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Reshard a Collection

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New in version 5.0.

The ideal shard key allows MongoDB to distribute documents evenly throughout the cluster while facilitating common query patterns. A suboptimal shard key can lead to performance or scaling issues due to uneven data distribution. Starting in MongoDB 5.0, you can change the shard key for a collection to change the distribution of your data across a cluster.

Note

Before resharding your collection, read Troubleshoot Shard Keys for information on common performance and scaling issues and advice on how to fix them.

Before you reshard your collection, ensure that you meet the following requirements:

  • Your application can tolerate a period of two seconds where the collection that is being resharded blocks writes. During the time period where writes are blocked your application experiences an increase in latency. If your workload cannot tolerate this requirement, consider refining your shard key instead.
  • Your database meets these resource requirements:

    • Available storage space: Ensure that your available storage space is at least 1.2x the size of the collection that you want to reshard. For example, if the size of the collection you want to reshard is 1 TB, you should have at least 1.2 TB of free storage when starting the sharding operation.
    • I/O: Ensure that your I/O capacity is below 50%.
    • CPU load: Ensure your CPU load is below 80%.
    Important

    These requirements are not enforced by the database. A failure to allocate enough resources can result in:

    • the database running out of space and shutting down
    • decreased performance
    • the resharding operation taking longer than expected

    If your application has time periods with less traffic, reshard your collection during that time if possible.

  • You have rewritten your application code to update your queries to use both the current shard key and the new shard key.

    The following queries return an error if the query filter does not include both the current shard key or a unique field (like _id):

    For optimal performance, we recommend that you also rewrite other queries to include the new shard key.

    Once the resharding operation completes, you can remove the old shard key from the queries.

  • No index builds are in progress. Use db.currentOp() to check for any running index builds:

    db.adminCommand(
    {
    currentOp: true,
    $or: [
    { op: "command", "command.createIndexes": { $exists: true } },
    { op: "none", "msg" : /^Index Build/ }
    ]
    }
    )

    In the result document, if the inprog field value is an empty array, there are no index builds in progress:

    {
    inprog: [],
    ok: 1,
    '$clusterTime': { ... },
    operationTime: <timestamp>
    }
Warning

We strongly recommend that you check the Limitations and read the resharding process section in full before resharding your collection.

1

While connected to the mongos, issue a reshardCollection command that specifies the collection to be resharded and the new shard key:

db.adminCommand({
reshardCollection: "<database>.<collection>",
key: <shardkey>
})

MongoDB sets the max number of seconds to block writes to two seconds and begins the resharding operation.

2

To monitor the resharding operation, you can use the $currentOp pipeline stage:

db.getSiblingDB("admin").aggregate([
{ $currentOp: { allUsers: true, localOps: false } },
{
$match: {
type: "op",
"originatingCommand.reshardCollection": "<database>.<collection>"
}
}
])
Note

To see updated values, you need to continuously run the preceeding pipeline.

The $currentOp pipeline outputs:

  • totalOperationTimeElapsedSecs: elapsed operation time in seconds
  • remainingOperationTimeEstimatedSecs: estimate of the remaining time to complete the resharding operation
[
{
shard: '<shard>',
type: 'op',
desc: 'ReshardingRecipientService | ReshardingDonorService | ReshardingCoordinatorService <reshardingUUID>',
op: 'command',
ns: '<database>.<collection>',
originatingCommand: {
reshardCollection: '<database>.<collection>',
key: <shardkey>,
unique: <boolean>,
collation: { locale: 'simple' }
},
totalOperationTimeElapsedSecs: <number>,
remainingOperationTimeEstimatedSecs: <number>,
...
},
...
]
3

Throughout the resharding process, the estimated time to complete the resharding operation (remainingOperationTimeEstimatedSecs) decreases. When the estimated time is below two seconds, MongoDB blocks writes and completes the resharding operation. Until the estimated time to complete the resharing operation is below two seconds, the resharding operation does not block writes by default. During the time period where writes are blocked your application experiences an increase in latency.

Once the resharding process has completed, the resharding command returns ok: 1.

{
ok: 1,
'$clusterTime': {
clusterTime: <timestamp>,
signature: {
hash: Binary(Buffer.from("0000000000000000000000000000000000000000", "hex"), 0),
keyId: <number>
}
},
operationTime: <timestamp>
}

To see whether the resharding operation completed successfully, check the output of the sh.status() method:

sh.status()

The sh.status() method output contains a subsection for the databases. If resharding has completed successfully, the output lists the new shard key for the collection:

databases
[
{
database: {
_id: '<database>',
primary: '<shard>',
partitioned: true,
version: {
uuid: <uuid>,
timestamp: <timestamp>,
lastMod: <number>
}
},
collections: {
'<database>.<collection>': {
shardKey: <shardkey>,
unique: <boolean>,
balancing: <boolean>,
chunks: [],
tags: []
}
}
}
...
]
Note

If the resharded collection uses Atlas Search, the search index will become unavailable when the resharding operation completes. You need to manually rebuild the search index once the resharding operation completes.

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. The commitReshardCollection command blocks writes early and forces the resharding operation to complete. The command has the following syntax:

db.adminCommand({
commitReshardCollection: "<database>.<collection>"
})

You can abort the resharding operation during any stage of the resharding operation, even after running the commitReshardCollection, until shards have fully caught up.

For example, if the write unavailability duration estimate does not decrease, you can abort the resharding operation with the abortReshardCollection command:

db.adminCommand({
abortReshardCollection: "<database>.<collection>"
})

After canceling the operation, you can retry the resharding operation during a time window with lower write volume. If this is not possible, add more shards before retrying.

The minimum duration of a resharding operation is always 5 minutes.

Retryable writes initiated before or during resharding can be retried during and after the collection has been resharded for up to 5 minutes. After 5 minutes you may be unable to find the definitive result of the write and subsequent attempts to retry the write fail with an IncompleteTransactionHistory error.

If a primary failover on a replica set shard or config server occurs, the resharding operation aborts.

If a resharding operation aborts due to a primary failover, run the cleanupReshardCollection command before starting a new resharding operation:

db.runCommand({
cleanupReshardCollection: "<database>.<collection>"
})

The resharding operation fails if _id values are not globally unique to avoid corrupting collection data. Duplicate _id values can also prevent successful chunk migration. If you have documents with duplicate _id values, copy the data from each into a new document, and then delete the duplicate documents.

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