payment_service_1.rs

    // Spawn the "payment" service.
    let _ = thread::spawn(move || loop {
        match payment_request_receiver.recv() {
            Ok(PaymentRequest::NewOrder(order_id)) => {
                // Process the payment for a new order.
                let _ = payment_result_sender.send(PaymentResult(order_id, true));
            }
            Ok(PaymentRequest::ShutDown) => {
                break;
            }
            Err(_) => panic!("Error receiving a payment request."),
        }
    });