code: def get(self):
      
      # Get all the values for parameter "cluster".
      clusters = self.get_arguments(constants.PARAM_CLUSTER)
  
      # Get all the values for parameter "environ".
      environs = self.get_arguments(constants.PARAM_ENVIRON)
  
      role = self.get_argument_role()
  
      ret = {}
      topologies = self.tracker.topologies
      for topology in topologies:
        cluster = topology.cluster
        environ = topology.environ
        if not cluster or not environ:
          continue
  
        # This cluster is not asked for.
        # Note that "if not clusters", then
        # we show for all the clusters.
        if clusters and cluster not in clusters:
          continue
  
        # This environ is not asked for.
        # Note that "if not environs", then
        # we show for all the environs.
        if environs and environ not in environs:
          continue
  
        if cluster not in ret:
          ret[cluster] = {}
        if environ not in ret[cluster]:
          ret[cluster][environ] = {}
        try:
          topology_info = self.tracker.getTopologyInfo(topology.name, cluster, role, environ)
          if topology_info and "execution_state" in topology_info:
            ret[cluster][environ][topology.name] = topology_info["execution_state"]
        except Exception:
          # Do nothing
          pass
      self.write_success_response(ret)
docs: get method