reshardCollection
Definition
reshardCollection
New in version 5.0.
The
reshardCollection
command changes the shard key for a collection and changes the distribution of your data.Tip
In
mongosh
, this command can also be run through thesh.reshardCollection()
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
Note
This command is supported in all MongoDB Atlas clusters. For information on all commands, see Unsupported Commands.
MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB
Syntax
The command has the following syntax:
{ reshardCollection: "<database>.<collection>", key: <shardkey>, unique: <boolean>, numInitialChunks: <integer>, collation: { locale: "simple" }, zones: [ { min: <document with same shape as shardkey>, max: <document with same shape as shardkey>, zone: <string> | null }, ... ] }
Command Fields
The command takes the following fields:
Field | Type | Description |
---|---|---|
reshardCollection | string | The namespace of the collection to be resharded. Takes
the form <database>.<collection> . |
key | document | The document that specifies the new field or fields to use as the shard key.
Set the field values to either:
|
unique | boolean | Optional. Specify whether there is a uniqueness constraint on the shard key. Only
false is supported. Defaults to false . |
numInitialChunks | integer | Optional. Specifies the initial number of chunks to create
across all shards in the cluster when resharding a collection.
The default is the number of chunks that exist for the
collection under the current shard key pattern. MongoDB will
then create and balance chunks across the cluster. The
numInitialChunks must result in less than 8192 per shard. |
collation | document | Optional. If the collection specified to reshardCollection
has a default collation,
you must include a collation document with
{ locale : "simple" } , or the reshardCollection
command fails. |
zones | array | Optional. To maintain or add zones,
specify the zones for your collection in an array. |
The mongosh
provides a wrapper method
sh.reshardCollection()
.
Resharding Process
In a collection resharding operation, a shard can be a:
donor, which currently stores chunks for the sharded collection.
recipient, which stores new chunks for the sharded collection based on the shard keys and zones.
A shard can be donor and a recipient at the same time. The set of donor shards is identical to the recipient shards, unless you use zones.
The config server primary is always the resharding coordinator and starts each phase of the resharding operation.
Initialization Phase
During the initialization phase, the resharding coordinator determines the new data distribution for the sharded collection.
Index Phase
During the index phase:
Each shard recipient creates a new, empty sharded collection with the same collection options as the existing sharded collection. This new sharded collection is the target for where recipient shards write the new data.
Each shard recipient builds the necessary new indexes. These include all existing indexes on the sharded collection and an index compatible with the new shard key pattern if such an index doesn’t already exist on the sharded collection.
Clone, Apply, and Catch-up Phase
During the clone, apply, and catch-up phase:
Each shard recipient clones an initial copy of the documents it would own under the new shard key.
Each shard recipient begins applying oplog entries from operations that happened after the recipient cloned the data.
When the estimate for the time remaining to complete the resharding operation is under two seconds, the resharding coordinator blocks writes for the collection.
Note
If desired, you can manually force the resharding operation to complete by issuing the
commitReshardCollection
command. This is useful if the current time estimate to complete the resharding operation is an acceptable duration for your collection to block writes. ThecommitReshardCollection
command blocks writes early and forces the resharding operation to complete. During the time period where writes are blocked your application experiences an increase in latency.
Commit Phase
Once the resharding process reaches the commit phase, it may no longer be aborted with
abortReshardCollection
.When all shards have reached strict consistency, the resharding coordinator commits the resharding operation and installs the new routing table.
The resharding coordinator instructs each donor and recipient shard primary, independently, to rename the temporary sharded collection. The temporary collection becomes the new resharded collection.
Each donor shard drops the old sharded collection.
Example
Reshard a Collection
The following example reshards the sales.orders
collection with the
new shard key { order_id: 1 }
:
db.adminCommand({ reshardCollection: "sales.orders", key: { order_id: 1 } })
MongoDB returns the following:
{ ok: 1, '$clusterTime': { clusterTime: Timestamp(1, 1624887954), signature: { hash: Binary(Buffer.from("0000000000000000000000000000000000000000", "hex"), 0), keyId: 0 } }, operationTime: Timestamp(1, 1624887947) }