public Optional<Integer> loadInstanceWorkerId(final String instanceId) {
        try {
            String workerId = repository.query(ComputeNode.getInstanceWorkerIdNodePath(instanceId));
            return Strings.isNullOrEmpty(workerId) ? Optional.empty() : Optional.of(Integer.valueOf(workerId));
        } catch (final NumberFormatException ex) {
            log.error("Invalid worker id for instance: {}", instanceId);
        }
        return Optional.empty();
    }
@Test
    void assertLoadInstanceWorkerId() {
        InstanceMetaData instanceMetaData = new ProxyInstanceMetaData("foo_instance_id", 3307);
        final String instanceId = instanceMetaData.getId();
        new ComputeNodePersistService(repository).loadInstanceWorkerId(instanceId);
        verify(repository).query(ComputeNode.getInstanceWorkerIdNodePath(instanceId));
    }