diff --git a/docs/architecture/transaction-sequence.md b/docs/architecture/transaction-sequence.md
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+sequenceDiagram
+    participant User
+    participant PaymentService
+    participant PaymentRepository
+    participant ExternalAPI
+
+    User->>PaymentService: 결제 요청
+    PaymentService->>PaymentRepository: 결제 정보 저장
+    alt 정상 흐름
+        PaymentRepository-->>PaymentService: 저장 성공
+        PaymentService->>ExternalAPI: 결제 처리 요청
+        ExternalAPI-->>PaymentService: 결제 성공
+        PaymentService-->>User: 결제 완료
+    else 외부 실패
+        PaymentRepository-->>PaymentService: 저장 성공
+        PaymentService-->>User: 결제 실패 (외부 API 오류)
+    else DB 실패
+        PaymentRepository-->>PaymentService: 저장 실패
+        PaymentService-->>User: 결제 실패 (DB 오류)
+    end
+    note over PaymentService: 커밋 또는 롤백 경계
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