public static Set<String> getClustersByServiceName(String serviceName) {
        return serviceNameMapping.getOrDefault(serviceName, Collections.EMPTY_SET);
    }
@Test
    public void testGetClustersByServiceName() {
        Map<String, Set<String>> mapping = new HashMap<>();
        Set<String> clusters = new HashSet<>();
        clusters.add("cluster");
        mapping.put("serviceA", clusters);
        Set<String> result;

        // serviceNameMapping is empty
        XdsDataCache.updateServiceNameMapping(new HashMap<>());
        result = XdsDataCache.getClustersByServiceName("serviceA");
        Assert.assertNotNull(result);
        Assert.assertEquals(0, result.size());

        // serviceNameMapping is not empty, get un cached service
        XdsDataCache.updateServiceNameMapping(mapping);
        result = XdsDataCache.getClustersByServiceName("serviceB");
        Assert.assertNotNull(result);
        Assert.assertEquals(0, result.size());

        // serviceNameMapping is not null, get cached service
        XdsDataCache.updateServiceNameMapping(mapping);
        result = XdsDataCache.getClustersByServiceName("serviceA");
        Assert.assertNotNull(result);
        Assert.assertEquals(1, result.size());
        Assert.assertTrue(result.contains("cluster"));
    }