```mermaid
sequenceDiagram
    participant User as User
    participant Checkout as Checkout Service
    participant Payment as Payment Gateway
    participant DB as Database

    %% Normal flow
    User->>Checkout: Initiate Checkout
    Checkout->>DB: Save Order
    DB-->>Checkout: Order Confirmation
    Checkout->>Payment: Process Payment
    Payment-->>Checkout: Payment Success
    Checkout->>User: Payment Successful

    %% External failure
    User->>Checkout: Initiate Checkout
    Checkout->>DB: Save Order
    DB-->>Checkout: Order Confirmation
    Checkout->>Payment: Process Payment
    Payment-->>Checkout: Payment Failure
    Checkout->>DB: Rollback Order
    DB-->>Checkout: Order Rollback
    Checkout->>User: Payment Failed

    %% DB failure
    User->>Checkout: Initiate Checkout
    Checkout->>DB: Save Order
    DB-->>Checkout: Order Confirmation
    Checkout->>Payment: Process Payment
    Payment-->>Checkout: Payment Success
    Checkout->>DB: Commit Order
    DB-->>Checkout: Error
    Checkout->>DB: Rollback Order
    DB-->>Checkout: Order Rollback
    Checkout->>User: Payment Successful, Order Error
```