数据库内核月报－ 2019/06 - MySQL · 最佳实践 · RDS MySQL 8.0 语句级并发控制 - 《数据库内核月报》

背景
规则设计

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。