```mermaid
sequenceDiagram
    participant User
    participant PaymentGateway
    participant Database

    User->>PaymentGateway: Initiate payment
    PaymentGateway->>User: Payment status (success)
    User->>Database: Save transaction
    Database-->>User: Transaction saved

    %% External failure scenario
    User->>PaymentGateway: Initiate payment
    PaymentGateway-->>User: Payment status (failure)
    User->>Database: Log failure reason

    %% Database failure scenario
    User->>PaymentGateway: Initiate payment
    PaymentGateway->>User: Payment status (success)
    User->>Database: Save transaction
    Database-->>User: Transaction failed (DB error)
    Note right of User: Rollback transaction
```