ERROR 1040 (HY000): Too many connections
连接管理
在MySQL 8.0版本中，对连接管理这一块，是先后做了两个比较大的改变：一个是允许额外连接，另一个是专用的管理端口。
额外连接
在MySQL 8.0版本中，在当前连接数达到最大连接数时，服务端允许1个额外连接，可以让具有CONNECTION_ADMIN权限的用户连接进来，下面简单测试一下。
(1)为了方便测试，先调整最大连接数
mysql> set global max_connections=3;
Query OK, 0 rows affected (0.00 sec)
(2)多开几个会话，以达到最大连接数
mysql> show processlist;
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
| 15 | event_scheduler | localhost | NULL | Daemon | 154190 | Waiting on empty queue | NULL |
| 54 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 55 | test | 127.0.0.1:59120 | NULL | Sleep | 19 | | NULL |
| 56 | test | 127.0.0.1:59136 | NULL | Sleep | 9 | | NULL |
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
4 rows in set (0.00 sec)
mysql> show global status like 'threads_connected';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Threads_connected | 3 |
+-------------------+-------+
4 rows in set (0.01 sec)
(3)普通用户test尝试连接，报错too many connections
$ mysql -utest -p -h127.0.0.1 -P10080
Enter password:
ERROR 1040 (08004): Too many connections
(4)超级用户root尝试连接成功
$ mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 60
Server version: 8.0.20 MySQL Community Server - GPL
Copyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.
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>
(5)再次查看当前连接数，为max_connections+1
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
| 15 | event_scheduler | localhost | NULL | Daemon | 155064 | Waiting on empty queue | NULL |
| 54 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 55 | test | 127.0.0.1:59120 | NULL | Sleep | 893 | | NULL |
| 56 | test | 127.0.0.1:59136 | NULL | Sleep | 883 | | NULL |
| 60 | root | localhost | NULL | Sleep | 141 | | NULL |
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
5 rows in set (0.00 sec)
mysql> show global status like 'threads_connected';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Threads_connected | 4 |
+-------------------+-------+
4 rows in set (0.00 sec)
(6)超级用户root再次尝试连接，也报错too many connections
$ mysql -uroot -p
Enter password:
ERROR 1040 (HY000): Too many connections
通过上面测试可知，在MySQL 8.0中，允许的连接数为max_connections+1，其中这1个额外连接，只允许具有CONNECTION_ADMIN权限的用户使用。通过这1个额外连接，DBA可以使用超级用户root连接，进行kill会话等管理操作，以避免直接重启实例，降低成本，提高效率。
管理端口
额外连接，在一定程度上，提供了出现too many connection问题时的临时解决手段，但额外数量只有1个，难免会有一些意外，出现类似"连接被抢用"、“终端异常掉线”等情况。因此，在MySQL 8.0.14版本中，又推出了一个非常重要的新特性——管理端口；它允许具有SERVICE_CONNECTION_ADMIN权限的用户，通过特定的IP和PORT连接上来，且没有连接数限制。
(1)先介绍下相关参数
admin_address：监听IP地址
admin_port：监听端口
create_admin_listener_thread：是否创建一个单独的线程来监听管理连接
(2)通过配置上述参数，即可启用管理端口
mysql> show global variables like 'admin%';
+---------------+-----------+
| Variable_name | Value |
+---------------+-----------+
| admin_address | 127.0.0.1 |
| admin_port | 33062 |
+---------------+-----------+
2 rows in set (0.00 sec)
# netstat -lntp | grep 33062
tcp 0 0 127.0.0.1:33062 0.0.0.0:* LISTEN 20042/mysqld
(3)接下来进行测试
mysql> show processlist;
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
| 15 | event_scheduler | localhost | NULL | Daemon | 168750 | Waiting on empty queue | NULL |
| 54 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 55 | test | 127.0.0.1:59120 | NULL | Sleep | 14579 | | NULL |
| 56 | test | 127.0.0.1:59136 | NULL | Sleep | 14569 | | NULL |
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
4 rows in set (0.00 sec)
mysql> show global status like 'threads_connected';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Threads_connected | 3 |
+-------------------+-------+
1 row in set (0.00 sec)
(4)普通用户test尝试连接，报错too many connections
$ mysql -utest -p -h127.0.0.1 -P10080
Enter password:
ERROR 1040 (08004): Too many connections
(5)超级用户root尝试通过管理端口连接成功
$ mysql -uroot -p -h127.0.0.1 -P33062
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 62
Server version: 8.0.20 MySQL Community Server - GPL
Copyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.
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>
(6)继续多开几个会话，使用超级用户root，通过管理端口连接成功，不受最大连接数max_connections限制
mysql> show processlist;
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
| 15 | event_scheduler | localhost | NULL | Daemon | 169035 | Waiting on empty queue | NULL |
| 54 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 55 | test | 127.0.0.1:59120 | NULL | Sleep | 14864 | | NULL |
| 56 | test | 127.0.0.1:59136 | NULL | Sleep | 14854 | | NULL |
| 62 | root | 127.0.0.1:47660 | NULL | Sleep | 151 | | NULL |
| 63 | root | 127.0.0.1:47760 | NULL | Sleep | 52 | | NULL |
| 64 | root | 127.0.0.1:47768 | NULL | Sleep | 43 | | NULL |
| 65 | root | 127.0.0.1:47780 | NULL | Sleep | 35 | | NULL |
| 66 | root | 127.0.0.1:47790 | NULL | Sleep | 24 | | NULL |
| 67 | root | 127.0.0.1:47800 | NULL | Sleep | 16 | | NULL |
| 68 | root | 127.0.0.1:47808 | NULL | Sleep | 8 | | NULL |
+----+-----------------+-----------------+------+---------+--------+------------------------+------------------+
11 rows in set (0.00 sec)
mysql> show global status like 'threads_connected';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| Threads_connected | 10 |
+-------------------+-------+
1 row in set (0.00 sec)
可以说，有了管理端口这个新功能，DBA再也不用担心too many connections的问题。
总结
在MySQL 8.0版本中，为了应对too many connections的场景，先后推出了额外连接和管理端口两个新功能，可以让DBA方便、快速地解决问题；不过，这始终是一个临时应急手段，最根本的原因还是要排查应用端的配置(并发限流、SQL性能、连接池配置等等)，以彻底规避此类问题。
