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balancerCollectionStatus

Definition

balancerCollectionStatus

New in version 4.4.

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.

The command takes the following form:

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

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

The mongo shell 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:

Value Description
"draining"

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

Note

If the "firstComplianceViolation" returns "draining", there may also be pending chunk migration due to "zoneViolation".

"zoneViolation"

Chunks violate the defined zone ranges for a shard.

Note

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

"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.

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)
      }
   }
}