写在前面
使用数据库时，想要较高的吞吐、较低的延迟，但又想在高并发下可以一致地读写数据，因此需要高效的锁机制。
InnoDB中的锁可以分为：
- latch：程序上的锁机制，用来锁定内部对象，没有死锁检测；
- lock：用来锁定数据库中的对象，比如表、页、行，有死锁检测机制；
可以使用show engine innodb mutex来查看latch的情况（一般不怎么关心），下面我们来重点看lock。
锁机制
基础概念
锁的目的是将一个资源占住，不让其他人操作。更准确地可以描述为：
禁止其他人对某个资源执行某个操作。
数据库操作无非是读、写，那么相应的锁类型为：
- 共享锁（S）：允许事务读数据；
- 排他锁（X）：允许事务修改数据；
锁并非都互斥（有你没他），比如两个事务可以对同一条记录加S锁来读数据。共存即兼容：
| 兼容性 | X | S |
|---|---|---|
| X | 不兼容 | 不兼容 |
| S | 不兼容 | 兼容 |
此外还支持一种额外的锁方式（意向锁）：
- 意向共享锁（IS）
- 意向排他锁（IX）
其含义是：
在行上加普通锁之前，需要在更粗的粒度上加对应类型的意向锁，也就是意向锁反映了行锁的情况，能提升加锁效率。
比如，事务A想对表加S锁，需要判断表中是否有行有X锁。以前需要逐行判断，而现在直接判断表上有没有IX锁即可。数据库中层次结构如下：
包含意向锁的兼容性为：
| 兼容性 | IS | IX | S | X |
|---|---|---|---|---|
| IS | 兼容 | 兼容 | 兼容 | 不兼容 |
| IX | 兼容 | 兼容 | 不兼容 | 不兼容 |
| S | 兼容 | 不兼容 | 兼容 | 不兼容 |
| X | 不兼容 | 不兼容 | 不兼容 | 不兼容 |
加锁算法
不同算法的区别在于范围：
- Record Lock：锁定一行记录；
- Gap Lock：锁定一个范围，不包含记录；
- Next-Key Lock：锁定一个范围，包含记录；
死锁
在两个及以上的事务在执行过程中，可能因为争夺资源而互相等待：
- T1拥有资源A，尝试获取资源B；
- T2拥有资源B，尝试获取资源A；
如果不暴力终结某个事务，T1、T2会永远等下去，这就是死锁。数据库通过wait-for graph（等待图）来检测是否存在死锁，当发现有回路时，引擎会选择回滚undo量最小的事务。
程序上为防止死锁一般会统一更新顺序。
加锁分析
下面对常见的几条SQL做加锁分析，建表如下：
CREATE TABLE `my_test` (
`id` bigint(20) NOT NULL AUTO_INCREMENT,
`a` bigint(20) NOT NULL,
`b` bigint(20) NOT NULL,
`c` bigint(20) NOT NULL,
`d` bigint(20) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `unique_a_b`(`a`,`b`),
KEY `idx_c`(`c`)
);初始化数据如下：
在执行完成SQL后，可以从information_schema中的：
- INNODB_TRX
- INNODB_LOCKS
- INNODB_LOCK_WAITS
来查看锁和事务的状态，此外也可以用show engine innodb status来看整体情况，需要增加锁监控：
create table innodb_lock_monitor(x int) engine=innodb;才可以看到比较详细的锁信息。
RC+主键
select * from my_test where id = 1 for update;加锁信息：
2 lock struct(s), heap size 360, 1 row lock(s)
MySQL thread id 2, OS thread handle 0x1648, query id 148 localhost 127.0.0.1 root cleaning up
TABLE LOCK table `mysql`.`my_test` trx id 2863 lock mode IX
RECORD LOCKS space id 38 page no 3 n bits 72 index `PRIMARY` of table `mysql`.`my_test` trx id 2863 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 7; compact format; info bits 0加锁逻辑为：
- 表上加IX锁；
- 行上加X锁；
RC+唯一约束
select * from my_test where a = 1 and b = 1 for update;加锁信息如下：
3 lock struct(s), heap size 360, 2 row lock(s)
MySQL thread id 2, OS thread handle 0x1648, query id 152 localhost 127.0.0.1 root cleaning up
TABLE LOCK table `mysql`.`my_test` trx id 2864 lock mode IX
RECORD LOCKS space id 38 page no 4 n bits 72 index `unique_a_b` of table `mysql`.`my_test` trx id 2864 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 3; compact format; info bits 0
0: len 8; hex 8000000000000001; asc ;;
1: len 8; hex 8000000000000001; asc ;;
2: len 8; hex 8000000000000001; asc ;;
RECORD LOCKS space id 38 page no 3 n bits 72 index `PRIMARY` of table `mysql`.`my_test` trx id 2864 lock_mode X locks rec but not gap
Record lock, heap no 2 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000001; asc ;;
1: len 6; hex 000000000b23; asc #;;
2: len 7; hex 9c000001500110; asc P ;;
3: len 8; hex 8000000000000001; asc ;;
4: len 8; hex 8000000000000001; asc ;;
5: len 8; hex 8000000000000001; asc ;;
6: len 8; hex 8000000000000001; asc ;;加锁逻辑为：
- 表上加IX锁；
- 唯一约束上加X锁；
- 行上加X锁；
RC+普通索引
select * from my_test where c = 3 for update;加锁信息如下：
3 lock struct(s), heap size 360, 5 row lock(s)
MySQL thread id 2, OS thread handle 0x1648, query id 166 localhost 127.0.0.1 root cleaning up
TABLE LOCK table `mysql`.`my_test` trx id 2872 lock mode IX
RECORD LOCKS space id 38 page no 5 n bits 72 index `idx_c` of table `mysql`.`my_test` trx id 2872 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 8; hex 8000000000000003; asc ;;
1: len 8; hex 8000000000000003; asc ;;
Record lock, heap no 5 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 8; hex 8000000000000003; asc ;;
1: len 8; hex 8000000000000004; asc ;;
RECORD LOCKS space id 38 page no 3 n bits 72 index `PRIMARY` of table `mysql`.`my_test` trx id 2872 lock_mode X locks rec but not gap
Record lock, heap no 4 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000003; asc ;;
1: len 6; hex 000000000b23; asc #;;
2: len 7; hex 9c000001500130; asc P 0;;
3: len 8; hex 8000000000000003; asc ;;
4: len 8; hex 8000000000000003; asc ;;
5: len 8; hex 8000000000000003; asc ;;
6: len 8; hex 8000000000000003; asc ;;
Record lock, heap no 5 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000004; asc ;;
1: len 6; hex 000000000b37; asc 7;;
2: len 7; hex ac0000015e0110; asc ^ ;;
3: len 8; hex 8000000000000004; asc ;;
4: len 8; hex 8000000000000004; asc ;;
5: len 8; hex 8000000000000003; asc ;;
6: len 8; hex 8000000000000004; asc ;;加锁逻辑为：
- 表上加IX锁；
- 索引上加X锁（2）；
- 行上加X锁（2）；
RC+无索引
select * from my_test where d = 1 for update;加锁信息：
2 lock struct(s), heap size 360, 5 row lock(s)
MySQL thread id 2, OS thread handle 0x1648, query id 172 localhost 127.0.0.1 root cleaning up
TABLE LOCK table `mysql`.`my_test` trx id 2874 lock mode IX
RECORD LOCKS space id 38 page no 3 n bits 72 index `PRIMARY` of table `mysql`.`my_test` trx id 2874 lock_mode X
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;
Record lock, heap no 2 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000001; asc ;;
1: len 6; hex 000000000b23; asc #;;
2: len 7; hex 9c000001500110; asc P ;;
3: len 8; hex 8000000000000001; asc ;;
4: len 8; hex 8000000000000001; asc ;;
5: len 8; hex 8000000000000001; asc ;;
6: len 8; hex 8000000000000001; asc ;;
Record lock, heap no 3 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000002; asc ;;
1: len 6; hex 000000000b23; asc #;;
2: len 7; hex 9c000001500120; asc P ;;
3: len 8; hex 8000000000000002; asc ;;
4: len 8; hex 8000000000000002; asc ;;
5: len 8; hex 8000000000000002; asc ;;
6: len 8; hex 8000000000000002; asc ;;
Record lock, heap no 4 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000003; asc ;;
1: len 6; hex 000000000b23; asc #;;
2: len 7; hex 9c000001500130; asc P 0;;
3: len 8; hex 8000000000000003; asc ;;
4: len 8; hex 8000000000000003; asc ;;
5: len 8; hex 8000000000000003; asc ;;
6: len 8; hex 8000000000000003; asc ;;
Record lock, heap no 5 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000004; asc ;;
1: len 6; hex 000000000b37; asc 7;;
2: len 7; hex ac0000015e0110; asc ^ ;;
3: len 8; hex 8000000000000004; asc ;;
4: len 8; hex 8000000000000004; asc ;;
5: len 8; hex 8000000000000003; asc ;;
6: len 8; hex 8000000000000004; asc ;;加锁逻辑：
- 表上加X锁；
- 行上加X锁（4）；
RR
在事务1中执行：
delete from my_test where c = 3;在事务2中执行：
insert into my_test values(6, 6, 6, 3, 6);查看事务2的锁信息如下：
mysql tables in use 1, locked 1
LOCK WAIT 2 lock struct(s), heap size 360, 1 row lock(s), undo log entries 1
MySQL thread id 3, OS thread handle 0x2378, query id 198 localhost 127.0.0.1 root update
insert into my_test values(6, 6, 6, 3, 6)
------- TRX HAS BEEN WAITING 20 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 38 page no 5 n bits 72 index `idx_c` of table `mysql`.`my_test` trx id 2897 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;查看锁等待信息select * from information_schema.innodb_locks如下：
而对于lock_data：
If a gap lock is taken for key values or ranges above the largest value in the index, LOCK_DATA reports “supremum pseudo-record”.
根据上面的Next-Key算法分析，锁定的不只是单个记录，而是一个范围：[3, 4)。
死锁分析
首先模拟常见的死锁，事务1：
begin;
select * from my_test where id = 1 for update;事务2：
begin;
select * from my_test where id = 2 for update;事务1：
select * from my_test where id = 2 for update;此时处于锁等待，接着事务2：
select * from my_test where id = 1 for update;此时事务2会报死锁，然后将事务回滚，查看死锁信息如下：
------------------------
LATEST DETECTED DEADLOCK
------------------------
2016-11-22 21:45:37 2378
*** (1) TRANSACTION:
TRANSACTION 2899, ACTIVE 48 sec starting index read
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 360, 2 row lock(s)
MySQL thread id 2, OS thread handle 0x1648, query id 217 localhost 127.0.0.1 root statistics
select * from my_test where id = 2 for update
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 38 page no 3 n bits 80 index `PRIMARY` of table `mysql`.`my_test` trx id 2899 lock_mode X locks rec but not gap waiting
Record lock, heap no 3 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000002; asc ;;
1: len 6; hex 000000000b23; asc #;;
2: len 7; hex 9c000001500120; asc P ;;
3: len 8; hex 8000000000000002; asc ;;
4: len 8; hex 8000000000000002; asc ;;
5: len 8; hex 8000000000000002; asc ;;
6: len 8; hex 8000000000000002; asc ;;
*** (2) TRANSACTION:
TRANSACTION 2900, ACTIVE 28 sec starting index read, thread declared inside InnoDB 5000
mysql tables in use 1, locked 1
3 lock struct(s), heap size 360, 2 row lock(s)
MySQL thread id 3, OS thread handle 0x2378, query id 218 localhost 127.0.0.1 root statistics
select * from my_test where id = 1 for update
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 38 page no 3 n bits 80 index `PRIMARY` of table `mysql`.`my_test` trx id 2900 lock_mode X locks rec but not gap
Record lock, heap no 3 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000002; asc ;;
1: len 6; hex 000000000b23; asc #;;
2: len 7; hex 9c000001500120; asc P ;;
3: len 8; hex 8000000000000002; asc ;;
4: len 8; hex 8000000000000002; asc ;;
5: len 8; hex 8000000000000002; asc ;;
6: len 8; hex 8000000000000002; asc ;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 38 page no 3 n bits 80 index `PRIMARY` of table `mysql`.`my_test` trx id 2900 lock_mode X locks rec but not gap waiting
Record lock, heap no 2 PHYSICAL RECORD: n_fields 7; compact format; info bits 0
0: len 8; hex 8000000000000001; asc ;;
1: len 6; hex 000000000b23; asc #;;
2: len 7; hex 9c000001500110; asc P ;;
3: len 8; hex 8000000000000001; asc ;;
4: len 8; hex 8000000000000001; asc ;;
5: len 8; hex 8000000000000001; asc ;;
6: len 8; hex 8000000000000001; asc ;;
*** WE ROLL BACK TRANSACTION (2)从死锁信息上能看到事务分别拥有哪些锁、在申请哪些锁的时候互相等待了。
INSERT + Unique Key
还是沿用上面的表，开启事务1：
begin;
insert into my_test values (7, 7, 7, 0, 0);开启事务2：
begin;
insert into my_test values (8, 7, 7, 0, 0);此时会锁等待，再开启事务3：
begin;
insert into my_test values (9, 7, 7, 0, 0);此时的锁情况为：
原因：
事务1在插入是需要加X锁，而事务2、事务3在插入前需要做唯一性检查，所以需要加S锁，因为与X锁不兼容所以事务2、3处于等待状态。
接着事务1进行rollback话，事务2会插入成功，事务3会报死锁：
2016-11-22 22:36:36 1ea4
*** (1) TRANSACTION:
TRANSACTION 2904, ACTIVE 300 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 4 lock struct(s), heap size 1184, 2 row lock(s), undo log entries 1
MySQL thread id 3, OS thread handle 0x2378, query id 238 localhost 127.0.0.1 root update
insert into my_test values (8, 7, 7, 0, 0)
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 38 page no 4 n bits 72 index `unique_a_b` of table `mysql`.`my_test` trx id 2904 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;
*** (2) TRANSACTION:
TRANSACTION 2905, ACTIVE 290 sec inserting, thread declared inside InnoDB 1
mysql tables in use 1, locked 1
4 lock struct(s), heap size 1184, 2 row lock(s), undo log entries 1
MySQL thread id 6, OS thread handle 0x1ea4, query id 239 localhost 127.0.0.1 root update
insert into my_test values (9, 7, 7, 0, 0)
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 38 page no 4 n bits 72 index `unique_a_b` of table `mysql`.`my_test` trx id 2905 lock mode S
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 38 page no 4 n bits 72 index `unique_a_b` of table `mysql`.`my_test` trx id 2905 lock_mode X insert intention waiting
Record lock, heap no 1 PHYSICAL RECORD: n_fields 1; compact format; info bits 0
0: len 8; hex 73757072656d756d; asc supremum;;
*** WE ROLL BACK TRANSACTION (2)原因：
在事务1回滚后，事务2、3都获得了S锁，接着要执行INSERT要去获取X锁，事务2申请X锁时发现与事务3的S锁不兼容，事务3也是同样的情况，于是出现死锁。
特殊的唯一约束：主键也有相同的问题。
总结
对InnoDB了解的还是很少，最近把锁相关的突击了一下，抽时间把底层的实现也了解一下，其他部分再慢慢补上。