在以前，当需要对MySQL数据库进行维护操作时，通常需要先进行主从切换，然后修改设置并重启实例，关闭网络监听，只允许从本地socket方式登入，再进行相应的维护操作；有时候甚至还要修改相应的防火墙，或者干脆关闭前端业务服务，总体比较麻烦。
从MySQL 5.7开始，支持设置为离线模式（offline_mode），再有维护操作需求就不用这么麻烦了。只需在线动态修改，可立即生效，非常的简单粗暴：
- mysql> set global offline_mode = on; -- 打开离线模式，拒绝外部请求
- mysql> set global offline_mode = off; -- 关闭离线模式，允许外部连接请求
当设置为离线模式后，普通用户将无法继续发起连接请求，甚至当前正在执行的SQL也会立即被终止并被断开连接。
1. 无法创建新连接
- $ mysql -h127.0.0.1 -uyejr -pxx -P4306 sbtest
- mysql: [Warning] Using a password on the command line interface can be insecure.
- ERROR 3032 (HY000): The server is currently in offline mode
2. 即便是普通用户通过本地socket连接，当启用离线模式后，也会被断开
- $ mysql -S/data/MySQL/mysql.sock
- ...
- mysql> show processlist;
- +-----+------+-----------+------+---------+------+-------+------------------+
- | Id | User | Host | db | Command | Time | State | Info |
- +-----+------+-----------+------+---------+------+-------+------------------+
- | 304 | yejr | localhost | NULL | Query | 0 | init | show processlist |
- +-----+------+-----------+------+---------+------+-------+------------------+
- 1 row in set (0.00 sec)
- [sbtest]>select *,sleep(10) from t1 limit 3; -- 正在运行的SQL会立即被终止，并断开连接
- ERROR 2013 (HY000): Lost connection to MySQL server during query
- No connection. Trying to reconnect...
- ERROR 3032 (HY000): The server is currently in offline mode
- ERROR:
- Can't connect to the server
3. 正在运行中的sysbench压测，也会被立即断开
- [ 1s ] thds: 16 tps: 442.02 qps: 9078.28 (r/w/o: 6382.37/1795.94/899.96) lat (ms,99%,99%,99.9%): 150.29/150.29/150.29 err/s: 0.00 reconn/s: 0.00
- [ 2s ] thds: 16 tps: 471.23 qps: 9387.56 (r/w/o: 6576.19/1868.91/942.46) lat (ms,99%,99%,99.9%): 61.08/61.08/65.65 err/s: 0.00 reconn/s: 0.00
- [ 3s ] thds: 16 tps: 386.03 qps: 7712.68 (r/w/o: 5399.48/1541.14/772.07) lat (ms,99%,99%,99.9%): 82.96/82.96/84.47 err/s: 0.00 reconn/s: 0.00
- [ 4s ] thds: 16 tps: 547.00 qps: 10894.97 (r/w/o: 7609.98/2190.99/1094.00) lat (ms,99%,99%,99.9%): 65.65/65.65/68.05 err/s: 0.00 reconn/s: 0.00
- FATAL: mysql_drv_query() returned error 2013 (Lost connection to MySQL server during query) for query 'COMMIT'
- (last message repeated 1 times)
- FATAL: mysql_drv_query() returned error 2013 (Lost connection to MySQL server during query) for query 'SELECT c FROM sbtest1 WHERE id=4822870'
- FATAL: mysql_drv_query() returned error 2013 (Lost connection to MySQL server during query) for query 'COMMIT'
- FATAL: mysql_drv_query() returned error 2013 (Lost connection to MySQL server during query) for query 'UPDATE sbtest1 SET k=k+1 WHERE id=2265001'
- FATAL: mysql_drv_query() returned error 2013 (Lost connection to MySQL server during query) for query 'SELECT c FROM sbtest8 WHERE id BETWEEN 3389984 AND 3390083'
另外，从MySQL 8.0开始，对于离线模式又做了些改进和完善，比如新引入 CONNECTION_ADMIN权限等，细化离线模式的权限管理模式。
简单几点小结关于离线模式：
- 必须要有 CONNECTION_ADMIN 以及 CONNECTION_ADMIN权限 或者 SUPER权限（SUPER权限在未来会被废弃，而细分成更多细粒度权限），才能在线设置离线模式。
- 复制线程不会受到离线模式影响，还能正常工作。
- 当设置为离线模式时，没有授予 CONNECTION_ADMIN 或 SUPER 权限的普通用户，正在执行的SQL会被立即终止，连接也会被立即断开。
- 当设置为离线模式时，拥有 CONNECTION_ADMIN 或 SUPER 权限的用户，不会被断开连接。
- 当设置离线模式的用户不具备 SYSTEM_USER 权限（只拥有 CONNECTION_ADMIN 以及 CONNECTION_ADMIN权限）的话，拥有 SYSTEM_USER 权限的活跃用户连接不会被断开（因为想要断开 SYSTEM_USER 权限级别用户连接同样需要至少有 SYSTEM_USER 权限），详见下面的案例。
有 u1 和 u2 两个用户，授权模式不同
- mysql> show grants for u1;
- +----------------------------------------+
- | Grants for u1@% |
- +----------------------------------------+
- | GRANT USAGE ON *.* TO `u1`@`%` |
- | GRANT SELECT ON `sbtest`.* TO `u1`@`%` |
- +----------------------------------------+
- mysql> show grants for u2;
- +----------------------------------------+
- | Grants for u2@% |
- +----------------------------------------+
- | GRANT USAGE ON *.* TO `u2`@`%` |
- | GRANT SYSTEM_USER ON *.* TO `u2`@`%` |
- | GRANT SELECT ON `sbtest`.* TO `u2`@`%` |
- +----------------------------------------+
用户 yejr 的授权模式如下
- +--------------------------------------------------------------------+
- | Grants for yejr@% |
- +--------------------------------------------------------------------+
- | GRANT USAGE ON *.* TO `yejr`@`%` |
- | GRANT CONNECTION_ADMIN,SYSTEM_VARIABLES_ADMIN ON *.* TO `yejr`@`%` |
- | GRANT ALL PRIVILEGES ON `sbtest`.* TO `yejr`@`%` |
- +--------------------------------------------------------------------+
当 yejr 用户设置离线模式后，u2 用户的连接不会被断开（但不能再建立新连接），而 u1 用户的连接会被断开
- # 三个用户先分别建立连接
- $ jobs
- [1] Stopped mysql -h127.0.0.1 -uyejr -pxx -P4306 sbtest
- [2]- Stopped mysql -h127.0.0.1 -uu2 -pxx -P4306 sbtest
- [3]+ Stopped mysql -h127.0.0.1 -uu1 -pxx -P4306 sbtest
- # 设置离线模式
- $ fg 1
- mysql -h127.0.0.1 -uyejr -pxx -P4306 sbtest
- [yejr@db160] [sbtest]>set global offline_mode=on;
- Query OK, 0 rows affected (0.00 sec)
- # u1用户被断开连接
- $ fg 3
- mysql -h127.0.0.1 -uu1 -pxx -P4306 sbtest
- [u1@db160] [sbtest]>select 1;
- ERROR 2013 (HY000): Lost connection to MySQL server during query
- No connection. Trying to reconnect...
- ERROR 3032 (HY000): The server is currently in offline mode
- ERROR:
- Can't connect to the server
- # u2用户不会被断开连接
- $ fg 2
- mysql -h127.0.0.1 -uu2 -pxx -P4306 sbtest
- [u2@db160] [sbtest]>select 1;
- +---+
- | 1 |
- +---+
- | 1 |
- +---+
- # 但u1/u2用户均不能再建立新链接
- $ mysql -h127.0.0.1 -uu2 -pxx -P4306 sbtest
- mysql: [Warning] Using a password on the command line interface can be insecure.
- ERROR 3032 (HY000): The server is currently in offline mode
- $ mysql -h127.0.0.1 -uu1 -pxx -P4306 sbtest
- mysql: [Warning] Using a password on the command line interface can be insecure.
- ERROR 3032 (HY000): The server is currently in offline mode
是不是有点好玩呀~
结合前面的两篇文章 MySQL 8.0不再担心被垃圾SQL搞爆内存 以及 InnoDB buffer pool size进度更透明 可以看到MySQL 8.0在各个细节方面做的是越来越好了。
延伸阅读
- #sysvar_offline_mode, https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html
- Changes in MySQL 8.0.31, https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-31.html
原文地址：https://mp.weixin.qq.com/s/5atNn31T6ol5NLCn9OT1nQ
