Can mongodb replica set be easily expanded to be fragmented without downtime?
The method of fragmentation uses a little more machines. Can you use one replica set, and then establish multiple replica sets when fragmentation is needed to improve performance, and then use routing to control the servers to form a fragmented cluster?
Official tutorials can be found here:Convert a Replica Set to a Replicated Sharded Cluster
The prerequisite for smooth migration is that config server is required and all requests go to mongos. In view of the fact that the subject already has a replica-set, the prerequisite is that it already has, so smooth expansion should be no problem.