[toc]
---
## 背景
应用端需要生成依次递增的序列来做流水序号等，方案有1、redis /MySQL SEQUENCE引擎生成序列；2、MySQL中myisam表 replace into方式；3、MySQL中innodb表INSERT ... ON DUPLICATE KEY方式
## 分析
- redis /MySQL SEQUENCE引擎生成序列，但多个MySQL集群都有生成序列的需求，若出问题，影响范围大；redis /MySQL SEQUENCE中生成序列也增加了研发修改代码的成本，新项目可以使用这种方式
---
- MySQL中myisam表 replace into 是我们目前使用生成序列的方式（虽然是表锁，每秒生成的序列也满足得了需求），使用方式为
```
CREATE TABLE `test_sequence` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`val` tinyint(1) DEFAULT '0',
PRIMARY KEY (`id`),
UNIQUE KEY `val` (`val`)
) ENGINE=MyISAM;
>replace into test_sequence(val) values(99);
Query OK, 1 row affected (0.00 sec)
>select last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
| 1 |
+------------------+
1 row in set (0.00 sec)
>replace into test_sequence(val) values(99);
Query OK, 2 rows affected (0.00 sec)
>select last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
| 2 |
+------------------+
1 row in set (0.00 sec)
```
> 但存在问题：
> myisam表非事务存储引擎，备份存在不一致（恢复还原数据有不一致风险）；
> myisam也不是crash-safe的；
> gtid模式下，同一个事务中不能操作myisam表和innodb表
>
> 为什么不用innodb表replace into方式了？
> 该方式并发大时，存在发生死锁的风险
---
- MySQL中事务性 innodb表INSERT ... ON DUPLICATE KEY，是crash-safe ，看起来myisam生成序列的存在的问题它都没有！实际情况了？
使用方式：
```
CREATE TABLE `test_sequence2` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`val` tinyint(1) DEFAULT '0',
PRIMARY KEY (`id`),
UNIQUE KEY `val` (`val`)
) ENGINE=InnoDB;
00>insert into test_sequence2(val) values(99) on duplicate key update id=id+1;
Query OK, 1 row affected (0.00 sec)
39>select id from test_sequence2;
+---------+
| id |
+---------+
| 1 |
+---------+
1 row in set (0.00 sec)
22>insert into test_sequence2(val) values(99) on duplicate key update id=id+1;
Query OK, 2 rows affected (0.00 sec)
25>select id from test_sequence2;
+---------+
| id |
+---------+
| 2 |
+---------+
1 row in set (0.00 sec)
```
## 测试
普通机械磁盘机器
MySQL5.7.16
RR隔离级别
sysbench 自定义sql语句测试tps（每秒生成多少序列）
- myisam replace into 方式
```
cd /usr/share/sysbench/tests
sysbench ./test_myisam.lua --mysql-host=127.0.0.1 --mysql-port=3701 --mysql-db=test --mysql-user=sysbench --mysql-password=sysbench --tables=1 --threads=10 --time=30 --report-interval=5 run
```
- innodb INSERT ... ON DUPLICATE KEY UPDATE方式
```
cd /usr/share/sysbench/tests
sysbench ./test_innodb.lua --mysql-host=127.0.0.1 --mysql-port=3701 --mysql-db=test --mysql-user=sysbench --mysql-password=sysbench --tables=1 --threads=10 --time=30 --report-interval=5 run
```
| |myisam replace into |innodb insert..on duplicate |
| --- | --- | --- |
| 1并发线程 | 124 tps | 122 tps |
| 10并发线程 | 123 tps | 121 tps |
|20并发线程 | 125 tps |104 tps |
|30并发线程 |127 tps |67 tps |
|40并发线程 |127 tps | 33 tps |
- **可见myisam随着并发线程数的增加，replace into tps保持不变，原因是myisam是表锁，同一时刻，该表只能写或者只能读**
- **innodb表随着并发数的上升，insert..on duplicate tps不升反降，行锁之前的争用变大了 造成锁等待**
- **本次测试机器配置差，结果有些参考性，线上机器配置更好**
---
***注意 mysqlslap 压测innodb表40个并发线程时可能会出现死锁（RC隔离级别也是），死锁详细见最后***
***为什么sysbench40 并发线程测试没有出现过死锁？难道sysbench并发线程不是同一时刻发出的？~_~***
```
/usr/local/mysql/bin/mysqlslap -usysbench -h127.0.0.1 -P3701 -p --concurrency=40 --iterations=1 --create-schema=test --query='insert into test_sequence2(val) values(99) on duplicate key update id=id+1;select id from test_sequence2;'
/usr/local/mysql/bin/mysqlslap: Cannot run query insert into test_sequence2(val) values(99) on duplicate key update id=id+1;select id from test_sequence2; ERROR : Deadlock found when trying to get lock; try restarting transaction
```
## 结论
- ***myisam表 replace into生成序列是稳定的方法，不管并发线程数多少，生成序列速度是稳定的，但myisam表存在缺陷问题***
- ***innodb表 inert on duplicate 生成序列适合并发线程数少情况，并发线程数多会出现死锁 生成序列速度下降情况***
- ***若要求生成序列的速度快，可用redis /MySQL SEQUENCE方式***
**死锁日志**
```
LATEST DETECTED DEADLOCK
------------------------
2020-02-11 11:03:11 0x7f6a0c643700
*** (1) TRANSACTION:
TRANSACTION 39260727, ACTIVE 1 sec inserting
mysql tables in use 1, locked 1
LOCK WAIT 28 lock struct(s), heap size 3520, 26 row lock(s), undo log entries 1
MySQL thread id 460828, OS thread handle 140093451958016, query id 21296424 127.0.0.1 root update
insert into test_sequence2(val) values(99) on duplicate key update id=id+1
*** (1) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 48 page no 4 n bits 72 index val of table `test`.`test_sequence2` trx id 39260727 lock_mode X waiting
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 1; hex e3; asc ;;
1: len 8; hex 000000000000001a; asc ;;
*** (2) TRANSACTION:
TRANSACTION 39260729, ACTIVE 1 sec updating or deleting, thread declared inside InnoDB 5000
mysql tables in use 1, locked 1
29 lock struct(s), heap size 3520, 27 row lock(s), undo log entries 1
MySQL thread id 460835, OS thread handle 140093451155200, query id 21296425 127.0.0.1 root update
insert into test_sequence2(val) values(99) on duplicate key update id=id+1
*** (2) HOLDS THE LOCK(S):
RECORD LOCKS space id 48 page no 4 n bits 72 index val of table `test`.`test_sequence2` trx id 39260729 lock_mode X
Record lock, heap no 4 PHYSICAL RECORD: n_fields 2; compact format; info bits 0
0: len 1; hex e3; asc ;;
1: len 8; hex 000000000000001a; asc ;;
*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 48 page no 3 n bits 168 index PRIMARY of table `test`.`test_sequence2` trx id 39260729 lock_mode X waiting
Record lock, heap no 37 PHYSICAL RECORD: n_fields 4; compact format; info bits 0
0: len 8; hex 000000000000001b; asc ;;
1: len 6; hex 000002571237; asc W 7;;
2: len 7; hex b6000001680110; asc h ;;
3: len 1; hex e3; asc ;;
*** WE ROLL BACK TRANSACTION (1)
```
**自定义sysbench脚本**
less test_myisam/innodb.lua
```
require("oltp_common")
function thread_init(thread_id)
drv=sysbench.sql.driver()
con=drv:connect()
end
function event(thread_id)
local vid1
local dbprefix
con:query('replace into test_sequence(val) values(99)')
con:query('select last_insert_id()')
##innodb insert..on duplicate 语句
#con:query('insert into test_sequence2(val) values(99) on duplicate key update id=id+1')
#con:query('select id from test_sequence2;')
end
function thread_done()
con:disconnect()
end
```
