@Nullable
    public static TNetworkAddress getBackendHost(ImmutableMap<Long, ComputeNode> backendMap,
                                                 Reference<Long> backendIdRef) {
        ComputeNode node = getBackend(backendMap);
        if (node != null) {
            backendIdRef.setRef(node.getId());
            return new TNetworkAddress(node.getHost(), node.getBePort());
        }
        return null;
    }
@Test
    public void testEmptyBackendList() throws InterruptedException {
        Reference<Long> idRef = new Reference<>();
        TNetworkAddress address = SimpleScheduler.getBackendHost(null, idRef);
        Assert.assertNull(address);

        ImmutableMap.Builder<Long, ComputeNode> builder = ImmutableMap.builder();
        address = SimpleScheduler.getBackendHost(builder.build(), idRef);
        Assert.assertNull(address);
    }