```mermaid
sequenceDiagram
    participant User
    participant PaymentGateway
    participant Bank

    %% Normal flow
    User ->> PaymentGateway: Request Payment
    PaymentGateway ->> Bank: Process Payment
    Bank -->> PaymentGateway: Payment Approved
    PaymentGateway -->> User: Payment Confirmation

    %% External failure branch
    User ->> PaymentGateway: Request Payment
    PaymentGateway ->> Bank: Process Payment
    Bank -->> PaymentGateway: Payment Declined
    PaymentGateway -->> User: Payment Declined Message

    %% DB failure branch
    User ->> PaymentGateway: Request Payment
    PaymentGateway ->> Bank: Process Payment
    Bank -->> PaymentGateway: DB Error
    PaymentGateway -->> User: Payment Failed Due to Error
    Note over PaymentGateway: Rollback transaction if needed
```