MySQL 8.0新特性 — 管理端口的使用简介

前言

下面这个报错，相信大多数童鞋都遇见过；那么碰到这个问题，我们应该怎么办呢？在MySQL 5.7及之前版本，出现“too many connection”报错，超级用户root也无法登录上去，除了重启实例，没有其他更好的解决办法；不过在MySQL 8.0版本中，是对连接管理做了一些优化，下面我们就来看一下。

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。

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