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ClickHouse29 Hot0 comments7 min read
Read your writes: WAIT FOR in PostgreSQL 19
AI summary
Trong PostgreSQL, các replica bất đồng bộ thường trả về dữ liệu cũ vì chúng chỉ áp dụng WAL sau một khoảng trễ, gây ra hiện tượng đọc không thấy bản ghi vừa ghi trên primary.
Phiên bản PostgreSQL 19 bổ sung lệnh WAIT FOR cho phép một truy vấn READ chỉ định vị trí WAL (log sequence number) mà nó muốn chờ trước khi trả về kết quả.
Khi lệnh được thực hiện, phiên bản server sẽ tạm dừng việc đọc cho đến khi WAL đã được replay tới vị trí đó trên replica, từ đó đảm bảo read‑your‑writes mà không cần chuyển sang synchronous replication hoặc tăng toàn bộ độ trễ hệ thống.
Điều đáng học: thay vì phải bật synchronous replication toàn cluster hoặc chịu đọc stale data, разработчики có thể sử dụng WAIT FOR cho các giao dịch cụ thể nơi cần nhất quán đọc‑ghi, tiết kiệm tài nguyên và kiểm soát độ trễ chi tiết.
Ví dụ thực tế, một lệnh như SELECT ... WAIT FOR LSN '0/3000000' sẽ chỉ trả về sau khi replica đã áp dụng WAL tới LSN đó, cho phép ứng dụng xác nhận ngay lập tức việc ghi vừa thực hiện trên primary mà không ảnh hưởng đến các truy vấn khác.
AI summaries can be wrong. Read the original before relying on anything important.
Why read it: Bài này giúp lập trình viên hiểu cách sử dụng WAIT FOR trong PostgreSQL 19 để đảm bảo tính nhất quán read-your-writes trên bản sao bất đồng bộ.
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Source: https://clickhouse.com/blog/postgresql-19-wait-for-read-your-writes. 8 Sync News only summarizes and links out; content copyright belongs to the authors and original sources.
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