继续吹 MySQL 8.0~
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
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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 sbtestmysql: [Warning] Using a password on the command line interface can be insecure.ERROR 3032 (HY000): The server is currently in offline mode
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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 queryNo connection. Trying to reconnect...ERROR 3032 (HY000): The server is currently in offline modeERROR:Can't connect to the server
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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: 0.00FATAL: 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'
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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`@`%` |+----------------------------------------+
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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`@`%` |+--------------------------------------------------------------------+
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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 1mysql -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 3mysql -h127.0.0.1 -uu1 -pxx -P4306 sbtest[u1@db160] [sbtest]>select 1;ERROR 2013 (HY000): Lost connection to MySQL server during queryNo connection. Trying to reconnect...ERROR 3032 (HY000): The server is currently in offline modeERROR:Can't connect to the server
# u2用户不会被断开连接$ fg 2mysql -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 sbtestmysql: [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 sbtestmysql: [Warning] Using a password on the command line interface can be insecure.ERROR 3032 (HY000): The server is currently in offline mode
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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
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