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In a sharded cluster, this command reassigns the database’s primary shard, which holds all un-sharded collections in the database. movePrimary is an administrative command that is only available for mongos instances. Only use movePrimary when removing a shard from a sharded cluster.

movePrimary changes the primary shard for a database in the cluster metadata, and migrates all un-sharded collections to the specified shard. Use the command with the following form:

{ movePrimary : "test", to : "shard0001" }

When the command returns, the database’s primary location will shift to the designated shard. To fully decommission a shard, use the removeShard command.



Only use movePrimary when:

  • the database does not contain any collections, or
  • you have drained all sharded collections using the removeShard command.


If you use the movePrimary command to move un-sharded collections, you must either restart all mongos instances, or use the flushRouterConfig command on all mongos instances before writing any data to the cluster. This action notifies the mongos of the new shard for the database.

If you do not update the mongos instances’ metadata cache after using movePrimary, the mongos may not write data to the correct shard, to recover you must manually intervene.

Additional Information

See Remove Shards from an Existing Sharded Cluster for a complete procedure.

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