```mermaid
sequenceDiagram
    participant User
    participant PaymentService
    participant PaymentGateway
    participant Database

    User->>PaymentService: 결제 요청
    PaymentService->>Database: 거래 시작
    Database-->>PaymentService: 거래 ID
    PaymentService->>PaymentGateway: 결제 진행
    alt Success
        PaymentGateway-->>PaymentService: 결제 성공
        PaymentService->>Database: 거래 확정
    else Failure
        PaymentGateway-->>PaymentService: 결제 실패
        PaymentService->>Database: 거래 롤백
    end
```