MariaDB主从异步复制详解

一 异步复制(Asynchronous replication)

1.MariaDB本身支持单向的、异步的复制。异步复制意味着在把数据从一台机器拷贝到另一台机器时有一个延时，最重要的是这意味着当应用系统的事务提交已经确认时数据并不能在同一时刻拷贝，应用到从机。通常这个延时是由网络带宽、资源可用性和系统负载决定的。然而，使用正确的组件并且调优，复制能做到接近瞬时完成。

2.MariaDB默认的复制即是异步的，主库在执行完客户端提交的事务后会立即将结果返给给客户端，并不关心从库是否已经接收并处理，这样就会有一个问题，主如果crash掉了，此时主上已经提交的事务可能并没有传到从上，如果此时，强行将从提升为主，可能导致新主上的数据不完整。

二 实验过程

1.时间要进行同步

2.注意复制的开始位置
 从0开始
 从备份中恢复到从节点后启动的复制，复制的起始点备份操作时主节点所处的日志文件及其实践位置

3.配置文件
主节点：

[root@node1 ~]# vim /etc/my.cnf.d/server.cnf
skip_name_resolve=ON
innodb_file_per_table=ON
server_id=1
log_bin=log-bin

从节点：

[root@node2 ~]# vim /etc/my.cnf.d/server.cnf/etc/my.cnf.d/server.cnf
skip_name_resolve=ON
innodb_file_per_table=ON
server_id=2
relay_log=relay-log

4.启动服务，并进入MariaDB

[root@node1 ~]# systemctl start mariadb.service
[root@node2 ~]# systemctl start mariadb.service

5.对数据库库进行操作
在主节点上创建用户并授权

MariaDB [(none)]> CREATE USER 'repluser'@'172.18.67.%' IDENTIFIED BY 'replpass';
MariaDB [(none)]> GRANT REPLICATION SLAVE,REPLICATION CLIENT ON *.* TO 'repluser'@'172.18.67.%' IDENTIFIED BY 'replpass';
MariaDB [(none)]> FLUSH PRIVILEGES;
MariaDB [(none)]> SHOW MASTER STATUS;
+----------------+----------+--------------+------------------+
| File           | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+----------------+----------+--------------+------------------+
| log-bin.000001 |      497 |              |                  |
+----------------+----------+--------------+------------------+

为从节点关联主节点

MariaDB [(none)]> CHANGE MASTER TO MASTER_HOST='172.18.67.11',MASTER_USER='repluser',MASTER_PASSWORD='replpass',MASTER_LOG_FILE='log-bin.000001',MASTER_LOG_POS=497;

查看从节点状态

MariaDB [(none)]> SHOW SLAVE STATUS\G;
*************************** 1. row ***************************
               Slave_IO_State:
                  Master_Host: 172.18.67.11
                  Master_User: repluser
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: log-bin.000001
          Read_Master_Log_Pos: 497
               Relay_Log_File: relay-log.000001
                Relay_Log_Pos: 4
        Relay_Master_Log_File: mysql-bin.000001
             Slave_IO_Running: No
            Slave_SQL_Running: No
              Replicate_Do_DB:
          Replicate_Ignore_DB:
           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: 497
              Relay_Log_Space: 245
              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: NULL
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: 0
1 row in set (0.00 sec)

开启IO和SQL线程

MariaDB [(none)]> HELP START SLAVE;
MariaDB [(none)]> HELP STOP SLAVE;
MariaDB [(none)]> START SLAVE IO_THREAD;
MariaDB [(none)]> START SLAVE SQL_THREAD;
MariaDB [(none)]> SHOW SLAVE STATUS\G;
*************************** 1. row ***************************
               Slave_IO_State:
                  Master_Host: 172.18.67.11
                  Master_User: repluser
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: log-bin.000001
          Read_Master_Log_Pos: 497
               Relay_Log_File: relay-log.000001
                Relay_Log_Pos: 4
        Relay_Master_Log_File: mysql-bin.000001
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB:
          Replicate_Ignore_DB:
           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: 497
              Relay_Log_Space: 245
              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: NULL
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: 0
1 row in set (0.00 sec)

查看从节点状态，IO和SQL线程已经启动：Slave_IO_Running: Yes，Slave_SQL_Running: Yes
在主节点上创建数据库mydb

MariaDB [(none)]> CREATE DATABASE mydb CHARACTER SET 'utf8';
MariaDB [(none)]> SHOW MASTER STATUS;
+----------------+----------+--------------+------------------+
| File           | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+----------------+----------+--------------+------------------+
| log-bin.000001 |      601 |              |                  |
+----------------+----------+--------------+------------------+
MariaDB [(none)]> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| mydb               |
| mysql              |
| performance_schema |
| test               |
+--------------------+

查看从节点的状态

MariaDB [(none)]> SHOW SLAVE STATUS\G;
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 172.18.67.11
                  Master_User: repluser
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: log-bin.000001
          Read_Master_Log_Pos: 601
               Relay_Log_File: relay-log.000002
                Relay_Log_Pos: 631
        Relay_Master_Log_File: log-bin.000001
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB:
          Replicate_Ignore_DB:
           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: 601
              Relay_Log_Space: 919
              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: 1
1 row in set (0.00 sec)

此时我们可以从从节点上观察到Read_Master_Log_Pos由497变为了631。为了使实验更加深入，准备了一个数据库，我们将其导入到主节点

[root@node1 ~]# mysql < hellodb.sql
MariaDB [(none)]> SHOW MASTER STATUS;
+----------------+----------+--------------+------------------+
| File           | Position | Binlog_Do_DB | Binlog_Ignore_DB |
+----------------+----------+--------------+------------------+
| log-bin.000001 |     8011 |              |                  |
+----------------+----------+--------------+------------------+

再次查看从节点状态

MariaDB [(none)]> SHOW SLAVE STATUS\G;
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 172.18.67.11
                  Master_User: repluser
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: log-bin.000001
          Read_Master_Log_Pos: 8011
               Relay_Log_File: relay-log.000002
                Relay_Log_Pos: 8041
        Relay_Master_Log_File: log-bin.000001
             Slave_IO_Running: Yes
            Slave_SQL_Running: Yes
              Replicate_Do_DB:
          Replicate_Ignore_DB:
           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: 8011
              Relay_Log_Space: 8329
              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: 1
1 row in set (0.00 sec)
MariaDB [(none)]> use hellodb;
MariaDB [hellodb]> SHOW TABLES;
+-------------------+
| Tables_in_hellodb |
+-------------------+
| classes           |
| coc               |
| courses           |
| scores            |
| students          |
| teachers          |
| toc               |
+-------------------+
MariaDB [hellodb]> SELECT * FROM students;
+-------+---------------+-----+--------+---------+-----------+
| StuID | Name          | Age | Gender | ClassID | TeacherID |
+-------+---------------+-----+--------+---------+-----------+
|     1 | Shi Zhongyu   |  22 | M      |       2 |         3 |
|     2 | Shi Potian    |  22 | M      |       1 |         7 |
|     3 | Xie Yanke     |  53 | M      |       2 |        16 |
|     4 | Ding Dian     |  32 | M      |       4 |         4 |
|     5 | Yu Yutong     |  26 | M      |       3 |         1 |
|     6 | Shi Qing      |  46 | M      |       5 |      NULL |
|     7 | Xi Ren        |  19 | F      |       3 |      NULL |
|     8 | Lin Daiyu     |  17 | F      |       7 |      NULL |
|     9 | Ren Yingying  |  20 | F      |       6 |      NULL |
|    10 | Yue Lingshan  |  19 | F      |       3 |      NULL |
|    11 | Yuan Chengzhi |  23 | M      |       6 |      NULL |
|    12 | Wen Qingqing  |  19 | F      |       1 |      NULL |
|    13 | Tian Boguang  |  33 | M      |       2 |      NULL |
|    14 | Lu Wushuang   |  17 | F      |       3 |      NULL |
|    15 | Duan Yu       |  19 | M      |       4 |      NULL |
|    16 | Xu Zhu        |  21 | M      |       1 |      NULL |
|    17 | Lin Chong     |  25 | M      |       4 |      NULL |
|    18 | Hua Rong      |  23 | M      |       7 |      NULL |
|    19 | Xue Baochai   |  18 | F      |       6 |      NULL |
|    20 | Diao Chan     |  19 | F      |       7 |      NULL |
|    21 | Huang Yueying |  22 | F      |       6 |      NULL |
|    22 | Xiao Qiao     |  20 | F      |       1 |      NULL |
|    23 | Ma Chao       |  23 | M      |       4 |      NULL |
|    24 | Xu Xian       |  27 | M      |    NULL |      NULL |
|    25 | Sun Dasheng   | 100 | M      |    NULL |      NULL |
+-------+---------------+-----+--------+---------+-----------+

我们观察到主节点的数据发生变化，从节点的数据库都会同步。

三 复制时应该注意的问题

1.从服务设定为“只读”
在从服务器启动read_only，但仅对非SUPER权限的用户有效
阻止所有用户：MariaDB [(none)]> FLUSH TABLES WITH READ LOCK

2.尽量确保复制时的事务安全
在master节点启用参数：
 sync_binlog = ON
如果用到的是InnoDB存储引擎：
 innodb_flush_logs_at_trx_commit=ON
 innodb_support_xa=ON
3.从服务器意外中止时尽量避免自动启动复制线程