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Sharding

balancerCollectionStatus

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  • Definition
  • Compatibility
  • Syntax
  • Access Control
  • Output Document
  • Example

Definition

balancerCollectionStatus

Returns a document that contains information about whether the chunks of a sharded collection are balanced (i.e. do not need to be moved) as of the time the command is run or need to be moved because of draining shards, zone violation or imbalance of chunks across shards.

You can only issue the balancerCollectionStatus against the admin database.

Tip

In mongosh, this command can also be run through the sh.balancerCollectionStatus() 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

The command has the following syntax:

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

Specify the full namespace ("<db>.<collection>") of the sharded collection.

mongosh provides a wrapper method sh.balancerCollectionStatus().

Access Control

When running with access control, the user must have the enableSharding privilege actions on database and/or collection to run the command. That is, a user must have a role that grants the following privilege:

{ resource: { db: <database>, collection: <collection> }, actions: [ "enableSharding" ] }

The built-in clusterManager role provides the appropriate privileges.

Output Document

The following is an example of a document returned by the command:

{
   "balancerCompliant" : false,
   "firstComplianceViolation" : "chunksImbalance",
   "ok" : 1,
   "operationTime" : Timestamp(1583192967, 16),
   "$clusterTime" : {
      "clusterTime" : Timestamp(1583192967, 16),
      "signature" : {
         "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
         "keyId" : NumberLong(0)
      }
   }
}
Field
Description
"balancerCompliant"
A boolean that indicates whether the chunks do not need to be moved (true) or need to be moved (false).
"firstComplianceViolation"

A string that indicates the reason chunks for this namespace need to be moved. The field is only available if "balancerCompliant" is false.

Possible values are:

"chunksImbalance"
The difference in the number of chunks between the shard with the most chunks for the collection and the shard with the fewest chunks for the collection exceed the migration threshold.
"draining"

A remove shard operation is in progress and MongoDB must drain chunks off the removed shard to other shards.

If the "firstComplianceViolation" responds with "draining", there may also be pending chunk migrations due to "chunksImbalance".

"zoneViolation"

Chunks violate the defined zone ranges for a shard.

If the "firstComplianceViolation" responds with "zoneViolation", there may also be pending chunk migrations due to "chunksImbalance".

This field only returns information on the first violation observed by MongoDB. There may be additional pending chunk migrations due to a different reason than the one reported in firstComplianceViolation.

In addition to the command-specific return fields, the command also returns the ok status field, the operationTime field, and the $clusterTime field for the operation. For details on these fields, see Response.

Example

To check whether the chunks of a sharded collection test.contacts is currently in balance, connect to a mongos instance and issue the following command:

db.adminCommand( { balancerCollectionStatus: "test.contacts" } )

If the chunks for the collection do not need to be moved, the command returns an output similar to the following:

{
   "balancerCompliant" : true,
   "ok" : 1,
   "operationTime" : Timestamp(1583193238, 1),
   "$clusterTime" : {
      "clusterTime" : Timestamp(1583193238, 1),
      "signature" : {
         "hash" : BinData(0,"AAAAAAAAAAAAAAAAAAAAAAAAAAA="),
         "keyId" : NumberLong(0)
      }
   }
}

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