The index specification document to use as the shard key. The shard key determines how MongoDB distributes the documents among the shards.
Unless the collection is empty, the index must exist prior to the shardCollection command. If the collection is empty, MongoDB creates the index prior to sharding the collection if the index that can support the shard key does not already exist.
db.runCommand({ addshard:"localhost:27020" }) # 添加分片服务器1
db.runCommand({ addshard:"localhost:27021" }) # 添加分片服务器2
db.runCommand({ enablesharding:"student" }) # 为了分片存储一个集合，必须先enablesharding所属的数据库。
db.runCommand({ shardcollection: "student.info", key: { userid:1}}) # 设定分片存储的集合，并提供一个 shard key
注意：各个分片服务器虽然构成了一个集群，但是每一个还可单独作为一个服务器被连接。
