세 번의 리모델링 — 당근페이가 아키텍처를 갈아엎은 진짜 이유

src/
  controller/
    PaymentController.kt
    TransferController.kt
  service/
    PaymentService.kt
    TransferService.kt
  repository/
    PaymentRepository.kt
    TransferRepository.kt
  entity/
    Payment.kt
    Transfer.kt

class PaymentService(
    private val paymentRepository: PaymentRepository,
    private val userRepository: UserRepository,
    private val notificationClient: NotificationClient,
    private val fraudDetectionClient: FraudDetectionClient,
    private val ledgerClient: LedgerClient,
    // ... 10개 이상의 의존성
) {
    // 결제 생성, 취소, 환불, 부분 환불, 정산, 알림, 사기 탐지...
    // 수천 줄의 코드가 한 클래스에 집중
}

src/
  domain/
    Payment.kt
    PaymentPolicy.kt
  port/
    in/
      CreatePaymentUseCase.kt
      CancelPaymentUseCase.kt
    out/
      SavePaymentPort.kt
      LoadPaymentPort.kt
      SendNotificationPort.kt
  adapter/
    in/
      web/
        PaymentController.kt
      message/
        PaymentEventListener.kt
    out/
      persistence/
        PaymentJpaAdapter.kt
        PaymentJpaRepository.kt
      notification/
        FcmNotificationAdapter.kt

// Inbound Port — 외부에서 도메인으로 들어오는 요청
interface CreatePaymentUseCase {
    fun execute(command: CreatePaymentCommand): Payment
}
 
// Outbound Port — 도메인에서 외부로 나가는 요청
interface SavePaymentPort {
    fun save(payment: Payment): Payment
}
 
interface SendNotificationPort {
    fun send(userId: String, message: String)
}

class PaymentService(
    private val savePaymentPort: SavePaymentPort,
    private val sendNotificationPort: SendNotificationPort,
) : CreatePaymentUseCase {
 
    override fun execute(command: CreatePaymentCommand): Payment {
        val payment = Payment.create(
            amount = command.amount,
            userId = command.userId
        )
        val saved = savePaymentPort.save(payment)
        sendNotificationPort.send(command.userId, "결제가 완료되었습니다")
        return saved
    }
}

modules/
  payment/
    domain/
      Payment.kt
      PaymentPolicy.kt
    usecase/
      CreatePaymentUseCase.kt
      CancelPaymentUseCase.kt
    adapter/
      in/
        PaymentController.kt
      out/
        PaymentPersistenceAdapter.kt
  transfer/
    domain/
      Transfer.kt
    usecase/
      CreateTransferUseCase.kt
    adapter/
      in/
        TransferController.kt
      out/
        TransferPersistenceAdapter.kt
  settlement/
    domain/
      Settlement.kt
    usecase/
      CreateSettlementUseCase.kt
    adapter/
      ...
  shared/
    event/
      DomainEvent.kt
    util/
      MoneyUtils.kt

// payment 모듈의 build.gradle.kts
dependencies {
    implementation(project(":modules:shared"))
    // transfer 모듈에 직접 의존하지 않는다!
    // 도메인 간 통신은 이벤트를 통해서만
}

// payment 모듈
class CreatePaymentUseCase(
    private val savePaymentPort: SavePaymentPort,
    private val eventPublisher: DomainEventPublisher,
) {
    fun execute(command: CreatePaymentCommand): Payment {
        val payment = Payment.create(command.amount, command.userId)
        val saved = savePaymentPort.save(payment)
        eventPublisher.publish(PaymentCompletedEvent(saved.id, saved.amount))
        return saved
    }
}
 
// settlement 모듈 — payment에 직접 의존하지 않는다
class PaymentCompletedEventHandler(
    private val createSettlementUseCase: CreateSettlementUseCase,
) {
    fun handle(event: PaymentCompletedEvent) {
        createSettlementUseCase.execute(
            CreateSettlementCommand(event.paymentId, event.amount)
        )
    }
}