RDS MySQL 8.0 语句级并发控制
背景
为了应对突发的数据库请求流量，资源消耗过载的语句访问，SQL 访问模型的变化， 并保持 MySQL 实例持续稳定运行，AliSQL 版本设计了基于语句规则的并发控制，Statement Concurrency Control，以下简称 CCL，有效控制匹配某种规则的并发度，并提供了一组工具包（DBMS_CCL package) 方便快捷使用。
规则设计
CCL 规则一共定义了三个维度的特征：
1）SQL command
根据 statement 的类型，例如 ‘SELECT', 'UPDATE', 'INSERT', 'DELETE';
2) Object
根据 statement 操作的对象进行控制， 例如 TABLE，VIEW;
3）keywords
根据 statement 语句的关键字进行控制;
CCL 根据规则的定义，设计了一个系统表，mysql.concurrency_control 持久化保存 CCL rule：
Concurrency_control
CREATE TABLE `concurrency_control` (
`Id` bigint(20) NOT NULL AUTO_INCREMENT,
`Type` enum('SELECT','UPDATE','INSERT','DELETE') NOT NULL DEFAULT 'SELECT',
`Schema_name` varchar(64) COLLATE utf8_bin DEFAULT NULL,
`Table_name` varchar(64) COLLATE utf8_bin DEFAULT NULL,
`Concurrency_count` bigint(20) DEFAULT NULL,
`Keywords` text COLLATE utf8_bin,
`State` enum('N','Y') NOT NULL DEFAULT 'Y',
`Ordered` enum('N','Y') NOT NULL DEFAULT 'N',
PRIMARY KEY (`Id`)
) /*!50100 TABLESPACE `mysql` */ ENGINE=InnoDB
DEFAULT CHARSET=utf8 COLLATE=utf8_bin
STATS_PERSISTENT=0 COMMENT='Concurrency control'COLUMNS
-
"Type"
- 用来定义 SQL command
-
"Schema_name" && "Table_name"
- 用来定义 Object
-
"Keywords"
- 用来定义关键字，可使用 ';' 分隔符多个关键字
-
"Concurrency_count"
- 用来定义并发度
-
"State"
- 表示这条规则是否 active
-
"Ordered"
- 表示keywords中多个关键字是否按顺序匹配
用户可以直接操作这个表来定义规则，也可以使用 DBMS_CCL 工具包来操作 CCL rule。
管理接口
为了便捷的管理 CCL rule，AliSQL 在 DBMS_CCL package 中定义了四个 native procedure 来管理；
1）Add CCL rule
dbms_ccl.add_ccl_rule(type=>, schema=>, table=>, Concurrency_count=>, keywords=>);
增加规则（包括表和内存）例如：
1. 增加 SELECT 语句的并发度为 10；
mysql> call dbms_ccl.add_ccl_rule('SELECT', '', '', 10, '');
Query OK, 0 rows affected (0.00 sec)
2. 增加 SELECT 语句，并在语句中出现关键字 key1 的并发度为 20
mysql> call dbms_ccl.add_ccl_rule('SELECT', '', '', 20, 'key1');
Query OK, 0 rows affected (0.00 sec)
3. 增加 test.t 表的 SELECT 语句的并发读为 20；
mysql> call dbms_ccl.add_ccl_rule('SELECT', 'test', 't', 30, '');
Query OK, 0 rows affected (0.00 sec)规则的匹配按照 3 > 2 > 1 的优先级顺序进行匹配。
2）Delete CCL rule
dbms_ccl.del_ccl_rule(rule_id=> );
删除规则（包括内存和表中）例如：
1. 删除 rule id = 15 的 CCL rule
mysql> call dbms_ccl.del_ccl_rule(15);
Query OK, 0 rows affected (0.01 sec)
2. 如果删除的rule 不存在，语句报相应的 warning
mysql> call dbms_ccl.del_ccl_rule(100);
Query OK, 0 rows affected, 2 warnings (0.00 sec)
mysql> show warnings;
+---------+------+----------------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------------+
| Warning | 7514 | Concurrency control rule 100 is not found in table |
| Warning | 7514 | Concurrency control rule 100 is not found in cache |
+---------+------+----------------------------------------------------+3) Show CCL rule
dbms_ccl.show_ccl_rule();
展示在内存中 active rule 的情况，例如：
mysql> call dbms_ccl.show_ccl_rule();
+------+--------+--------+-------+-------+-------+-------------------+---------+---------+----------+----------+
| ID | TYPE | SCHEMA | TABLE | STATE | ORDER | CONCURRENCY_COUNT | MATCHED | RUNNING | WAITTING | KEYWORDS |
+------+--------+--------+-------+-------+-------+-------------------+---------+---------+----------+----------+
| 17 | SELECT | test | t | Y | N | 30 | 0 | 0 | 0 | |
| 16 | SELECT | | | Y | N | 20 | 0 | 0 | 0 | key1 |
| 18 | SELECT | | | Y | N | 10 | 0 | 0 | 0 | |
+------+--------+--------+-------+-------+-------+-------------------+---------+---------+----------+----------+除了 rule 本身的属性之外，增加了三个数字统计：
1）MATCHED
规则匹配成功次数
2）RUNNING
在此规则下，正在 run 的线程数
3）WAITING
在此规则下，正在 wait的线程数
4）Flush CCL rule
dbms_ccl.flush_ccl_rule();
如果直接操作了concurrency_control table 修改规则， 不能立即生效，可以调用 flush，重新生效。例如：
mysql> update mysql.concurrency_control set CONCURRENCY_COUNT = 15 where id = 18;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> call dbms_ccl.flush_ccl_rule();
Query OK, 0 rows affected (0.00 sec)压力测试
测试场景
1）设计三条规则
Rule 1：对 sbtest1 表 应用 Object rule 控制
call dbms_ccl.add_ccl_rule('SELECT', 'test', 'sbtest1', 3, '');Rule 2: 对sbtest2 表 应用 keyword rule 控制
call dbms_ccl.add_ccl_rule('SELECT', '', '', 2, 'sbtest2');Rule 3: 对sbtest3 表 应用 SQL command 控制
call dbms_ccl.add_ccl_rule('SELECT', '', '', 2, '');2）使用 sysbench 进行测试
- 64 threads
- 4 tables
- select.lua
查看规则并发使用情况，可以到到 running 和 waiting 的数量：
mysql> call dbms_ccl.show_ccl_rule();
+------+--------+--------+---------+-------+-------+-------------------+---------+---------+----------+----------+
| ID | TYPE | SCHEMA | TABLE | STATE | ORDER | CONCURRENCY_COUNT | MATCHED | RUNNING | WAITTING | KEYWORDS |
+------+--------+--------+---------+-------+-------+-------------------+---------+---------+----------+----------+
| 20 | SELECT | test | sbtest1 | Y | N | 3 | 389 | 3 | 9 | |
| 21 | SELECT | | | Y | N | 2 | 375 | 2 | 14 | sbtest2 |
| 22 | SELECT | | | Y | N | 2 | 519 | 2 | 34 | |
+------+--------+--------+---------+-------+-------+-------------------+---------+---------+----------+----------+
3 rows in set (0.00 sec)查看线程运行情况： 大部分处在 Concurrency control waitting 状态。
mysql> show processlist;
+-----+-----------------+-----------------+------+---------+------+------------------------------+--------------------------------------+
| Id | User | Host | db | Command | Time | State | Info |
+-----+-----------------+-----------------+------+---------+------+------------------------------+--------------------------------------+
| 72 | root | localhost:33601 | NULL | Query | 0 | starting | show processlist |
| 171 | u1 | localhost:60120 | test | Query | 2 | Concurrency control waitting | SELECT pad FROM sbtest3 WHERE id=51 |
| 172 | u1 | localhost:60128 | test | Query | 5 | Concurrency control waitting | SELECT pad FROM sbtest4 WHERE id=35 |
| 174 | u1 | localhost:60385 | test | Query | 4 | Concurrency control waitting | SELECT pad FROM sbtest3 WHERE id=54 |
| 178 | u1 | localhost:60136 | test | Query | 12 | Concurrency control waitting | SELECT pad FROM sbtest4 WHERE id=51 |
| 179 | u1 | localhost:60149 | test | Query | 5 | Concurrency control waitting | SELECT pad FROM sbtest2 WHERE id=51 |
| 182 | u1 | localhost:60124 | test | Query | 1 | Concurrency control waitting | SELECT pad FROM sbtest4 WHERE id=51 |
| 183 | u1 | localhost:60371 | test | Query | 5 | User sleep | SELECT pad FROM sbtest2 WHERE id=51 |
| 184 | u1 | localhost:60133 | test | Query | 4 | Concurrency control waitting | SELECT pad FROM sbtest3 WHERE id=51 |
| 190 | u1 | localhost:60406 | test | Query | 5 | Concurrency control waitting | SELECT pad FROM sbtest3 WHERE id=51 |
| 191 | u1 | localhost:60402 | test | Query | 1 | Concurrency control waitting | SELECT pad FROM sbtest4 WHERE id=51 |
| 192 | u1 | localhost:60131 | test | Query | 2 | User sleep | SELECT pad FROM sbtest1 WHERE id=51 |
......使用规则和风险
- Concurrency_control 被设计成不产生 BINLOG，所以对于 CCL 的操作只影响当前实例。
- 对于 DML 的并发控制，可能存在事务锁死锁的情况， 除了 CCL 提供了超时机制，
同时等待中的线程也会响应事务超时和线程 KILL 操作，以应对死锁可能。
