樂觀鎖、悲觀鎖、要實踐

http://chenzhou123520.iteye.com/blog/1860954 《mysql悲觀鎖總結和實踐》

http://chenzhou123520.iteye.com/blog/1863407 《mysql樂觀鎖總結和實踐》

http://outofmemory.cn/sql/optimistic-lock-and-pessimistic-lock

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注意，以下的表里面的列名，一定要用 `` 反引號來包括。

mysql> create table `t_goods` ( -> `id` bigint(11) NOT NULL AUTO_INCREMENT, -> `status` bigint(11) DEFAULT 0, -> `name` varchar(32) DEFAULT NULL, -> `version` bigint(11) DEFAULT 1, -> PRIMARY KEY (`id`) -> ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8; Query OK, 0 rows affected (0.13 sec)

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mysql> insert into t_goods (`name`) values ('weapon'); Query OK, 1 row affected (0.07 sec)mysql> insert into t_goods (`name`) values ('equipment'); Query OK, 1 row affected (0.10 sec)

mysql> select * from t_goods;
+----+--------+-----------+---------+
| id | status | name | version |
+----+--------+-----------+---------+
| 1 | 0 | weapon | 1 |
| 2 | 0 | equipment | 1 |
+----+--------+-----------+---------+
2 rows in set (0.00 sec)

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實驗1，select for update 指定主鍵，只鎖行：

首先要關閉autocommit：

mysql> set autocommit=0; Query OK, 0 rows affected (0.00 sec)mysql> show variables like 'autocommit'; +---------------+-------+ | Variable_name | Value | +---------------+-------+ | autocommit | OFF | +---------------+-------+ 1 row in set (0.00 sec)

如果不關閉，經過實驗，的確不會相互影響。

關閉autocommit之后，普通的sql不放在事務里面也可以。

console A:mysql> select * from t_goods where id = 1 for update; +----+--------+--------+---------+ | id | status | name | version | +----+--------+--------+---------+ | 1 | 0 | weapon | 1 | +----+--------+--------+---------+ 1 row in set (0.00 sec)console B: mysql> select * from t_goods where id = 1; +----+--------+--------+---------+ | id | status | name | version | +----+--------+--------+---------+ | 1 | 0 | weapon | 1 | +----+--------+--------+---------+ 1 row in set (0.00 sec)mysql> select * from t_goods where id = 2 for update; +----+--------+-----------+---------+ | id | status | name | version | +----+--------+-----------+---------+ | 2 | 0 | equipment | 1 | +----+--------+-----------+---------+ 1 row in set (0.00 sec)mysql> select * from t_goods where id = 1 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

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可以看出，不加for update不影響，加了for update不是同一行，不影響（僅對于主鍵查詢有關）。

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實驗2，select for update 指定非主鍵，鎖全部：

Console A: mysql> select * from t_goods where name = 'weapon' for update; +----+--------+--------+---------+ | id | status | name | version | +----+--------+--------+---------+ | 1 | 0 | weapon | 1 | +----+--------+--------+---------+ 1 row in set (0.00 sec)Console B: mysql> select * from t_goods; +----+--------+-----------+---------+ | id | status | name | version | +----+--------+-----------+---------+ | 1 | 0 | weapon | 1 | | 2 | 0 | equipment | 1 | +----+--------+-----------+---------+ 2 rows in set (0.00 sec)mysql> select * from t_goods where name = 'equipment' for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transactionmysql> select * from t_goods where id = 2 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

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實驗3，查詢主鍵沒查到，不鎖：

Console A: mysql> select * from t_goods where id=3 for update; Empty set (0.01 sec)Console B: mysql> select * from t_goods for update; +----+--------+-----------+---------+ | id | status | name | version | +----+--------+-----------+---------+ | 1 | 0 | weapon | 1 | | 2 | 0 | equipment | 1 | +----+--------+-----------+---------+ 2 rows in set (0.00 sec)

說明，主鍵沒查到數據，不加鎖。

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實驗4，查詢非主鍵，沒查到，鎖全部，table lock.

Console A: mysql> select * from t_goods where name = 'abc' for update; Empty set (0.00 sec)Console B: mysql> select * from t_goods for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transactionmysql> select * from t_goods where id = 1 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

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實驗5：查詢主鍵不明確，為范圍，大于小于，只鎖相關的行；

Console A: mysql> select * from t_goods where id > 1 for update; +----+--------+-----------+---------+ | id | status | name | version | +----+--------+-----------+---------+ | 2 | 0 | equipment | 1 | +----+--------+-----------+---------+ 1 row in set (0.00 sec)Console B: mysql> select * from t_goods where id = 1 for update; +----+--------+--------+---------+ | id | status | name | version | +----+--------+--------+---------+ | 1 | 0 | weapon | 1 | +----+--------+--------+---------+ 1 row in set (0.00 sec)mysql> select * from t_goods where id = 2 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

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實驗5：查詢主鍵不明確，!=或者<>，鎖全部；

Console A: mysql> select * from t_goods where id != 1 for update; +----+--------+-----------+---------+ | id | status | name | version | +----+--------+-----------+---------+ | 2 | 0 | equipment | 1 | +----+--------+-----------+---------+ 1 row in set (0.00 sec)Console B: mysql> select * from t_goods where id = 1 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transactionConsole A: mysql> select * from t_goods where id <> 1 for update; +----+--------+-----------+---------+ | id | status | name | version | +----+--------+-----------+---------+ | 2 | 0 | equipment | 1 | +----+--------+-----------+---------+ 1 row in set (0.00 sec)Console B: mysql> select * from t_goods where id = 1 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

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實驗6，對于普通索引，也類似于主鍵的效果：

未加索引之前，鎖全表： console A: mysql> select * from t_goods where status = 1 for update; +----+--------+--------+---------+ | id | status | name | version | +----+--------+--------+---------+ | 1 | 1 | weapon | 1 | +----+--------+--------+---------+ 1 row in set (0.00 sec)console B: mysql> select * from t_goods where id = 2 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction加了索引之后，只鎖行： console A: mysql> alter table t_goods add index index_name(`status`); Query OK, 0 rows affected (0.15 sec) Records: 0 Duplicates: 0 Warnings: 0mysql> select * from t_goods where status = 1 for update; +----+--------+--------+---------+ | id | status | name | version | +----+--------+--------+---------+ | 1 | 1 | weapon | 1 | +----+--------+--------+---------+ 1 row in set (0.00 sec)console B: mysql> select * from t_goods where id = 2 for update; +----+--------+-----------+---------+ | id | status | name | version | +----+--------+-----------+---------+ | 2 | 0 | equipment | 1 | +----+--------+-----------+---------+ 1 row in set (0.00 sec)mysql> select * from t_goods where id = 1 for update; ERROR 1205 (HY000): Lock wait timeout exceeded; try restarting transaction

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樂觀鎖：

從業務層面加鎖，一般是加上version字段，然后sql以如下形式處理：

update t_goods set status=2,version=version+1 where id=#{id} and version=#{version};

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自旋鎖與互斥鎖

http://blog.csdn.net/a675311/article/details/49096435

自旋鎖就是不聽的忙檢測，拿不到鎖就返回。

pthread中提供的鎖有：pthread_mutex_t, pthread_spinlock_t, pthread_rwlock_t。pthread_mutex_t是互斥鎖，同一瞬間只能有一個線程能夠獲取鎖，其他線程在等待獲取鎖的時候會進入休眠狀態。因此pthread_mutex_t消耗的CPU資源很小，但是性能不高，因為會引起線程切換。 pthread_spinlock_t是自旋鎖，同一瞬間也只能有一個線程能夠獲取鎖，不同的是，其他線程在等待獲取鎖的過程中并不進入睡眠狀態，而是在CPU上進入“自旋”等待。自旋鎖的性能很高，但是只適合對很小的代碼段加鎖（或短期持有的鎖），自旋鎖對CPU的占用相對較高。 pthread_rwlock_t是讀寫鎖，同時可以有多個線程獲得讀鎖，同時只允許有一個線程獲得寫鎖。其他線程在等待鎖的時候同樣會進入睡眠。讀寫鎖在互斥鎖的基礎上，允許多個線程“讀”，在某些場景下能提高性能。 諸如pthread中的pthread_cond_t, pthread_barrier_t, semaphone等，更像是一種同步原語，不屬于單純的鎖。

http://www.cnblogs.com/hdflzh/p/3716156.html

http://blog.csdn.net/pi9nc/article/details/39177343

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Java鎖相關

http://blog.csdn.net/Evankaka/article/details/44153709 (這一篇要重點看，講了Thread Runnable等)

http://blog.csdn.net/Evankaka/article/details/51866242（Java鎖技術內幕上）

http://blog.csdn.net/evankaka/article/details/51932044（Java鎖技術內幕中）

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