MySQL登录之socket与TCP

　　在一台测试服务器上部署了2个实例，一个端口是默认的3306，另一个端口是3376。MySQL的版本是5.6.35

[root@MySQL56_L1 ~]# ps -ef | grep mysql | grep -v grep
mysql         : pts/    :: /usr/local/mysql/bin/mysqld --defaults-file=/etc/my.cnf
mysql         : pts/    :: /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3376/my3376.cnf

　　my3376.cnf的部分参数配置如下

[root@MySQL56_L1 ~]# more /etc/my.cnf 

[client]
port            =
socket          = /tmp/mysql.sock

# The MySQL server
[mysqld]
# Basic
port            =
user        = mysql
basedir         = /usr/local/mysql
datadir         = /data/mysql/mysql3306/data
tmpdir          = /data/mysql/mysql3306/tmp
socket          = /tmp/mysql.sock

　　my3376.cnf的部分参数配置如下

[root@MySQL56_L1 ~]# more /data/mysql/mysql3376/my3376.cnf 

[client]
port            =
socket          = /tmp/mysql3376.sock

# The MySQL server
[mysqld]
# Basic
port            =
user        = mysql
basedir         = /usr/local/mysql
datadir         = /data/mysql/mysql3376/data
tmpdir          = /data/mysql/mysql3376/tmp
socket          = /tmp/mysql3376.sock

　　两个数据库中的账号及密码如下

(product)root@localhost [(none)]> select host, user, password from mysql.user ;
+-----------+------+----------+
| host      | user | password |
+-----------+------+----------+
| localhost | root |          |
| 127.0.0.1 | root |          |
+-----------+------+----------+

　　当使用账号、密码、端口的方式方式登录到端口为3376的实例时，发现登录的却是3306端口的实例下

[root@MySQL56_L1 ~]# mysql  -uroot -p -P3376
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log MySQL Community Server (GPL)

Copyright (c) , , 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.

(product)root@localhost mysql.sock [(none)]> 

　　请注意上面代码块中的红色加粗字体，这是端口为3306实例的socket文件，3376端口实例使用socket方式登录如下：

[root@MySQL56_L1 ~]# mysql -S /tmp/mysql3376.sock
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log MySQL Community Server (GPL)

Copyright (c) , , 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.

(product)root@localhost mysql3376.sock [(none)]> 

　　在需要登录到3376端口实例下时，已经明确指定了端口，为什么还是登录到3306端口实例下呢？

　　难道是因为没有加上“-h”?

[root@MySQL56_L1 ~]# mysql -hlocalhost -uroot -p -P3376
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log MySQL Community Server (GPL)

Copyright (c) , , 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.

(product)root@localhost mysql.sock [(none)]> 

　　可以看到，使用“-hlocalhost”是还是登录到3306端口实例下，若是修改为“-h IP”，又会怎么样呢？

[root@MySQL56_L1 ~]# mysql -h127.0.0. -uroot -p -P3376
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is
Server version: 5.6.-log MySQL Community Server (GPL)

Copyright (c) , , 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.

(product)root@127.0.0.1 3376 [(none)]> 

　　这次就能如愿登录到所期望的3376端口实例下了。

　　小结：安装了多实例的情况下，需要登录到特定端口实例下的方法

　　1、使用“-S” 参数，通过指定实例下的socket文件来登录到指定的实例

　　2、使用“-h”参数，注意，这里必须是使用'TCP/IP'的方式，不能是'localhost'，因为'localhost'会使用默认的socket文件登录

　　那么，为什么指定了端口“-P3376”，还是会登录到3306端口实例下呢？

　　原因是因为没有指定“-h”使用'TCP/IP'方式来登录时，是默认使用socket方式登录，问题又来了，是使用哪个socket文件呢？

[root@MySQL56_L1 ~]# mysql --verbose --help | grep socket
  --protocol=name     The protocol to use for connection (tcp, socket, pipe,
  -S, --socket=name   The socket file to use for connection.
                      The buffer size for TCP/IP and socket communication.
socket                            /tmp/mysql.sock

　　这个文件是在哪里指定呢？

[root@MySQL56_L1 ~]# mysql --verbose --help | grep my.cnf
                      order of preference, my.cnf, $MYSQL_TCP_PORT,
/etc/my.cnf /etc/mysql/my.cnf /usr/local/mysql/etc/my.cnf ~/.my.cnf 

　　注：以上的4个“my.cnf”后面的参数会覆盖前面的。

　　若是产生怀疑，可以试验一下在“~/.my.cnf”中添加如下代码

[root@MySQL56_L1 ~]# vi ~/.my.cnf  

[client]
socket          = /tmp/mysql3306.sock

　　再查看一下mysql读取的socket文件

[root@MySQL56_L1 ~]# mysql --verbose --help | grep socket
  --protocol=name     The protocol to use for connection (tcp, socket, pipe,
  -S, --socket=name   The socket file to use for connection.
                      The buffer size for TCP/IP and socket communication.
socket                            /tmp/mysql3306.sock

　　从上面的测试就可以看出是读取socket文件的路径

　　注意：测试完之后需要改回正确的socket文件名

　　看完上面的说明，现在应该能明白为什么为什么指定了“-P 3376”，却登录到了3306端口是实例下了。其实，正确的应该是这个路径“/etc/my.cnf /etc/mysql/my.cnf /usr/local/mysql/etc/my.cnf ~/.my.cnf”下的socket参数对应的是哪个端口，就会登录到哪个端口的实例下。

　　看完了这么多，可能还是不明白socket是什么？

　　socket是用于同一台主机的进程间通讯（IPC），不需要经过网络协议栈，不需要打包拆包、计算校验和、维护序号和应答等，只是将应用层数据从一个进程拷贝到另外一个进程，消息既不会丢失也不会顺序错乱。可用于两个没有亲缘关系的进程，是全双工的。

　　以上，若有错误，请不吝指出。