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movePrimary

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  • Considerations
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movePrimary

In a sharded cluster, movePrimary reassigns the primary shard which holds all un-sharded collections in the database. movePrimary first changes the primary shard in the cluster metadata, and then migrates all un-sharded collections to the specified shard. Use the command with the following form:

db.adminCommand( { movePrimary: <databaseName>, to: <newPrimaryShard> } )

For example, the following command moves the primary shard from test to shard0001:

db.adminCommand( { movePrimary : "test", to : "shard0001" } )

When the command returns, the database's primary shard location has switched to the specified shard. To fully decommission a shard, use the removeShard command.

movePrimary is an administrative command that is only available for mongos instances.

Warning

After starting movePrimary, do not perform any read or write operations against any unsharded collection in that database until the command completes. Read or write operations issued against those collections during the migration can result in unexpected behavior, including potential failure of the migration operation or loss of data.

Considerations

mongos uses "majority" write concern for movePrimary.

Issue movePrimary During A Maintenance Window

movePrimary may require a significant time to complete depending on the size of the database and factors such as network health or machine resources. Read or write operations issued against the database during the migration can result in unexpected behavior, including potential failure of the migration operation or loss of data.

Consider scheduling a maintenance window during which applications stop all reads and writes to the cluster. Issuing movePrimary during planned downtime mitigates the risk of encountering undefined behavior due to interleaving reads or writes to the unsharded collections in the database.

Using movePrimary To Move Unsharded Collections

For MongoDB 4.2 and previous, if using the movePrimary command on a database that contains an unsharded collection, you must perform the following additional steps.

Note

MongoDB 4.4 does not require these additional steps when moving databases that contain unsharded collections.

  • For MongoDB 4.2, you must either:

    • Restart all mongos instances and all mongod shard members (including the secondary members);

    • Use the flushRouterConfig command on all mongos instances and all mongod shard members (including the secondary members) before reading or writing any data to any unsharded collections that were moved.

  • For MongoDB 4.0 and earlier, you must either:

    • Restart all mongos instances;

    • Use the flushRouterConfig command on all mongos instances before reading or writing any data to any unsharded collections that were moved.

These steps ensure that all cluster nodes refresh their metadata cache, which includes the location of the primary shard. Otherwise, you may miss data on reads, and may not write data to the correct shard. To recover, you must manually intervene.

Namespace Conflicts Prevent Migration

movePrimary fails if the destination shard contains a conflicting collection namespace. For example:

  1. An administrator issues movePrimary to change the primary shard for the hr database.

  2. A user or application issues a write operation against an unsharded collection in hr while movePrimary is moving that collection. The write operation creates the collection in the original primary shard.

  3. An administrator later issues movePrimary to restore the original primary shard for the hr database.

  4. movePrimary fails due to the conflicting namespace left behind from the interleaving write operation.

Destination Shard Must Rebuild Indexes

As part of the movePrimary operation, the destination shard must rebuild indexes on the migrated collections after becoming the primary shard. This may require a significant amount of time depending on the number of indexes per collection and the amount of data to index.

See Index Builds on Populated Collections for more information on the index build process.

Additional Information

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

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