MYSQL 通过管理端口处理ERROR 1040 (HY000): Too many connections 问题
适用范围
MYSQL 8.0.14+
问题概述
应用连接DB时，出现 “ERROR 1040 (HY000): Too many connections” 错误，此时在低版要么是手动停掉应用释放连接 ，要么是重启DB以释放连接。
问题原因
出现这个报错的原因有两种情况：
- 一种是单个用户的连接数超过“max_user_connections”参数定义值。
- 另一种情况是，所有应用的连接数超过“max_connections”参数定义值。
解决方案
在MySQL8以前的版本，由于应用用户和管理用户共同使用同一个端口服务，没有进行隔离，如果使用不规范时，很容易造成DBA无法用root用户连接数据库，进行故障定位。
到MySQL8的版本，MySQL官方考虑到这个问题，于是就给数据库管理人员独立起了一个管理端口服务，这样应用用户和管理用户访问的端口进行隔离，互不影响。
由于管理端口默认是33062 ，如果在多实例环境则需要手动去更改该参数。在my.cnf配置文件中添加3个参数
admin_address=127.0.0.1
admin_port=33306
create_admin_listener_thread=1
重启DB即可
测试效果
下面就来测试一下，当报"ERROR 1040 (HY000): Too many connections"错误之后，DBA是否还能使用管理端口连接数据库
1.模拟会话总数已经达到max_connections参数定义阀值
mysql> show variables like '%connections%';
+------------------------+-------+
| Variable_name | Value |
+------------------------+-------+
| max_connections | 3 |
| max_user_connections | 0 |
| mysqlx_max_connections | 100 |
+------------------------+-------+
3 rows in set (0.01 sec)
mysql> show global status like 'Threads_connected';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Threads_connected | 3 |
+-------------------+-------+
1 row in set (0.01 sec)
#模拟连接数过多
[root@s2ahuoracle02 ~]# mysql -u root -proot -h 127.0.0.1 -P3333
mysql: [Warning] Using a password on the command line interface can be insecure.
ERROR 1040 (HY000): Too many connections
2.使用MySQL数据库额外提供的端口，root连接数据库，不占用连接数
[root@s2ahuoracle02 ~]# mysql -u root -proot -h 127.0.0.1 -P33306
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 223
Server version: 8.0.30-commercial MySQL Enterprise Server - Commercial
Copyright (c) 2000, 2022, Oracle and/or its affiliates.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> show global status like 'Threads_connected';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Threads_connected | 4 |
+-------------------+-------+
1 row in set (0.01 sec)
可以看到root用户还可以通过管理端口33306进行登录了，当前的连接数总数也到了4个，超过了max_connections定义的3个。
3.通过SQL统计当前会话连接，以分析出应用连接问题
#通过用户分组统计连接数
mysql> select USER, count(*) CONNECT_COUNT FROM information_schema.processlist GROUP BY USER ORDER BY count(*) DESC ;
+-----------------+---------------+
| USER | CONNECT_COUNT |
+-----------------+---------------+
| root | 4 |
| event_scheduler | 1 |
+-----------------+---------------+
2 rows in set (0.07 sec)
#查看每个host的当前连接数和总连接数
mysql> SELECT * FROM performance_schema.hosts;
+-----------+---------------------+-------------------+
| HOST | CURRENT_CONNECTIONS | TOTAL_CONNECTIONS |
+-----------+---------------------+-------------------+
| NULL | 75 | 93 |
| localhost | 2 | 8 |
| 127.0.0.1 | 3 | 400 |
+-----------+---------------------+-------------------+
3 rows in set (0.24 sec)
# 按照登录用户+登录服务器查看登录信息
mysql> SELECT
USER as login_user,
LEFT(HOST,POSITION(':' IN HOST)-1) AS login_ip,
count(1) as login_count
FROM information_schema.PROCESSLIST P
-- WHERE P.USER IN('root')
GROUP BY USER,LEFT(HOST,POSITION(':' IN HOST)-1);
+-----------------+-----------+-------------+
| login_user | login_ip | login_count |
+-----------------+-----------+-------------+
| root | 127.0.0.1 | 2 |
| event_scheduler | | 1 |
| root | | 1 |
+-----------------+-----------+-------------+
3 rows in set (0.01 sec)
# 按照登录用户+数据库+登录服务器查看登录信息
SELECT
DB as database_name,
USER as login_user,
LEFT(HOST,POSITION(':' IN HOST)-1) AS login_ip,
count(1) as login_count
FROM information_schema.PROCESSLIST P
-- WHERE P.USER IN('root')
GROUP BY DB,USER,LEFT(HOST,POSITION(':' IN HOST)-1);
+---------------+-----------------+-----------+-------------+
| database_name | login_user | login_ip | login_count |
+---------------+-----------------+-----------+-------------+
| NULL | root | 127.0.0.1 | 2 |
| mysql | root | 127.0.0.1 | 1 |
| NULL | event_scheduler | | 1 |
| NULL | root | | 1 |
+---------------+-----------------+-----------+-------------+
4 rows in set (0.00 sec)
参考文档
https://dev.mysql.com/doc/refman/8.0/en/administrative-connection-interface.html
