The _shardingOnTransitionToPrimaryHook() and  _shardingOnStepDownHook() private methods of ReplicationCoordinatorExternalStateImpl contain logic to initialize/start/enable a series of node subsystems supporting sharded cluster topologies upon node transition from/to primary role. This choice breaks the separation of concerns between replication and sharding subsystem for no real advantage, since the methods share no state with their parent class.

On the other hand, the related callback methods of the ShardingInitializationMongoD service represent a more natural fit.