在使用雙主互為主從的模式前提是互為主從的兩個數據庫，表的主鍵必須是自增的。
環境不多說了 ，多台mysql單實例或單台多實例都可以。多實例的配置請參考：https://www.cnblogs.com/qiuhom-1874/p/9757061.html
實驗步驟是在主從復制已經配置好的兩台數據庫上配置。說下思想吧，其實很簡單就是把原來的從庫當主庫，把原來的主庫當從庫。按照主從配置的方法從前把主庫當從庫，把從庫當主庫的配置一遍。
1.配置my.cnf
主庫1配置my.cnf：
auto_increment_increment = 2 auto_increment_offset = 2 log-bin =/mysql_multi_case/3306/mysqld-bin log-slave-updates = 1
主庫2配置my.cnf：
auto_increment_increment = 2 auto_increment_offset = 1 log-bin =/mysql_multi_case/3307/mysqld-bin log-slave-updates = 1
提示：以上參數都是配置mysqld下。
2.創建用戶，給予replication slave 權限。
grant replication slave on *.* to 'rep_3306'@'127.0.0.1' identified by 'admin'; ####（3307上創建）
3.導出數據（原來的從庫庫數據）
mysqldump -uroot -padmin -S /mysql_multi_case/3307/mysql.sock -A -B --master-data=1 -x --events > /work/bak/3307all.sql
4.導入數據（導入原來的主庫）
mysql -uroot -padmin -S /mysql_muitl_case/3306/mysql.sock < /work/bak/3307all.sql
5.在主庫1上執行change master to .. 語句
change master to master_host='127.0.0.1', master_port=3307 master_user='rep_3306', master_password='admin';
提示：因為全備指定選項--master-data=1 所以我們在3306上執行change master to語句就不用指定master_log_file和master_log_pos的值。
mysql> change master to master_host='127.0.0.1',master_port=3307,master_user='rep_3306',master_password='admin'; Query OK, 0 rows affected (0.53 sec)
提示：我們可以去3306下的data目錄下查看master-info文件
[root@qiuhom data]# cat /mysql_multi_case/3306/data/master.info 18 mysqld-bin.000007 1240 127.0.0.1 rep_3306 admin 3307 60 0 0 1800.000 0
提示：我們可以看出master_log_file是mysqld-bin.000007，pos是1240，說明--master-data=1 記錄了我們binlog日志文件名和位置點。我們在開啟同步看看slave status里是否一致。
6.開啟同步和檢查slave狀態
mysql> start slave;
Query OK, 0 rows affected (0.00 sec)
mysql> show slave status\G
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 127.0.0.1
Master_User: rep_3306
Master_Port: 3307
Connect_Retry: 60
Master_Log_File: mysqld-bin.000007
Read_Master_Log_Pos: 1240
Relay_Log_File: relay-bin.000008
Relay_Log_Pos: 1387
Relay_Master_Log_File: mysqld-bin.000007
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Replicate_Do_DB:
Replicate_Ignore_DB: mysql
Replicate_Do_Table:
Replicate_Ignore_Table:
Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
Last_Errno: 0
Last_Error:
Skip_Counter: 0
Exec_Master_Log_Pos: 1240
Relay_Log_Space: 1837
Until_Condition: None
Until_Log_File:
Until_Log_Pos: 0
Master_SSL_Allowed: No
Master_SSL_CA_File:
Master_SSL_CA_Path:
Master_SSL_Cert:
Master_SSL_Cipher:
Master_SSL_Key:
Seconds_Behind_Master: 0
Master_SSL_Verify_Server_Cert: No
Last_IO_Errno: 0
Last_IO_Error:
Last_SQL_Errno: 0
Last_SQL_Error:
Replicate_Ignore_Server_Ids:
Master_Server_Id: 2
1 row in set (0.00 sec)
接下來我們再來看看兩個庫的線程情況
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3306/mysql.sock -e "show processlist;" +----+-------------+-----------------+------+-------------+------+-----------------------------------------------------------------------------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------------+-----------------+------+-------------+------+-----------------------------------------------------------------------------+------------------+ | 1 | rep | 127.0.0.1:49248 | NULL | Binlog Dump | 1148 | Master has sent all binlog to slave; waiting for binlog to be updated | NULL | | 5 | system user | | NULL | Connect | 516 | Waiting for master to send event | NULL | | 6 | system user | | NULL | Connect | 1029 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL | | 8 | root | localhost | NULL | Query | 0 | NULL | show processlist | +----+-------------+-----------------+------+-------------+------+-----------------------------------------------------------------------------+------------------+ [root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3307/mysql.sock -e "show processlist;" +----+-------------+-----------------+------+-------------+------+-----------------------------------------------------------------------------+------------------+ | Id | User | Host | db | Command | Time | State | Info | +----+-------------+-----------------+------+-------------+------+-----------------------------------------------------------------------------+------------------+ | 1 | system user | | NULL | Connect | 1156 | Waiting for master to send event | NULL | | 2 | system user | | NULL | Connect | 702 | Slave has read all relay log; waiting for the slave I/O thread to update it | NULL | | 5 | rep_3306 | 127.0.0.1:43773 | NULL | Binlog Dump | 524 | Master has sent all binlog to slave; waiting for binlog to be updated | NULL | | 7 | root | localhost | NULL | Query | 0 | NULL | show processlist | +----+-------------+-----------------+------+-------------+------+-----------------------------------------------------------------------------+------------------+
提示：可以看到3306和3307分別有3個線程，一個主io,一個從io,一個sql線程。和我們配置主從的線程是不是不一樣了。這樣兩個互為主從的實例就配置好了。接下來我們測試下寫入數據是什么情況。
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3306/mysql.sock -e "create database qiuhom;use qiuhom;create table test(id int not null auto_increment primary key,name char(10) not null);"
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3306/mysql.sock -e "show databases;use qiuhom;show tables;"
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| qiuhom |
+--------------------+
+------------------+
| Tables_in_qiuhom |
+------------------+
| test |
+------------------+
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3306/mysql.sock -e "use qiuhom;insert into test(name) values('aa'),('bb'),('cc');"
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3307/mysql.sock -e "show databases;"
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| qiuhom |
+--------------------+
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3307/mysql.sock -e "use qiuhom;show tables;"
+------------------+
| Tables_in_qiuhom |
+------------------+
| test |
+------------------+
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3307/mysql.sock -e "use qiuhom;insert into test(name) values('dd'),('ee'),('ff');"
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3306/mysql.sock -e "use qiuhom;insert into test(name) values('gg'),('hh'),('ii');"
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3306/mysql.sock -e "select * from qiuhom.test;"
+----+------+
| id | name |
+----+------+
| 2 | aa |
| 4 | bb |
| 6 | cc |
| 7 | dd |
| 9 | ee |
| 11 | ff |
| 12 | gg |
| 14 | hh |
| 16 | ii |
+----+------+
[root@qiuhom data]# mysql -uroot -padmin -S /mysql_multi_case/3307/mysql.sock -e "select * from qiuhom.test;"
+----+------+
| id | name |
+----+------+
| 2 | aa |
| 4 | bb |
| 6 | cc |
| 7 | dd |
| 9 | ee |
| 11 | ff |
| 12 | gg |
| 14 | hh |
| 16 | ii |
+----+------+
提示：從上面的測試看我們發現我們倆個庫都可以插入數據，況且插入數據有個規律，主庫1插入的數據主鍵都是偶數，主庫2插入的數據的主鍵都是奇數。這就是我們配置參數auto_increment_offset 有很大的關系，我們設置3306等於2所有它會從表里最后一個位置的主鍵開始偏移2開始計，因為我測試的表示空表所以3306開始插入的數據主鍵是從2開始的，然后兩個主鍵相隔兩個數，又計下一個主鍵。3307我們設置的=1 所3307插入的數據的主鍵就是從最后插入數據的主鍵位置偏移1開始計。所以從7開始計，它插入的數據主鍵也是相隔兩數字，所以第二次插入數據的主鍵就是9以此類推。
我們查看3307里面的qiuhom庫下的test表的數據和3306的一樣，這樣我們就實現了雙主互為主從，不管在哪個庫寫另外一庫就復制當從的角色。其實我們從字面意思也可以理解互為主從就是說一個數據庫又是主庫又是從庫的意思。