From 71e25467e37c328e21403f0e294969c9301e969a Mon Sep 17 00:00:00 2001
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Date: Sun, 12 Jul 2026 04:41:19 +0000
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 docs/architecture/transaction-sequence.md | 25 +++++++++++++++++++++++
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 create mode 100644 docs/architecture/transaction-sequence.md

diff --git a/docs/architecture/transaction-sequence.md b/docs/architecture/transaction-sequence.md
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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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