一、死锁案例
MySQL版本：Percona MySQL Server 5.7.19
隔离级别：可重复读（RR）
业务逻辑：并发下按某个索引字段先delete记录，再insert记录
比如：
begin;
delete from tb where order_id = xxx;
insert into tb(order_id) values(xxx);
commit;
二、MySQL锁基本概念
S：共享锁（行级锁）
X：排他锁（行级锁）
IS：意向共享锁（表级锁）
IX：意向排他锁（表级锁）
以上4种锁的兼容性见下表：
锁模式兼容性表
- gap锁与gap锁之间不冲突
- rec insert intention(插入意向锁）与gap锁冲突。
三、模拟复现死锁
打开参数，从innodb status获取更多的锁信息。
set GLOBAL innodb_status_output_locks=ON;
表结构：
CREATE TABLE `tb` (
`order_id` int(11) DEFAULT NULL,
KEY `idx_order_id` (`order_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
表中数据：
mysql> select * from tb;
+----------+
| order_id |
+----------+
| 10 |
| 20 |
+----------+
2 rows in set (0.00 sec)
事务执行步骤：
| session1 | session2 |
|---|---|
| begin | |
| begin | |
| delete from tb where order_id=15; | |
| delete from tb where order_id=15; | |
| insert into tb select 15;(等待锁) | |
| insert into tb select 15;(死锁) |
- 当session1执行delete from tb where order_id=15;，由于条件order_id=15的记录不存在，session1 获得2个锁结构，分别是意向排他锁IX（表级锁）、gap锁（行级锁），如下：
---TRANSACTION 1055191443, ACTIVE 20 sec
2 lock struct(s), heap size 1136, 1 row lock(s)
MySQL thread id 315642, OS thread handle 139960342456064, query id 150462030 localhost root
TABLE LOCK table `db`.`tb` trx id 1055191443 lock mode IX
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191443 lock_mode X locks gap before rec
- 当session2执行delete from tb where order_id=15;，同样由于order_id=15的记录不存在，session2 也获得2个锁结构，分别是意向排他锁IX（表级锁）、gap锁（行级锁），如下：
---TRANSACTION 1055191444, ACTIVE 3 sec
2 lock struct(s), heap size 1136, 1 row lock(s)
MySQL thread id 315336, OS thread handle 139960562685696, query id 150462412 localhost root
TABLE LOCK table `db`.`tb` trx id 1055191444 lock mode IX
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191444 lock_mode X locks gap before rec
- 当session2执行insert into tb select 15;， session2 已经获取到IX锁，gap锁，等待 rec insert intention（插入意向锁）
---TRANSACTION 1055191444, ACTIVE 68 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 315336, OS thread handle 139960562685696, query id 150462778 localhost root executing
insert into tb select 15
------- TRX HAS BEEN WAITING 2 SEC FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191444 lock_mode X locks gap before rec insert intention waiting
------------------
TABLE LOCK table `db`.`tb` trx id 1055191444 lock mode IX
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191444 lock_mode X locks gap before rec
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191444 lock_mode X locks gap before rec insert intention waiting
- 当session1执行insert into tb select 15;，session1 已获取到IX锁，gap锁， 等待rec insert intention（插入意向锁）， session1, session2 都在等待插入意向锁， 插入意向锁与gap锁冲突，双方都没有释放gap锁，又都在等待插入意向锁，死锁发生。
LATEST DETECTED DEADLOCK
------------------------
2018-11-03 17:15:11 0x7f4b0e7ea700
*** (1) TRANSACTION:
TRANSACTION 1055191444, ACTIVE 135 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 315336, OS thread handle 139960562685696, query id 150462778 localhost root executing
insert into tb select 15
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191444 lock_mode X locks gap before rec insert intention waiting
*** (2) TRANSACTION:
TRANSACTION 1055191443, ACTIVE 201 sec inserting, thread declared inside InnoDB 5000
mysql tables in use 1, locked 1
3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 315642, OS thread handle 139960342456064, query id 150463172 localhost root executing
insert into tb select 15
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191443 lock_mode X locks gap before rec
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191443 lock_mode X locks gap before rec insert intention waiting
*** WE ROLL BACK TRANSACTION (2)
四、案例扩展
以上死锁案例，业务代码逻辑是多线程并发下，有可能多个线程会执行相同order_id的job，比如两个线程执行的order_id 都是15。
另外一种情况，多个线程间，不会执行到相同order_id的情况，也可能发生死锁。比如一个线程order_id=15，另外一个线程order_id=16，如下所示：
事务执行步骤：
| session1 | session2 |
|---|---|
| begin | |
| begin | |
| delete from tb where order_id=15; | |
| delete from tb where order_id=16; | |
| insert into tb select 16;(等待锁) | |
| insert into tb select 15;(死锁) |
锁情况与上述相同，不再赘述，死锁信息如下：
LATEST DETECTED DEADLOCK
------------------------
2018-11-03 17:28:30 0x7f4b0e667700
*** (1) TRANSACTION:
TRANSACTION 1055191450, ACTIVE 18 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 316221, OS thread handle 139960338228992, query id 150467652 localhost root executing
insert into tb select 16
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191450 lock_mode X locks gap before rec insert intention waiting
*** (2) TRANSACTION:
TRANSACTION 1055191449, ACTIVE 28 sec inserting, thread declared inside InnoDB 5000
mysql tables in use 1, locked 1
3 lock struct(s), heap size 1136, 2 row lock(s), undo log entries 1
MySQL thread id 316222, OS thread handle 139960340870912, query id 150467681 localhost root executing
insert into tb select 15
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191449 lock_mode X locks gap before rec
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 1337 page no 4 n bits 72 index idx_order_id of table `db`.`tb` trx id 1055191449 lock_mode X locks gap before rec insert intention waiting
*** WE ROLL BACK TRANSACTION (2)
五、解决方案
- 修改隔离级别为提交读（RC）
- 修改业务代码逻辑，删除记录之前，先select，确认该记录存在，再执行delete删除该记录。
