Convert Replica Set to an Embedded Config Shard

Put the replica set in maintenance mode.

Perform a rolling restart on the replica set. Start each node as a config server in maintenance mode:

1. Stop each node, beginning with the secondaries. To shut down a server, use the db.shutdownServer() method.

   db.shutdownServer()

2. Restart the node as a config shard in maintenance mode.

   mongod --config /etc/mongodb.conf --configsvr \
      --replicaSetConfigShardMaintenanceMode \

   --configsvr

   Starts the mongod instance as a config server.

   --replicaSetConfigShardMaintenanceMode

   Enables the config shard maintenance mode, which disables some startup checks and allows you to convert a server into an embedded config shard.

3. Wait for the node to rejoin the replica set before restarting the next.

Find the primary node.

Use the rs.status() method to identify the new primary node. If you have a large replica set, use the db.aggregate() method to narrow your search.

db.aggregate( [
   { $documents: rs.status().members },
   { $match: { stateStr: "PRIMARY" } },
   { $project: { _id: 1, name: 1 }
] )

[ { _id: 3, name: "192.0.2.3:27017" } ]

Convert the primary into a config shard.

Connect to the primary node and reconfigure it to operate as an embedded config shard:

1. Use the rs.conf() method to get the current configuration and store it in a variable:

   var conf = rs.conf()

2. Set the configsvr field:

   conf.configsvr = true

3. Increment the version field:

   conf.version += 1

4. Reconfigure the node:

   rs.reconfig(conf)

5. Wait for secondaries to replicate the change. You can check the status by running an aggregation pipeline on the member documents:

   db.aggregate( [
      { $documents: rs.status().members },
      { $group: {
         _id: null,
         allConfigSvr: {
            $min: { $eq: ["$configsvr", true] }
         }
      } }
   ] )

   HIDE OUTPUT

   { _id: null, allConfigSvr: true }

   When allConfigSvr shows true, it indicates that the reconfiguation has propagated to all nodes in the cluster.

Restart the cluster.

Perform a rolling upgrade to restart the replica set as a config server:

1. Stop each node, beginning with the secondaries. To shut down a server, use the db.shutdownServer() method.

   db.shutdownServer()

2. Restart the node as a config server, no longer in maintenance mode:

   mongod --config /etc/mongodb.conf --configsvr

Start the router.

Start the mongos router:

mongos --config /etc/mongodb.conf

Transition the config server to an embedded config shard.

Connect to mongos and run the transitionFromDedicatedConfigServer command to add the config shard to the cluster.

db.adminCommand( { transitionFromDedidcatedConfigServer: 1 } )

Point application at router.

Update your application to connect to the mongos router instead of the replica set members.