Reshard a Collection

Requirements

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 must perform one of these tasks:

  • rewrite your application's queries to use both the current shard key and the new shard key

  • stop your application and then:

    • rewrite your application's queries to use the new shard key

    • wait until the resharding of the collection completes (to monitor the resharding process, use a $currentOp pipeline stage)

    • deploy your rewritten application

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

  • deleteOne()

  • findAndModify()

  • findOneAndDelete()

  • findOneAndReplace()

  • findOneAndUpdate()

  • replaceOne()

  • updateOne()

  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>
  }

Limitations

• Only one collection can be resharded at a time.

• writeConcernMajorityJournalDefault must be true.

• Resharding a collection that has a uniqueness constraint is not supported.

• The new shard key cannot have a uniqueness constraint.

• The following commands and corresponding shell methods are not supported on the collection that is being resharded while the resharding operation is in progress:

  • collMod

  • convertToCapped

  • createIndexes

  • createIndex()

  • drop

  • drop()

  • dropIndexes

  • dropIndex()

  • renameCollection

  • renameCollection()

• The following commands and methods are not supported on the cluster while the resharding operation is in progress:

  • addShard

  • removeShard

  • db.createCollection()

  • dropDatabase

  Warning

  Using any of the preceding commands during a resharding operation causes the resharding operation to fail.

• If the collection to be resharded 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't reshard a sharded time series collection.

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.

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

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

[
  {
    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>,
    ...
  },
  ...
]

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

sh.status()

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

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

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

Behavior

Error Case