code: def on_cluster_add(self, cluster):
          
          if cluster.discovery not in self.configurables[Discovery]:
              return
  
          discovery = self.configurables[Discovery][cluster.discovery]
  
          self.launch_thread(
              cluster.name, discovery.start_watching,
              cluster, self.sync_balancer_files
          )
docs: Once a cluster is added we tell its associated discovery method to
          start watching for changes to the cluster's child nodes (if the
          discovery method is configured and available).