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+```mermaid
+sequenceDiagram
+    participant User
+    participant Controller
+    participant Service
+    participant Repository
+    participant ExternalGateway
+
+    User->>Controller: 결제 요청
+    Controller->>Service: 결제 처리 요청
+    Service->>Repository: 결제 정보 저장
+    alt 정상 흐름
+        Repository-->>Service: 저장 성공
+        Service->>ExternalGateway: 결제 요청
+        ExternalGateway-->>Service: 결제 성공
+        Service-->>Controller: 성공 응답
+        Controller-->>User: 결제 완료
+    else 외부 실패
+        Repository-->>Service: 저장 성공
+        Service->>ExternalGateway: 결제 요청
+        ExternalGateway-->>Service: 결제 실패
+        Service-->>Controller: 실패 응답
+        Controller-->>User: 결제 실패
+    else DB 실패
+        Repository-->>Service: 저장 실패
+        Service-->>Controller: 실패 응답
+        Controller-->>User: 결제 실패
+    end
+```
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