```mermaid
sequenceDiagram
    participant User
    participant PaymentGateway
    participant TransactionDB
    User->>PaymentGateway: Initiate Payment
    alt Successful Payment
        PaymentGateway-->>TransactionDB: Store Transaction
        TransactionDB-->>PaymentGateway: Confirm Transaction Stored
        PaymentGateway-->>User: Payment Successful
    else External Failure
        PaymentGateway-->>User: Payment Gateway Error
    else DB Failure
        PaymentGateway-->>User: Payment Processing Error
        TransactionDB-->>PaymentGateway: DB Error Occurred
    end
    opt Commit Transaction
        TransactionDB-->>User: Commit Success
    end
    opt Rollback Transaction
        TransactionDB-->>User: Rollback Success
    end
```