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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: { <field1>: <1|"hashed">, ... },
   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 document that specifies the field or fields to use as the shard key.

{ <field1>: <1|"hashed">, ... }

Set the field values to either:

shard key must be supported by an index. 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. Defaults to false.
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.

Changed in version 4.0.3: The option has no effect if zones and zone ranges have been defined for the empty collection. See Zone Sharding and Initial Chunk Distribution.

Changed in version 3.4: If the collection is not empty or the shard key is not a hashed key, the operation returns an error.

collation document Optional. If the collection specified to shardCollection has a default collation, you must include a collation document with { 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.

Once a collection has been sharded, MongoDB provides no method to unshard a sharded collection.

Shard Keys

Choosing the best shard key to effectively distribute load among your shards requires some planning.

  • Starting in MongoDB 4.4, you can refine a collection’s shard key by adding a suffix field or fields to the existing key.
  • Starting in MongoDB 4.2, you can update a document’s shard key value (unless the shard key field is the immutable _id field).

For more information, see Shard Keys.

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

Zone Sharding and Initial Chunk Distribution

The shard collection operation (i.e. shardCollection command and the sh.shardCollection() helper) can perform initial chunk creation and distribution for an empty or a non-existing collection if zones and zone ranges have been defined for the collection. Initial chunk distribution allows for a faster setup of zoned sharding. After the initial distribution, the balancer manages the chunk distribution going forward per usual.

If the numInitialChunks option has no effect if zones and zone ranges have been defined for the empty collection.

See Pre-Define Zones and Zone Ranges for an Empty or Non-Existing Collection for an example.

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 exist.
  • 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, the shardCollection command must include a collation parameter with the value { locale: "simple" }. For non-empty collections with a default 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.

Write Concern

mongos uses "majority" for the write concern of the shardCollection command and its helper sh.shardCollection().

Example

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

db.adminCommand( { shardCollection: "records.people", key: { zipcode: 1 } } )