Sharding operates on the collection level. You can shard multiple collections within a database or have multiple databases with sharding enabled. [2] However, in production deployments, some databases and collections will use sharding, while other databases and collections will only reside on a single shard.
Sharding is the only solution for some classes of deployments. Use sharded clusters if:
New in version 2.4.
Hashed shard keys use a hashed index of a single field as the shard key to partition data across your sharded cluster.
Consider the use of a phone-number field as a shard key:
Phone number has a high cardinality, because users will generally have a unique value for this field, MongoDB will be able to split as many chunks as needed.
An easily divisible shard key makes it easy for MongoDB to distribute content among the shards. Shard keys that have a limited number of possible values can result in chunks that are “unsplittable.”