code: private void doListNode(final Message<JsonObject> message) {
      context.execute(new Action<Collection<String>>() {
        @Override
        public Collection<String> perform() {
          List<String> nodes = new ArrayList<>();
          for (String group : groups.keySet()) {
            nodes.addAll(groups.get(group));
          }
          return nodes;
        }
      }, new Handler<AsyncResult<Collection<String>>>() {
        @Override
        public void handle(AsyncResult<Collection<String>> result) {
          if (result.failed()) {
            message.reply(new JsonObject().putString("status", "error").putString("message", result.cause().getMessage()));
          } else {
            message.reply(new JsonObject().putString("status", "ok").putArray("result", new JsonArray(result.result().toArray(new String[result.result().size()]))));
          }
        }
      });
    }
docs: Lists nodes in the cluster.