public List<Node> getNodes(String clusterName, String group) {
        return clusterNodes.computeIfAbsent(clusterName, k -> new ConcurrentHashMap<>()).get(group);
    }
@Test
    public void testGetNodes() {
        Assertions.assertEquals(new ArrayList<>(), metadata.getNodes("cluster"));
        Assertions.assertNull(metadata.getNodes("cluster", "group"));
    }