public void processOrder(String productId, long quantity) {
    Product productById = productRepository.getProductById(productId);

    if (productById.getUnitsInStock() < quantity) {
      throw new IllegalArgumentException(
          "Out of stock. Available units in stock" + productById.getUnitsInStock());
    }

    productById.setUnitsInStock(productById.getUnitsInStock() - quantity);
  }