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+```mermaid
+sequenceDiagram
+    participant User
+    participant PaymentGateway
+    participant Bank
+
+    %% Normal flow
+    User ->> PaymentGateway: Request Payment
+    PaymentGateway ->> Bank: Process Payment
+    Bank -->> PaymentGateway: Payment Approved
+    PaymentGateway -->> User: Payment Confirmation
+
+    %% External failure branch
+    User ->> PaymentGateway: Request Payment
+    PaymentGateway ->> Bank: Process Payment
+    Bank -->> PaymentGateway: Payment Declined
+    PaymentGateway -->> User: Payment Declined Message
+
+    %% DB failure branch
+    User ->> PaymentGateway: Request Payment
+    PaymentGateway ->> Bank: Process Payment
+    Bank -->> PaymentGateway: DB Error
+    PaymentGateway -->> User: Payment Failed Due to Error
+    Note over PaymentGateway: Rollback transaction if needed
+```
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