Cho mình hỏi về việc thay đổi data trong hệ thống microservice

Đối với các hệ thống ngân hàng, bạn không nên áp dụng cache và Async việc update DB. Bởi vì các giao dịch ngân hàng bắt buộc phải strong consistency. Nếu lỡ như bạn đã gửi lại thông tin cho user là chuyển tiền thành công nhưng việc update DB lỗi hoặc lỗi máy ATM thì thông tin user nhận là sai. Ngoài ra còn gặp những vấn đề liên quan tới race condition nữa. Lưu trên cache cũng gặp rủi ro nếu như redis cache sập thì dữ liệu sẽ mất hết. Cách giải quyết có thể như sau:

Bước 1: Giữ chỗ (Hold/Freeze/Authorize) - Đồng bộ (Synchronous)

• Ví dụ khi bạn bấm rút 2 triệu: Service tiếp nhận lệnh và gọi đồng bộ xuống DB.
• Hệ thống sẽ thực hiện một lệnh Lock số dư của User đó (ví dụ: SELECT balance FROM accounts WHERE id = x FOR UPDATE).
• Kiểm tra số dư. Nếu đủ, hệ thống không trừ tiền ngay, mà chuyển 2 triệu đó vào trạng thái "Hold" (Tạm giữ/Phong tỏa).
• Lúc này, số dư khả dụng (Available Balance) giảm đi 2 triệu, nhưng số dư thực tế (Current Balance) vẫn giữ nguyên. Thao tác này diễn ra bằng một Transaction trong DB cực kỳ nhanh (vài mili-giây).

Bước 2: Ra lệnh cho ATM (Synchronous) Sau khi đã Hold tiền thành công trong DB, backend trả kết quả Đồng bộ về cho ATM: "Tôi đã giữ tiền an toàn, anh hãy nhả tiền mặt cho khách đi".

Bước 3: Đẩy tiền và Hoàn tất (Settle) hoặc Hủy (Rollback)

• Kịch bản 1 (Thành công): ATM nhả tiền thành công -> ATM gửi tín hiệu Confirm về Backend -> Backend thực hiện Deduct (Trừ hẳn) 2 triệu đang bị Hold trong DB (Thao tác này có thể chạy Async hoặc Sync tùy kiến trúc kiến tạo).
• Kịch bản 2 (Thất bại/Kẹt tiền): ATM không nhả được tiền -> Gửi tín hiệu Cancel về Backend -> Backend thực hiện Unhold (Giải phóng) 2 triệu đó lại cho khách hàng.

Nếu bạn đang hỏi về việc thay đổi dữ liệu (data) trong hệ thống microservices và cách xử lý "đúng", thì đây là một trong những vấn đề quan trọng nhất của kiến trúc microservices.

Có một số nguyên tắc phổ biến: Mỗi service sở hữu database của riêng mình Không service nào được update trực tiếp database của service khác. Ví dụ: User Service → User DB Order Service → Order DB Payment Service → Payment DB Muốn thay đổi dữ liệu của service khác thì gọi API

Thay vì:

UPDATE user SET status='VIP'

từ Order Service.

Thì:

Order Service ↓ Call User Service API ↓ User Service update DB

Dùng Event-driven khi cần đồng bộ dữ liệu Ví dụ:

Order Created ↓ Publish Event (Kafka/RabbitMQ) ↓ Payment Service Inventory Service Notification Service

Mỗi service tự cập nhật dữ liệu của mình sau khi nhận event.

Chấp nhận Eventual Consistency Trong microservices, dữ liệu thường không đồng bộ ngay lập tức giữa các service. Thay vào đó, hệ thống hướng đến eventual consistency, tức là sau một khoảng thời gian ngắn, các service sẽ đạt trạng thái nhất quán. Xử lý transaction Không nên dùng transaction xuyên nhiều database như trong monolith. Thay vào đó, thường sử dụng: Saga Pattern Compensation Transaction Outbox Pattern để đảm bảo event không bị mất khi ghi dữ liệu và phát sự kiện. Ví dụ

Giả sử hệ thống thương mại điện tử:

Customer đặt hàng │ ▼ Order Service │ ├── Lưu Order └── Publish OrderCreated │ ┌──────────┼──────────┐ ▼ ▼ ▼ Inventory Payment Notification Service Service Service Order Service chỉ cập nhật dữ liệu đơn hàng. Inventory Service tự trừ tồn kho. Payment Service tự tạo giao dịch thanh toán. Notification Service gửi email hoặc SMS.

Các service không cập nhật trực tiếp database của nhau mà phối hợp thông qua API hoặc event.

Nếu bạn đang chuẩn bị cho phỏng vấn, một câu trả lời ngắn gọn có thể là:

"Trong kiến trúc microservices, mỗi service sở hữu dữ liệu của riêng mình. Khi cần thay đổi dữ liệu liên quan đến service khác, service sẽ gọi API hoặc phát event thay vì truy cập trực tiếp database của service đó. Để đảm bảo tính nhất quán, hệ thống thường sử dụng Event-driven Architecture kết hợp các mẫu như Saga hoặc Outbox Pattern, chấp nhận eventual consistency thay vì transaction phân tán."

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