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balancerCollectionStatus

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  • Definition
  • Syntax
  • Access Control
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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 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().

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.

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
"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.
"defragmentingChunks"
The queried namespace is currently going through the chunk defragmentation process. Defragmentation can be triggered by the configureCollectionBalancing command.
"draining"
A remove shard operation is in progress and MongoDB must drain chunks off the removed shard to other shard(s).
"zoneViolation"
Chunks violate the defined zone ranges for a shard.

Note

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.

"details"

An object containing information on the ongoing defragmentation process. This object indicates the current phase of the defragmentation and how many chunks are left to process in that phase. For example output, see Ongoing Defragmentation Process.

This field is only returned when firstComplianceViolation is defragmentingChunks.

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.

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)
}
}
}

If the queried namespace is going through chunk defragmentation, the balancerCollectionStatus command returns output similar to the following:

{
"balancerCompliant": false,
"firstComplianceViolation": "defragmentingChunks",
"details": {
"currentPhase": "moveAndMergeChunks",
"progress": { "remainingChunksToProcess": 1 }
}
}

Note

Chunk defragmentation occurs in multiple phases. The progress field only pertains to the current phase.

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