docs: def on_cluster_add(self, cluster):
          """
          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).
          """
          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
          )