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sh.shardCollection(namespace, key, unique)

Shards a collection using the key as a the shard key. sh.shardCollection() takes the following arguments:

Parameter Type Description
namespace string The namespace of the collection to shard in the form <database>.<collection>.
key document

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.

See also Shard Key Indexes

unique boolean When true, the unique option ensures that the underlying index enforces a unique constraint. Hashed shard keys do not support unique constraints.


MongoDB provides no method to deactivate sharding for a collection after calling shardCollection. Additionally, after shardCollection, you cannot change shard keys or modify the value of any field used in your shard key index.

Shard Keys

Choosing the best shard key to effectively distribute load among your shards requires some planning. Review Shard Keys regarding choosing a shard key and restrictions.

Hashed Shard Keys

Hashed shard keys use a hashed index of a single field as the shard key.

Use the form {field: "hashed"} to specify a hashed shard key. Hashed shard keys may not be compound indexes.


If chunk migrations are in progress while creating a hashed shard key collection, the initial chunk distribution may be uneven until the balancer automatically balances the collection.

See also

Hashed Sharding


If specifying unique: true:

  • If the collection is empty, sh.shardCollection() creates the unique index on the shard key if such an index does not already exists.
  • If the collection is not empty, you must create the index first before using sh.shardCollection().

Although you can have a unique compound index where the shard key is a prefix, if using unique parameter, the collection must have a unique index that is on the shard key.

See also Sharded Collection and Unique Indexes


Given the people collection in the records database, the following command shards the collection by the zipcode field:

sh.shardCollection("records.people", { zipcode: 1} )