MySQl中隔离级别和悲观锁乐观锁

1.MySql的事物支持

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：

1. MyISAM：不支持事务，用于只读程序提高性能
2. InnoDB：支持ACID事务、行级锁、并发
3. Berkeley DB：支持事务

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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

1. 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
2. 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
3. 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。
4. 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：

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.

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3.乐观锁和悲观锁的策略

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$$    

一个讲解比较清楚的博客推荐:http://blog.csdn.net/csh624366188/article/details/7654996