Question

我有一个MySQL表（使用InnoDB作为存储引擎）来存储用户事务。

// OS.hpp -------------------------------------------------
// !!!! HERE I WILL NOT INCLUDE APPLICATION.HPP !!!!
// I JUST USE THE APPLICATION DEFINITION TO ACCESS THE 
// STATIC FUNCTION
extern class Application;

class OS {
public:
    OS() {}
    ~OS() {}
    int getInfo() { return Application::get(); }
}
// Middleware.hpp -----------------------------------------
#include "OS.hpp"

class Middleware {
    OS os;
public:
    Middleware() {}
    ~Middleware() {}
    void doSomething() { int a = os.getInfo(); }
}
// Application.hpp ----------------------------------------
#include "Middleware.hpp"

class Application {
    Middleware mw;
public:
    Application() {}
    ~Application() {}
    void run() { mw.doSomething(); }
    static int get() { return 0; } // !!!! this function (static) i would like to access from OS
}

// main.cpp -----------------------------------------------
#include "Application.hpp"

int main() {
    Application app;
    app.run();
}

我在多线程环境中工作，并希望确保：

没有两个线程可以同时为同一个用户插入事务
如果某个线程正在为用户插入事务，则其他任何线程都无法读取该用户的事务

我提出了以下解决方案：

当线程想要为用户插入事务时，获取与该用户对应的行的独占锁

CREATE TABLE `transactions` (
  `id` int(11) NOT NULL,
  `correlation_id` char(36) NOT NULL,
  `user_id` char(36) NOT NULL,
  `currency` char(3) NOT NULL,
  `time_created` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `transaction_amount` double NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

ALTER TABLE `transactions`
  ADD PRIMARY KEY (`id`),
  ADD UNIQUE KEY `correlation_id_unique` (`correlation_id`), 
  ADD INDEX (`user_id`);

当线程想要读取用户余额时（通常通过对用户的所有事务求和），它首先获取与该用户对应的行的共享锁

BEGIN;

-- Acquire an exclusive lock on the rows with user_id=1
SELECT * FROM transactions WHERE user_id = 1 FOR UPDATE;

-- Insert transactions
...

COMMIT;

但是，似乎独占锁是锁定整个表，而不仅仅是SELECT ... FOR UPDATE语句返回的行。这是一个例子。

主题1：

SELECT SUM(transaction_amount) 
FROM transactions 
WHERE user_id=1 
LOCK IN SHARE MODE;

主题2：

mysql> select user_id, transaction_amount from transactions;
+---------+--------------------+
| user_id | transaction_amount |
+---------+--------------------+
| 1       |                 10 |
| 1       |                 -2 |
| 2       |                  5 |
| 2       |                 10 |
+---------+--------------------+
4 rows in set (0.00 sec)

mysql> BEGIN;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM transactions WHERE user_id = 1 FOR UPDATE;
+----+----------------+---------+----------+---------------------+--------------------+
| id | correlation_id | user_id | currency | time_created        | transaction_amount |
+----+----------------+---------+----------+---------------------+--------------------+
|  1 | 1              | 1       | CHF      | 2018-03-06 09:54:28 |                 10 |
|  2 | 2              | 1       | CHF      | 2018-03-06 09:54:28 |                 -2 |
+----+----------------+---------+----------+---------------------+--------------------+
2 rows in set (0.01 sec)

阅读MySQL's documentation之后，我原本预计会这样做：

选择...锁定共享模式

在读取的任何行上设置共享模式锁定。 其他会话可以读取行，但在事务提交之前无法修改它们

SELECT ... FOR UPDATE

对于搜索遇到的索引记录，锁定行和任何关联的索引条目，就像为这些行发出UPDATE语句一样。阻止其他交易更新那些行，从进行SELECT ... LOCK IN SHARE MODE，或者从某些事务隔离级别读取数据。

现在，我找到this topic，说明在我的情况下，-- Retrieve transactions of user 2 mysql> SELECT * FROM transactions WHERE user_id = 2 LOCK IN SHARE MODE; [[Hangs]]字段应该有一个索引 - 而且确实如此。

我觉得问题可能是由请求user_id没有使用索引引起的：

SELECT * FROM transactions WHERE user_id=1

有什么想法吗？

Answer 1

我用MySQL 5.6.31测试了你的表，并在其中填充了50万行随机值，介于1和1000之间。

即使强迫指数也没有帮助：

mysql> EXPLAIN SELECT * FROM `transactions` force index (user_id) where user_id=1;
+----+-------------+--------------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table        | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+--------------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | transactions | ALL  | user_id       | NULL | NULL    | NULL | 520674 | Using where |
+----+-------------+--------------+------+---------------+------+---------+------+--------+-------------+

但是，即使没有索引提示，搜索整数字符串仍然有效：

mysql> EXPLAIN SELECT * FROM `transactions`  where user_id='1';
+----+-------------+--------------+------+---------------+---------+---------+-------+------+-----------------------+
| id | select_type | table        | type | possible_keys | key     | key_len | ref   | rows | Extra                 |
+----+-------------+--------------+------+---------------+---------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | transactions | ref  | user_id       | user_id | 36      | const |    1 | Using index condition |
+----+-------------+--------------+------+---------------+---------+---------+-------+------+-----------------------+

varchar列与二进制整数的比较似乎打败了可索引性。

MySQL独占锁（FOR UPDATE）锁定整个表

1 个答案: