is an ongoing task that keeps a live replica of its database's data
on another database.
The task keeps the two databases in sync by updating the destination
database with any deletion, addition, or modification made in the
origin database's data.
An external replication task can run on a non-sharded database,
and create a live replica of its data on a sharded database.
The destination (sharded) database will distribute incoming data
among its shards.
The sharded database can be kept and tested in a controlled environment
as long as necessary, as the original database keeps on serving its users.
When all is ready, the new database can be moved into production.
A live replica created using external replication includes all documents
and document extensions, but not server and cluster level features like
indexes and compare exchange entries. Find the full list here.