一、死鎖案例

MySQL版本：Percona MySQL Server 5.7.19
隔離級(jí)別：可重復(fù)讀（RR）
業(yè)務(wù)邏輯：并發(fā)下按某個(gè)索引字段先delete記錄，再insert記錄

比如：

begin;
delete from tb where order_id = xxx;
insert into tb(order_id) values(xxx);
commit;

二、MySQL鎖基本概念

S：共享鎖（行級(jí)鎖）
X：排他鎖（行級(jí)鎖）
IS：意向共享鎖（表級(jí)鎖）
IX：意向排他鎖（表級(jí)鎖）

以上4種鎖的兼容性見下表：

鎖模式兼容性表

• gap鎖與gap鎖之間不沖突
• rec insert intention(插入意向鎖）與gap鎖沖突。

三、模擬復(fù)現(xiàn)死鎖

打開參數(shù)，從innodb status獲取更多的鎖信息。
set GLOBAL innodb_status_output_locks=ON;

表結(jié)構(gòu)：

 CREATE TABLE `tb` (
  `order_id` int(11) DEFAULT NULL,
  KEY `idx_order_id` (`order_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

表中數(shù)據(jù)：

mysql> select * from tb;
+----------+
| order_id |
+----------+
|       10 |
|       20 |
+----------+
2 rows in set (0.00 sec)

事務(wù)執(zhí)行步驟：

1. 當(dāng)session1執(zhí)行delete from tb where order_id=15;，由于條件order_id=15的記錄不存在，session1 獲得2個(gè)鎖結(jié)構(gòu)，分別是意向排他鎖IX（表級(jí)鎖）、gap鎖（行級(jí)鎖），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

2. 當(dāng)session2執(zhí)行delete from tb where order_id=15;，同樣由于order_id=15的記錄不存在，session2 也獲得2個(gè)鎖結(jié)構(gòu)，分別是意向排他鎖IX（表級(jí)鎖）、gap鎖（行級(jí)鎖），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

3. 當(dāng)session2執(zhí)行insert into tb select 15;， session2 已經(jīng)獲取到IX鎖，gap鎖，等待 rec insert intention（插入意向鎖）

---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

4. 當(dāng)session1執(zhí)行insert into tb select 15;，session1 已獲取到IX鎖，gap鎖， 等待rec insert intention（插入意向鎖）， session1, session2 都在等待插入意向鎖， 插入意向鎖與gap鎖沖突，雙方都沒有釋放gap鎖，又都在等待插入意向鎖，死鎖發(fā)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、案例擴(kuò)展

以上死鎖案例，業(yè)務(wù)代碼邏輯是多線程并發(fā)下，有可能多個(gè)線程會(huì)執(zhí)行相同order_id的job，比如兩個(gè)線程執(zhí)行的order_id 都是15。
另外一種情況，多個(gè)線程間，不會(huì)執(zhí)行到相同order_id的情況，也可能發(fā)生死鎖。比如一個(gè)線程order_id=15，另外一個(gè)線程order_id=16，如下所示：

事務(wù)執(zhí)行步驟：

鎖情況與上述相同，不再贅述，死鎖信息如下：

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、解決方案

1. 修改隔離級(jí)別為提交讀（RC）
2. 修改業(yè)務(wù)代碼邏輯，刪除記錄之前，先select，確認(rèn)該記錄存在，再執(zhí)行delete刪除該記錄。