下面這個(gè)報(bào)錯(cuò)，相信大多數(shù)童鞋都遇見過；那么碰到這個(gè)問題，我們應(yīng)該怎么辦呢？在MySQL 5.7及之前版本，出現(xiàn)“too many connection”報(bào)錯(cuò)，超級(jí)用戶root也無法登錄上去，除了重啟實(shí)例，沒有其他更好的解決辦法；不過在MySQL 8.0版本中，是對(duì)連接管理做了一些優(yōu)化，下面我們就來看一下。

ERROR 1040 (HY000): Too many connections連接管理

在MySQL 8.0版本中，對(duì)連接管理這一塊，是先后做了兩個(gè)比較大的改變：一個(gè)是允許額外連接，另一個(gè)是專用的管理端口。

在MySQL 8.0版本中，在當(dāng)前連接數(shù)達(dá)到最大連接數(shù)時(shí)，服務(wù)端允許1個(gè)額外連接，可以讓具有CONNECTION_ADMIN權(quán)限的用戶連接進(jìn)來，下面簡單測試一下。

（1）為了方便測試，先調(diào)整最大連接數(shù)

mysql> set global max_connections=3;Query OK, 0 rows affected (0.00 sec)

（2）多開幾個(gè)會(huì)話，以達(dá)到最大連接數(shù)

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）普通用戶test嘗試連接，報(bào)錯(cuò)too many connections

$ mysql -utest -p -h127.0.0.1 -P10080Enter password: ERROR 1040 (08004): Too many connections

（4）超級(jí)用戶root嘗試連接成功

$ mysql -uroot -pEnter password: Welcome to the MySQL monitor. Commands end with ; or g.Your MySQL connection id is 60Server version: 8.0.20 MySQL Community Server - GPLCopyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type ’help;’ or ’h’ for help. Type ’c’ to clear the current input statement.mysql>

（5）再次查看當(dāng)前連接數(shù)，為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）超級(jí)用戶root再次嘗試連接，也報(bào)錯(cuò)too many connections

$ mysql -uroot -pEnter password: ERROR 1040 (HY000): Too many connections

通過上面測試可知，在MySQL 8.0中，允許的連接數(shù)為max_connections+1，其中這1個(gè)額外連接，只允許具有CONNECTION_ADMIN權(quán)限的用戶使用。通過這1個(gè)額外連接，DBA可以使用超級(jí)用戶root連接，進(jìn)行kill會(huì)話等管理操作，以避免直接重啟實(shí)例，降低成本，提高效率。

額外連接，在一定程度上，提供了出現(xiàn)too many connection問題時(shí)的臨時(shí)解決手段，但額外數(shù)量只有1個(gè)，難免會(huì)有一些意外，出現(xiàn)類似'連接被搶用'、“終端異常掉線”等情況。因此，在MySQL 8.0.14版本中，又推出了一個(gè)非常重要的新特性——管理端口；它允許具有SERVICE_CONNECTION_ADMIN權(quán)限的用戶，通過特定的IP和PORT連接上來，且沒有連接數(shù)限制。

（1）先介紹下相關(guān)參數(shù)

admin_address：監(jiān)聽IP地址admin_port：監(jiān)聽端口create_admin_listener_thread：是否創(chuàng)建一個(gè)單獨(dú)的線程來監(jiān)聽管理連接

（2）通過配置上述參數(shù)，即可啟用管理端口

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 33062tcp 0 0 127.0.0.1:33062 0.0.0.0:* LISTEN 20042/mysqld

（3）接下來進(jìn)行測試

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）普通用戶test嘗試連接，報(bào)錯(cuò)too many connections

$ mysql -utest -p -h127.0.0.1 -P10080Enter password: ERROR 1040 (08004): Too many connections

（5）超級(jí)用戶root嘗試通過管理端口連接成功

$ mysql -uroot -p -h127.0.0.1 -P33062Enter password: Welcome to the MySQL monitor. Commands end with ; or g.Your MySQL connection id is 62Server version: 8.0.20 MySQL Community Server - GPLCopyright (c) 2000, 2020, Oracle and/or its affiliates. All rights reserved.Oracle is a registered trademark of Oracle Corporation and/or itsaffiliates. Other names may be trademarks of their respectiveowners.Type ’help;’ or ’h’ for help. Type ’c’ to clear the current input statement.mysql>

（6）繼續(xù)多開幾個(gè)會(huì)話，使用超級(jí)用戶root，通過管理端口連接成功，不受最大連接數(shù)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)

可以說，有了管理端口這個(gè)新功能，DBA再也不用擔(dān)心too many connections的問題。

在MySQL 8.0版本中，為了應(yīng)對(duì)too many connections的場景，先后推出了額外連接和管理端口兩個(gè)新功能，可以讓DBA方便、快速地解決問題；不過，這始終是一個(gè)臨時(shí)應(yīng)急手段，最根本的原因還是要排查應(yīng)用端的配置(并發(fā)限流、SQL性能、連接池配置等等)，以徹底規(guī)避此類問題。

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