```mermaid
sequenceDiagram
    participant User as User
    participant Controller as Controller
    participant PaymentService as PaymentService
    participant Gateway as PaymentGateway
    participant DB as Database

    User ->> Controller: Initiate Payment
    Controller ->> PaymentService: Process Payment
    PaymentService ->> Gateway: Call Payment Gateway
    alt Successful Payment
        Gateway -->> PaymentService: Payment Success
        PaymentService ->> DB: Save Transaction
        DB -->> PaymentService: Transaction Saved
        PaymentService -->> Controller: Payment Processed
        Controller -->> User: Payment Successful
    else External Gateway Failure
        Gateway -->> PaymentService: Payment Failed
        PaymentService -->> Controller: Payment Failed
        Controller -->> User: Payment Failed
    else DB Failure
        Gateway -->> PaymentService: Payment Success
        PaymentService ->> DB: Save Transaction
        DB -->> PaymentService: DB Error
        PaymentService ->> Controller: Rollback Transaction
        Controller -->> User: Payment Failed
    end
```