the primary or secondary node of the MongoDB replica set. Using the secondary node of the source replica set for the read operations is recommended as it can result in reducing the overall load on the primary instance.
Option B is incorrect because CDC is required in online data migration. AWS DMS uses CDC to replicate changes to target DocumentDB.Option C is incorrect because AWS DMS uses the source MongoDB oplog to perform data migration. During online data migration, the oplog on each replica set should be large enough to contain all changes made during the entire duration of the data migration process.
Option D is CORRECT because AWS DMS does not migrate indexes. For large data migrations, it is most efficient to pre-create indexes in the target Amazon DocumentDB cluster before migrating the data.
Reference:
https://aws.amazon.com/blogs/database/migrating-to-amazon-documentdb-with-the-online-method/