shardCollection¶

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Definition
Considerations
Example

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:

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{
   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. 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 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.

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.

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.

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.

Example¶

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

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