DDD 이론 9: Service와 Supple Design

// Domain Service: 순수한 도메인 로직
class PricingService {
  calculateOrderTotal(
    items: OrderItem[],
    customer: Customer,
    coupon: Coupon | null
  ): PricingResult {
    // 1. 기본 가격 계산
    let subtotal = items.reduce(
      (sum, item) => sum.add(item.lineTotal),
      Money.zero()
    );

    // 2. 회원 등급 할인
    const memberDiscount = this.calculateMemberDiscount(
      subtotal, 
      customer.membershipLevel
    );

    // 3. 쿠폰 할인
    const couponDiscount = coupon 
      ? this.calculateCouponDiscount(subtotal, coupon)
      : Money.zero();

    // 4. 배송비 계산
    const shippingFee = this.calculateShippingFee(
      subtotal.subtract(memberDiscount).subtract(couponDiscount)
    );

    return new PricingResult(
      subtotal,
      memberDiscount,
      couponDiscount,
      shippingFee
    );
  }

  private calculateMemberDiscount(amount: Money, level: MembershipLevel): Money {
    const discountRate = {
      [MembershipLevel.BRONZE]: 0,
      [MembershipLevel.SILVER]: 0.03,
      [MembershipLevel.GOLD]: 0.05,
      [MembershipLevel.VIP]: 0.10
    };
    return amount.multiply(discountRate[level]);
  }

  private calculateShippingFee(amount: Money): Money {
    return amount.isGreaterThanOrEqual(Money.won(50000))
      ? Money.zero()
      : Money.won(3000);
  }
}

// Application Service: 유스케이스 조율
class OrderApplicationService {
  constructor(
    private readonly orderRepository: OrderRepository,
    private readonly customerRepository: CustomerRepository,
    private readonly productRepository: ProductRepository,
    private readonly pricingService: PricingService,
    private readonly eventPublisher: DomainEventPublisher
  ) {}

  @Transactional
  async createOrder(command: CreateOrderCommand): Promise<OrderId> {
    // 1. 필요한 데이터 조회
    const customer = await this.customerRepository.findById(
      CustomerId.from(command.customerId)
    );
    if (!customer) throw new CustomerNotFoundError(command.customerId);

    const products = await this.productRepository.findByIds(
      command.items.map(i => ProductId.from(i.productId))
    );

    // 2. 도메인 객체 생성
    const order = Order.create(
      customer.id,
      Address.from(command.shippingAddress)
    );

    // 3. 주문 항목 추가
    for (const item of command.items) {
      const product = products.find(p => p.id.value === item.productId);
      if (!product) throw new ProductNotFoundError(item.productId);
      
      order.addItem(
        product.id,
        product.name,
        Quantity.of(item.quantity),
        product.price
      );
    }

    // 4. 가격 계산 (Domain Service 사용)
    const pricing = this.pricingService.calculateOrderTotal(
      order.items,
      customer,
      command.couponCode ? await this.findCoupon(command.couponCode) : null
    );

    order.applyPricing(pricing);

    // 5. 저장
    await this.orderRepository.save(order);

    // 6. 이벤트 발행
    await this.eventPublisher.publishAll(order.domainEvents);

    return order.id;
  }
}