在用MySQL客户端对数据库进行操作时，如果一段时间没有操作，再次操作时，常常会报如下错误：

ERROR 2013 (HY000): Lost connection to MySQL server during query

ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...

这个报错信息就意味着当前的连接已经断开，需要重新建立连接。

那么，连接建立后，连接的时长是如何确定的呢？

在MySQL中，这个与两个参数interactive_timeout和wait_timeout的设置有关。

注:以下说明基于MySQL 5.7.

1.interactive_timeout和wait_timeout的定义

首先，看看官方文档对于这两个参数的定义。

interactive_timeout

The number of seconds the server waits for activity on an interactive connection before closing it. An interactive client is defined as a client that uses the CLIENT_INTERACTIVE option to mysql_real_connect(). See also wait_timeout.

interactive_timeout参数，定义了对于交互式连接，服务端等待数据的最大时间。如果超过这个时间，服务端仍然没有收到数据，则会关闭连接。

所谓交互式client，是指调用mysql_real_connect()函数建立连接时，设置了CLIENT_INTERACTIVE选项。比较常用的就是命令行终端。

查看interactive_timeout的值：

mysql> show global variables like  'interactive_timeout%';
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 28800 |
+---------------------+-------+
1 row in set (0.01 sec)

默认是28800，单位秒，即8个小时

wait_timeout

The number of seconds the server waits for activity on a noninteractive connection before closing it.

On thread startup, the session wait_timeout value is initialized from the global wait_timeout value or from the global interactive_timeout value, depending on the type of client (as defined by the CLIENT_INTERACTIVE connect option to mysql_real_connect()). See also interactive_timeout.

wait_timeout参数，定义对于非交互式连接，服务端等待数据的最长时间。如果超过这个时间，服务端仍然没有收到数据，则会关闭连接。

在连接线程启动的时候，根据连接的类型，决定会话级的wait_timeout的值是初始化为全局的wait_timeout，还是全局的interactive_timeout。即如果是交互式连接，会话变量wait_timeout初始化为全局的interactive_timeout，否则，初始化为全局的wait_timeout。

查看wait_timeout的值：

mysql> show global variables like  'wait_timeout%';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| wait_timeout  | 28800 |
+---------------+-------+
1 row in set (0.00 sec)

默认同样是28800s,即8小时。

根据上述定义，两者的区别显而易见：interactive_timeout针对交互式连接，wait_timeout针对非交互式连接。所谓的交互式连接，即在mysql_real_connect()函数中使用了CLIENT_INTERACTIVE选项。

 说得直白一点，通过mysql命令行终端连接数据库是交互式连接，通过jdbc等连接数据库是非交互式连接。 

下面来测试一下，确认如下问题：

• 控制连接最大空闲时长的是哪个参数。
• 会话变量wait_timeout的继承问题

2.控制连接最大空闲时长的是哪个参数

先给出答案：wait_timeout

接下来进行验证。

2.1 只修改会话的wait_timeout参数

查看当前会话的wait_timeout和interactive_timeout。

mysql> show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 28800 |
| wait_timeout        | 28800 |
+---------------------+-------+
2 rows in set (0.00 sec)

设置当前会话的wait_timeout为10s

mysql> set session wait_timeout=10;
Query OK, 0 rows affected (0.00 sec)

再次查看

mysql> show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 28800 |
| wait_timeout        | 10    |
+---------------------+-------+
2 rows in set (0.00 sec)

等待10s，再次查看

mysql> show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    8
Current database: *** NONE ***

+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 28800 |
| wait_timeout        | 28800 |
+---------------------+-------+
2 rows in set (0.01 sec)

可以看到，等待10s后再执行操作，原来的连接已经断开，并重新建立连接。

2.2 只修改会话的interactive_timeout参数

首先查看当前会话的interactive_timeout和wait_timeout.

mysql> show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 28800 |
| wait_timeout        | 28800 |
+---------------------+-------+
2 rows in set (0.00 sec)

接着，设置当前会话的interactive_timeout为10s

mysql> set session interactive_timeout=10;
Query OK, 0 rows affected (0.00 sec)

再次查看

mysql> show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 10    |
| wait_timeout        | 28800 |
+---------------------+-------+
2 rows in set (0.01 sec)

等待10s，再次查看

mysql> show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 10    |
| wait_timeout        | 28800 |
+---------------------+-------+
2 rows in set (0.00 sec)

可以看到，即使等待10后，连接是正常的。所以，设置interactive_timeout，对连接的时长没有影响。

3.会话变量wait_timeout的继承问题

上面已经提到，如果是交互式连接，则继承自全局变量interactive_timeout的值，如果是非交互式连接，则继承自全局变量wait_timeout的值。

下面进行验证。

3.1 只修改全局变量interactive_timeout的值

首先查看全局的interactive_timeout和wait_timeout。

mysql> show global  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 28800 |
| wait_timeout        | 28800 |
+---------------------+-------+
2 rows in set (0.00 sec)

接着，设置全局的interactive_timeout为10s。

mysql> set global INTERACTIVE_TIMEOUT=10;
Query OK, 0 rows affected (0.00 sec)

再次查看

mysql> show global  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 10    |
| wait_timeout        | 28800 |
+---------------------+-------+
2 rows in set (0.00 sec)

开启另外一个MySQL客户端，查看会话变量的值：

mysql> show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 10    |
| wait_timeout        | 10    |
+---------------------+-------+
2 rows in set (0.01 sec)

发现，WAIT_TIMEOUT的值已经变为10了。

等待10s后，再次查看，会发现原来的连接已经断开，连接的时长设置已经生效。

mysql> show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id:    70
Current database: *** NONE ***

+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 10    |
| wait_timeout        | 10    |
+---------------------+-------+
2 rows in set (0.01 sec)

但通过非终端测试，wait_timeout的值依旧是28800：

package main

import (
        "database/sql"
        "log"

        _ "github.com/go-sql-driver/mysql"
)

var DB *sql.DB
var dataBase = "root:Aa123456@tcp(127.0.0.1:3306)/?loc=Local&parseTime=true"

func mysqlInit() {
        var err error
        DB, err = sql.Open("mysql", dataBase)
        if err != nil {
                log.Fatalln("open db fail:", err)
        }

        DB.SetMaxOpenConns(1)

        err = DB.Ping()
        if err != nil {
                log.Fatalln("ping db fail:", err)
        }
}

func main() {
        mysqlInit()
        execSql()
}

func execSql() {
        var variableName string
        var value int
        sql := "show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout')"
        rows, err := DB.Query(sql)
        if err != nil {
                log.Println("query failed:", err)
                return
        }

        defer rows.Close()

        for rows.Next() {
                err = rows.Scan(&variableName, &value)
                if err != nil {
                        log.Println("rows.Scan failed:", err)
                        return
                }

                log.Println("variable_name:", variableName, ", value:", value)
        }
}

output:

2019/10/13 17:11:22 variable_name: interactive_timeout , value: 10
2019/10/13 17:11:22 variable_name: wait_timeout , value: 28800

结果输出如下

INTERACTIVE_TIMEOUT:  10
WAIT_TIMEOUT:  28800

3.2 只修改全局变量wait_timeout的值

首先查看全局的interactive_timeout和wait_timeout。
mysql> show global variables where Variable_name in ('interactive_timeout', 'wait_timeout'); +---------------------+-------+ | Variable_name | Value | +---------------------+-------+ | interactive_timeout | 28800 | | wait_timeout | 28800 | +---------------------+-------+ 2 rows in set (0.00 sec)

接着，将全局的WAIT_TIMEOUT设置为20s。

mysql> set global WAIT_TIMEOUT=20;
Query OK, 0 rows affected (0.07 sec)

再次查看

mysql> show global  variables where Variable_name in ('interactive_timeout', 'wait_timeout');
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| interactive_timeout | 28800 |
| wait_timeout        | 20    |
+---------------------+-------+
2 rows in set (0.00 sec)

开启另外一个mysql客户端，查看会话变量的值
mysql> show session variables where Variable_name in ('interactive_timeout', 'wait_timeout'); +---------------------+-------+ | Variable_name | Value | +---------------------+-------+ | interactive_timeout | 28800 | | wait_timeout | 28800 | +---------------------+-------+ 2 rows in set (0.00 sec)
WAIT_TIMEOUT的值依旧是28800.

查看非终端的代码执行的结果：

func execSql() {
        var variableName string
        var value int
        sql := "show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout')"
        rows, err := DB.Query(sql)
        if err != nil {
                log.Println("query failed:", err)
                return
        }

        defer rows.Close()

        for rows.Next() {
                err = rows.Scan(&variableName, &value)
                if err != nil {
                        log.Println("rows.Scan failed:", err)
                        return
                }

                log.Println("variable_name:", variableName, ", value:", value)
        }
}

output:

2019/10/13 17:23:10 variable_name: interactive_timeout , value: 28800
2019/10/13 17:23:10 variable_name: wait_timeout , value: 20

修改程序，执行sql语句后，等待25s后，再次执行sql语句，查看执行情况。

func main() {
        mysqlInit()
        for {
                execSql()
                time.Sleep(25*time.Second)
        }
}

func execSql() {
        var variableName string
        var value int
        sql := "show session  variables where Variable_name in ('interactive_timeout', 'wait_timeout')"
        rows, err := DB.Query(sql)
        if err != nil {
                log.Println("query failed:", err)
                return
        }

        defer rows.Close()

        for rows.Next() {
                err = rows.Scan(&variableName, &value)
                if err != nil {
                        log.Println("rows.Scan failed:", err)
                        return
                }

                log.Println("variable_name:", variableName, ", value:", value)
        }
}

output:

2019/10/13 17:26:46 variable_name: interactive_timeout , value: 28800
2019/10/13 17:26:46 variable_name: wait_timeout , value: 20
[mysql] 2019/10/13 17:27:11 packets.go:36: unexpected EOF
[mysql] 2019/10/13 17:27:11 packets.go:141: write tcp 127.0.0.1:53878->127.0.0.1:3306: write: broken pipe
2019/10/13 17:27:11 variable_name: interactive_timeout , value: 28800
2019/10/13 17:27:11 variable_name: wait_timeout , value: 20

可以看到，等待25s后，再次执行sql，此时连接已经断开。
底层又重新建立连接。

4.总结

• 控制连接最大空闲时长的wait_timeout参数。

• 关于wait_timeout的继承
  • 对于非交互式连接，类似于jdbc连接，wait_timeout的值继承自全局变量wait_timeout。
  • 对于交互式连接，类似于mysql命令行终端，wait_timeout的值继承全局变量interactive_timeout。

• 判断一个连接的空闲时间，可通过show processlist输出中Sleep状态的时间

mysql> show processlist;
+----+------+----------------------+------+---------+------+-------+------------------+
| Id | User | Host                 | db   | Command | Time | State | Info             |
+----+------+----------------------+------+---------+------+-------+------------------+
|  2 | root | localhost            | NULL | Query   |    0 | init  | show processlist |
|  6 | repl | 192.132.2.66:56001   | NULL | Sleep   | 1201 |       | NULL             |
+----+------+----------------------+------+---------+------+-------+------------------+
2 rows in set (0.03 sec)

5.参考

https://www.cnblogs.com/ivictor/p/5979731.html

http://www.cnblogs.com/cenalulu/archive/2012/06/20/2554863.html

http://www.cnblogs.com/Alight/p/4118515.html

http://ronaldbradford.com/blog/sqlstatehy000-general-error-2006-mysql-server-has-gone-away-2013-01-02/