Transition from Embedded to Dedicated Config Server

Ensure the balancer is enabled.

To migrate data from the config shard, the balancer process must be enabled. To check the balancer state, use the sh.getBalancerState() method:

sh.getBalancerState()

If the operation returns true, the balancer is enabled.

If the operation returns false, see Enable the Balancer.

Verify the config server is acting as a shard.

Run listShards to confirm that the config shard appears in the shard list:

db.adminCommand( { listShards: 1 } )

In the output, the shards._id field contains the shard names. The config shard typically has an _id of "config":

{
  shards: [
    {
      _id: 'config',
      ...
    },
    ...
  ],
  ok: 1
  ...
}

Start migrating sharded data off the config shard.

From the admin database, run the transitionToDedicatedConfigServer command:

use admin
db.adminCommand( { transitionToDedicatedConfigServer: 1 } )

The config shard enters the draining state and the balancer begins migrating chunks from the config shard to the remaining shards in the cluster. Depending on your network capacity and the amount of data, this operation can take anywhere from minutes to days to complete.

Move unsharded collections off the config shard.

Use the $listClusterCatalog aggregation stage to identify unsharded collections that still reside on the config shard:

use admin
db.aggregate([
  { $listClusterCatalog: { shards: true } },
  {
    $match: {
      sharded: false,
      shards: "config",
      type: { $nin: ["timeseries", "view"] },
      ns: { $not: { $regex: "^enxcol_\\..*(\\.esc|\\.ecc|\\.ecoc|\\.ecoc\\.compact)$" } },
      $or: [
        { ns: { $not: { $regex: "\\.system\\." } } },
        { ns: { $regex: "\\.system\\.buckets\\." } }
      ],
      db: { $nin: ["config", "admin"] }
    }
  },
  { $project: { _id: 0, ns: 1 } }
])

For each namespace in the output, use sh.moveCollection() to move the unsharded collection from the config shard to a recipient shard:

sh.moveCollection(
  "<database>.<collection>",
  "<ID of recipient shard>"
)

Repeat this step until no unsharded collections remain on the config shard.

Change the primary shard for databases that use the config shard.

From the admin database, run db.printShardingStatus():

use admin
db.printShardingStatus()

In the databases section of the output, check each database's primary field. For any application databases (databases other than config and admin) whose primary is the config shard, change the primary shard to another shard.

To change a database's primary shard, run movePrimary:

db.adminCommand({
  movePrimary: "<dbName>",
  to: "<recipientShard>"
})

Any collections that were not moved in the previous step are unavailable while movePrimary runs.

Check transition status.

To check the progress of the transition, re-run transitionToDedicatedConfigServer from the admin database:

use admin
db.adminCommand( { transitionToDedicatedConfigServer: 1 } )

Continue checking the status until the transition completes successfully, and the output resembles the following example:

{
  state: 'completed',
  msg: 'removeshard completed successfully',
  shard: 'config',
  ok: 1
}

Commit the transition to a dedicated config server.

After the config shard reports a state of 'completed', commit the transition from an embedded config server to a dedicated config server:

use admin
db.adminCommand( { commitTransitionToDedicatedConfigServer: 1 } )

A successful commit returns the following output:

{
   ok: 1,
   '$clusterTime': {
      ...
   },
   operationTime: ...
}

When successful, MongoDB removes the config shard from the cluster metadata and finalizes the transition to a dedicated config server. If the config shard is not completely drained, the command fails and you must continue checking the transition status for { state: 'completed' } before retrying.

After the transition is committed, listShards no longer includes the config shard in the shard list. The cluster now uses a dedicated config server and the config server no longer stores application data as a shard.