Implementing Service Layer

public interface OrderService {
    Order place(PlaceOrderCommand cmd);
    Order get(OrderId id);
}

@Service
public class DefaultOrderService implements OrderService { ... }

@Service
public class CheckoutService {
    private final OrderService orders;
    private final PaymentService payments;
    private final InventoryService inventory;
    @Transactional
    public CheckoutResult checkout(Cart cart) {
        inventory.reserve(cart);
        var order = orders.place(toCommand(cart));
        var receipt = payments.capture(order, cart.payment());
        return new CheckoutResult(order.id(), receipt.id());
    }
}