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Sharding

shardCollection

On this page

  • Definition
  • Compatibility
  • Syntax
  • Command Fields
  • 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.

Tip

In mongosh, this command can also be run through the sh.shardCollection() helper method.

Helper methods are convenient for mongosh users, but they may not return the same level of information as database commands. In cases where the convenience is not needed or the additional return fields are required, use the database command.

Compatibility

This command is available in deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Important

This command is not supported in serverless instances. For more information, see Unsupported Commands.

Syntax

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

The command takes the following form:

{
    shardCollection: "<database>.<collection>",
    key: { <field1>: <1|"hashed">, ... },
    unique: <boolean>,
    numInitialChunks: <integer>,
    presplitHashedZones: <boolean>,
    collation: { locale: "simple" }
}

Command Fields

The command takes 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

Specify true to ensure that the underlying index enforces a unique constraint. Defaults to false.

You cannot specify true when using hashed shard keys.

numInitialChunks
integer

Specifies the initial number of chunks to create across all shards in the cluster when sharding an empty collection with a hashed shard key. MongoDB will then create and balance chunks across the cluster. The numInitialChunks must result in less than 8192 per shard.

If the collection is not empty or the shard key does not contain a hashed field, the operation returns an error.

  • If sharding with presplitHashedZones: true, MongoDB attempts to evenly distribute the specified number of chunks across the zones in the cluster.

  • If sharding with presplitHashedZones: false or omitted and no zones and zone ranges are defined for the empty collection, MongoDB attempts to evenly distributed the specified number of chunks across the shards in the cluster.

  • If sharding with presplitHashedZones: false or omitted and zones and zone ranges have been defined for the empty collection, numInitChunks has no effect.

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.
boolean

Optional. Specify true to perform initial chunk creation and distribution for an empty or non-existing collection based on the defined zones and zone ranges for the collection. For hashed sharding only.

shardCollection with presplitHashedZones: true returns an error if any of the following are true:

Considerations

Shard Keys

While you can change your shard key later, it is important to carefully consider your shard key choice to avoid scalability and perfomance issues.

Tip

See also:

Hashed Shard Keys

Hashed shard keys use a hashed index or a compound hashed index as the shard key.

Use the form field: "hashed" to specify a hashed shard key field.

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.

Tip

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.

See Pre-Define Zones and Zone Ranges for an Empty or Non-Existing Collection for an example. If sharding a collection using a ranged or single-field hashed shard key, the numInitialChunks option has no effect if zones and zone ranges have been defined for the empty collection.

To shard a collection using a compound hashed index, see Zone Sharding and Compound Hashed Indexes.

Zone Sharding and Compound Hashed Indexes

MongoDB supports sharding collections on compound hashed indexes. When sharding an empty or non-existing collection using a compound hashed shard key, additional requirements apply in order for MongoDB to perform initial chunk creation and distribution.

The numInitialChunks option has no effect if zones and zone ranges have been defined for the empty collection and presplitHashedZones is false.

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

Tip

See also:

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 } } )

Tip

See also:

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