docs: @Override
      public ServiceInstance vote() {
          List<ModelServiceInstance> instances = getServiceInstanceList();
          if(instances == null || instances.isEmpty()){
              return null;
          }
          int i = index.getAndIncrement();
          int pos = i % instances.size();
          ModelServiceInstance instance = instances.get(pos);
          return ServiceInstanceUtils.transferFromModelServiceInstance(instance);
      }