shardCollection

Definition

shardCollection

Shards a collection to distribute its documents across shards. You must run enableSharding on a database before running the shardCollection command. The shardCollection command must be run against the admin database.

To run shardCollection, use the db.runCommand( { <command> } ) method.

shardCollection has the following form:

{
   shardCollection: "<database>.<collection>",
   key: <shardkey>,
   unique: <boolean>,
   numInitialChunks: <integer>,
   collation: { locale: "simple" }
}

shardCollection has the following fields:

Field Type Description
shardCollection 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.
numInitialChunks integer Specifies the number of chunks to create initially when sharding an empty collection with a hashed shard key. MongoDB will then create and balance chunks across the cluster. The numInitialChunks must be less than 8192 per shard. If the collection is not empty, numInitialChunks has no effect.
collation document Optional. If the collection specified to shardCollection has a default collation, you must set shardCollection to { locale : "simple"}, or the shardCollection command fails. At least one of the indexes whose fields support the shard key pattern must have the simple collation.

Considerations

Use

You can only shard a collection once.

Do not run more than one shardCollection command on the same collection at the same time.

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.

Note

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

Uniqueness

If specifying unique: true:

  • If the collection is empty, 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 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

Collation

Changed in version 3.4.

If the collection has a default collation, your shardCollection command must include a collation parameter with the value { locale: "simple" }. For non-empty collections with a collation, you must have at least one index with the simple collation whose fields support the shard key pattern.

You do not need to specify the collation option for collections without a collation. If you do specify the collation option for a collection with no collation, it will have no effect.

Example

The following operation enables sharding for the people collection in the records database and uses the zipcode field as the shard key:

use admin
db.runCommand( { shardCollection: "records.people", key: { zipcode: 1 } } )

See also

Sharding