A shard is a MongoDB instance that holds a subset of a collection’s
data. Each shard is either a single mongod instance or a
replica set. In production, all shards are replica
sets. For more information see Shards.
Each router is a mongos instance that routes the reads
and writes from applications to the shards. Applications do not
access the shards directly. For more information see
Sharded Cluster Query Routing.
Enable sharding in MongoDB on a per-collection basis. For each
collection you shard, you will specify a shard key for that