MySQL中的隔离级别和悲观锁及乐观锁示例

MySQL中的隔离级别和悲观锁及乐观锁示例

1，MySQL的事务支持
MySQL的事务支持不是绑定在MySQL服务器本身，而是与存储引擎相关：

MyISAM：不支持事务，用于只读程序提高性能

InnoDB：支持ACID事务、行级锁、并发

Berkeley DB：支持事务

2，隔离级别
隔离级别决定了一个session中的事务可能对另一个session的影响、并发session对数据库的操作、一个session中所见数据的一致性
ANSI标准定义了4个隔离级别，MySQL的InnoDB都支持：
Java代码

READ UNCOMMITTED：最低级别的隔离，通常又称为dirty read，它允许一个事务读取还没commit的数据，这样可能会提高性能，但是dirty read可能不是我们想要的

READ COMMITTED：在一个事务中只允许已经commit的记录可见，如果session中select还在查询中，另一session此时insert一条记录，则新添加的数据不可见

REPEATABLE READ：在一个事务开始后，其他session对数据库的修改在本事务中不可见，直到本事务commit或rollback。在一个事务中重复select的结果一样，除非本事务中update数据库。

SERIALIZABLE：最高级别的隔离，只允许事务串行执行。为了达到此目的，数据库会锁住每行已经读取的记录，其他session不能修改数据直到前一事务结束，事务commit或取消时才释放锁。

可以使用如下语句设置MySQL的session隔离级别：

1.	SET TRANSACTION ISOLATION LEVEL {READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE}

MySQL默认的隔离级别是REPEATABLE READ，在设置隔离级别为READ UNCOMMITTED或SERIALIZABLE时要小心，READ UNCOMMITTED会导致数据完整性的严重问题，而SERIALIZABLE会导致性能问题并增加死锁的机率

3，隔离级别
乐观所和悲观锁策略：
悲观锁：在读取数据时锁住那几行，其他对这几行的更新需要等到悲观锁结束时才能继续
乐观所：读取数据时不锁，更新时检查是否数据已经被更新过，如果是则取消当前更新
一般在悲观锁的等待时间过长而不能接受时我们才会选择乐观锁
悲观锁的例子：

CREATE PROCEDURE tfer_funds
(from_account INT, to_account INT,tfer_amount NUMERIC(10,2),
OUT status INT, OUT message VARCHAR(30))
BEGIN
DECLARE from_account_balance NUMERIC(10,2);

START TRANSACTION;

SELECT balance
INTO from_account_balance
FROM account_balance
WHERE account_id=from_account
FOR UPDATE;

IF from_account_balance>=tfer_amount THEN

UPDATE account_balance
SET balance=balance-tfer_amount
WHERE account_id=from_account;

UPDATE account_balance
SET balance=balance+tfer_amount
WHERE account_id=to_account;
COMMIT;

SET status=0;
SET message='OK';
ELSE
ROLLBACK;
SET status=-1;
SET message='Insufficient funds';
END IF;
END;

乐观锁的例子：

CREATE PROCEDURE tfer_funds
(from_account INT, to_account INT, tfer_amount NUMERIC(10,2),
OUT status INT, OUT message VARCHAR(30) )

BEGIN

DECLARE from_account_balance    NUMERIC(8,2);
DECLARE from_account_balance2   NUMERIC(8,2);
DECLARE from_account_timestamp1 TIMESTAMP;
DECLARE from_account_timestamp2 TIMESTAMP;

SELECT account_timestamp,balance
INTO from_account_timestamp1,from_account_balance
FROM account_balance
WHERE account_id=from_account;

IF (from_account_balance>=tfer_amount) THEN

-- Here we perform some long running validation that
-- might take a few minutes */
CALL long_running_validation(from_account);

START TRANSACTION;

-- Make sure the account row has not been updated since
-- our initial check
SELECT account_timestamp, balance
INTO from_account_timestamp2,from_account_balance2
FROM account_balance
WHERE account_id=from_account
FOR UPDATE;

IF (from_account_timestamp1 <> from_account_timestamp2 OR
from_account_balance    <> from_account_balance2)  THEN
ROLLBACK;
SET status=-1;
SET message=CONCAT("Transaction cancelled due to concurrent update",
" of account"  ,from_account);
ELSE
UPDATE account_balance
SET balance=balance-tfer_amount
WHERE account_id=from_account;

UPDATE account_balance
SET balance=balance+tfer_amount
WHERE account_id=to_account;

COMMIT;

SET status=0;
SET message="OK";
END IF;

ELSE
ROLLBACK;
SET status=-1;
SET message="Insufficient funds";
END IF;
END$$