code: public static Maybe<Set<String>> keys(CacheConfigBean cacheConfigBean, String key) {
          return SingleRxXian
                  .call(CacheService.CACHE_SERVICE, "cacheMapKeys", new JSONObject() {{
                      put("cacheConfig", cacheConfigBean);
                      put("key", key);
                  }})
                  .flatMapMaybe(unitResponse -> {
                      unitResponse.throwExceptionIfNotSuccess();
                      if (unitResponse.getData() == null)
                          return Maybe.empty();
                      else
                          return Maybe.just(Reflection.toTypedSet(unitResponse.getData(), String.class));
                  });
      }
docs: retrial all the keys of the cached map
  
  @param cacheConfigBean datasource configuration
  @param key             cache key
  @return all the keys of the cached map, or null if the key does not exist