docs: def healthy_services(self, role=None):
          '''
          Look up healthy services in the registry.
  
          A service is considered healthy if its 'last_heartbeat' was less than
          'ttl' seconds ago
  
          Args:
              role (str, optional): role name
  
          Returns:
              If `role` is supplied, returns list of healthy services for the
              given role, otherwise returns list of all healthy services. May
              return an empty list.
          '''
          try:
              query = self.rr.table(self.table)
              if role:
                  query = query.get_all(role, index='role')
              query = query.filter(
                  lambda svc: r.now().sub(svc["last_heartbeat"]) < svc["ttl"]   #.default(20.0)
              ).order_by("load")
              result = query.run()
              return result
          except r.ReqlNonExistenceError:
              return []