Intro

Concurrency control - 並行控制

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Database - 資料庫管理系統

悲觀鎖 - PCC

悲觀並行控制 - Pessimistic Concurrency Control，縮寫「PCC」

• 「先取鎖再訪問」
• 效率不佳，且有增加產生死鎖(deadlock)機會

MySQL 實現案例 (InnoDB Transaction)

# Row-level Lock for share
SELECT * FROM parent WHERE NAME = 'Jones' FOR SHARE;

# Row-level Lock for update
SELECT counter_field FROM child_codes FOR UPDATE;
UPDATE child_codes SET counter_field = counter_field + 1;

MySQL - Locking Reads
Postgresql Explicit Locking - Row-Level Locks

樂觀鎖 - OCC

樂觀並行控制 - Optimistic Concurrency Control，縮寫「OCC」

• 透過讀取資料時分派一個版本(時間戳)，通過校驗後寫入否則中斷(實作上可利用寫入連帶驗證版本達成)
• 適合交易之間資料競爭(data race)概率較小的應用情境

MySQL 實現範例

update `DB`.`table` set quantity = quantity -1 , version = version + 1 where id = 1 and version = 0;

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OS - 作業系統

IPC (Inter-Process Communication) - 行程間通訊

(TBC)