在mysql 5.7 中如何开启半同步复制

1.安装相关的插件

show plugins;  查看模块
help --uninstall; 查看卸载模块

master:
mysql> install plugin rpl_semi_sync_master soname 'semisync_master.so';  --安装 semisync_master.so插件 
Query OK, 0 rows affected (0.03 sec)
slave:
root@localhost [zw3306]>install plugin rpl_semi_sync_slave soname 'semisync_slave.so'; --安装 semisync_slave.so插件
Query OK, 0 rows affected (0.00 sec)
root@localhost [zw3306]>install plugin rpl_semi_sync_master soname 'semisync_master.so';
Query OK, 0 rows affected (0.00 sec)

2.修改的参数：

set global rpl_semi_sync_master_enabled=1;
set global rpl_semi_sync_master_timeout=1000;
set global rpl_semi_sync_slave_enabled=1;

也可以直接写到配置文件 [mysqld]

master:
[mysqld]
rpl_semi_sync_master_enabled = 1 
rpl_semi_sync_master_timeout = 1000 # 1 second
slave：
[mysqld]
rpl_semi_sync_slave_enabled = 1

修改了参数需要重启：

查看修改的参数

master: 
mysql> show global variables like '%rpl_semi%';

+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 1000       |
| rpl_semi_sync_master_trace_level          | 32         |
| rpl_semi_sync_master_wait_for_slave_count | 1          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
+-------------------------------------------+------------+
6 rows in set (0.00 sec)

slave:
root@localhost [(none)]>show global variables like '%rpl_semi%';

+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 1000       |
| rpl_semi_sync_master_trace_level          | 32         |
| rpl_semi_sync_master_wait_for_slave_count | 1          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | ON         |
| rpl_semi_sync_slave_trace_level           | 32         |
+-------------------------------------------+------------+
8 rows in set (0.00 sec)

如果：如果原来是已经建好的复制结构很简单：

stop slave io_thread;
start slave io_thread;

3.做同步

change master to master_host='192.168.26.233', master_port=3306, master_user='repl',master_password='repl', master_auto_position=1;
 
root@localhost [(none)]>start slave;
Query OK, 0 rows affected (0.01 sec)
root@localhost [(none)]>show slave status\G;
*************************** 1. row ***************************
               Slave_IO_State: Waiting for master to send event
                  Master_Host: 192.168.26.233
                  Master_User: repl
                  Master_Port: 3306
                Connect_Retry: 60
              Master_Log_File: mysql-bin.000002
          Read_Master_Log_Pos: 194
               Relay_Log_File: relay-bin.000004
                Relay_Log_Pos: 407
        Relay_Master_Log_File: mysql-bin.000002
             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: 194
              Relay_Log_Space: 861
              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: 3306100
                  Master_UUID: 7e354a2c-6f5f-11e6-997d-005056a36f08
             Master_Info_File: mysql.slave_master_info
                    SQL_Delay: 0
          SQL_Remaining_Delay: NULL
      Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates
           Master_Retry_Count: 86400
                  Master_Bind: 
      Last_IO_Error_Timestamp: 
     Last_SQL_Error_Timestamp: 
               Master_SSL_Crl: 
           Master_SSL_Crlpath: 
           Retrieved_Gtid_Set: 7e354a2c-6f5f-11e6-997d-005056a36f08:1-9
            Executed_Gtid_Set: 7e354a2c-6f5f-11e6-997d-005056a36f08:1-9,
ba0d5587-74d6-11e6-ab5c-005056a3f46e:1-2
                Auto_Position: 1
         Replicate_Rewrite_DB: 
                 Channel_Name: 
           Master_TLS_Version: 
1 row in set (0.00 sec)
ERROR: 
No query specified

5.查看slave是否有数据

root@localhost [zw3306]>show tables;

+------------------+
| Tables_in_zw3306 |
+------------------+
| t1               |
+------------------+
1 row in set (0.00 sec)

root@localhost [zw3306]>select * from t1;

+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
+------+
4 rows in set (0.00 sec)

6. 怎么确认是同步还是半同步？

show global variables like '%semi%';
show global status like '%semi%';

master:
root@localhost [zw3306]>show global status like '%semi%';

+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 1     | 有多少个Semi-sync的备库
| Rpl_semi_sync_master_net_avg_wait_time     | 0     | 事务提交后，等待备库响应的平均时间
| Rpl_semi_sync_master_net_wait_time         | 0     | 等待网络响应的总次数
| Rpl_semi_sync_master_net_waits             | 7     | 总的网络等待时间
| Rpl_semi_sync_master_no_times              | 0     | 一共有几次从Semi-sync跌回普通状态
| Rpl_semi_sync_master_no_tx                 | 0     | 库未及时响应的事务数,如果这个值很大就有问题
| Rpl_semi_sync_master_status                | ON    | 主库上Semi-sync是否正常开启
| Rpl_semi_sync_master_timefunc_failures     | 0     | 时间函数未正常工作的次数
| Rpl_semi_sync_master_tx_avg_wait_time      | 410   | 开启Semi-sync，事务返回需要等待的平均时间
| Rpl_semi_sync_master_tx_wait_time          | 2876  | 事务等待备库响应的总时间
| Rpl_semi_sync_master_tx_waits              | 7     | 事务等待备库响应的总次数
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     | 改变当前等待最小二进制日志的次数
| Rpl_semi_sync_master_wait_sessions         | 0     | 当前有几个线程在等备库响应
| Rpl_semi_sync_master_yes_tx                | 7     | Semi-sync模式下，成功的事务数
+--------------------------------------------+-------+
15 rows in set (0.00 sec)

root@localhost [zw3306]>show global variables like '%semi%';

+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 1000       |
| rpl_semi_sync_master_trace_level          | 32         |
| rpl_semi_sync_master_wait_for_slave_count | 1          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | ON         |
| rpl_semi_sync_slave_trace_level           | 32         |
+-------------------------------------------+------------+
8 rows in set (0.00 sec)

Rpl_semi_sync_master_no_tx 如果这个值很大就有问题

也有其他的原因；

mysql> show global status like '%semi%';

+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 1     |
| Rpl_semi_sync_master_net_avg_wait_time     | 0     |
| Rpl_semi_sync_master_net_wait_time         | 0     |
| Rpl_semi_sync_master_net_waits             | 17    |
| Rpl_semi_sync_master_no_times              | 1     |
| Rpl_semi_sync_master_no_tx                 | 10    | 不是用半同步复制的
| Rpl_semi_sync_master_status                | OFF   |
| Rpl_semi_sync_master_timefunc_failures     | 0     |
| Rpl_semi_sync_master_tx_avg_wait_time      | 410   |
| Rpl_semi_sync_master_tx_wait_time          | 2876  |
| Rpl_semi_sync_master_tx_waits              | 7     |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     |
| Rpl_semi_sync_master_wait_sessions         | 0     |
| Rpl_semi_sync_master_yes_tx                | 7     |
+--------------------------------------------+-------+
14 rows in set (0.01 sec)

操作10个事物，可以发现都是  Rpl_semi_sync_master_no_tx 
master接收到N个slave的应答后，才commit事物，等待1s用户可以设置应道slave的数量。
rpl_semi_sync_master_wait_for_slave_count=1 默认是1 
set global rpl_semi_sync_master_wait_for_slave_count=2;

意思是多少个半同步的slave;

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