mysql的半同步复制-semisync是基于默认的异步复制和完全同步复制之间，它是在master在执行完客户端提交的事务后不是立刻返回给客户端，而是等待至少一个slave收到并写到relay log中才返回给客户端。相对于异步复制，semisync提高了数据的安全性，但是又比完全同步性能好，所以master和slave之间的时间一定要一致，以免造成semisync失败。mysql的半同步复制的插件是由Google免费提供的在mysql的$basedir/lib/plugin下有相应的动态库文件，linux中是.so，DOS系统中是.dll，在默认情况下mysql是没有装载semisync的插件，所以要制作semisync要手动在mysql的shell中装载插件，下面就简单的说一说mysql的semisync配置,这里要注意要使用mysql的super用户来配置，并且mysql要5.5及以上版本才有，并且要在有异步复制的基础上才可以做配置

#master中mysql> install plugin rpl_semi_sync_master soname 'semisync_master.so';
Query OK, 0 rows affected (0.00 sec)
mysql> show global variables like '%semi%';
+------------------------------------+-------+
| Variable_name                      | Value |
+------------------------------------+-------+
| rpl_semi_sync_master_enabled       | OFF   |
| rpl_semi_sync_master_timeout       | 10000 |
| rpl_semi_sync_master_trace_level   | 32    |
| rpl_semi_sync_master_wait_no_slave | ON    |
+------------------------------------+-------+
4 rows in set (0.01 sec)
mysql> set global rpl_semi_sync_master_enabled = 1;#开启master上的semisync，设置超时时间，可以通过设置rpl_semi_sync_master_timeout变量来修改，默认单位为毫秒
Query OK, 0 rows affected (0.00 sec)
mysql> show status like '%Rpl_semi_sync%';
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 1     |
| Rpl_semi_sync_master_net_avg_wait_time     | 565   |
| Rpl_semi_sync_master_net_wait_time         | 3391  |
| Rpl_semi_sync_master_net_waits             | 6     |
| Rpl_semi_sync_master_no_times              | 1     |
| Rpl_semi_sync_master_no_tx                 | 12    |
| Rpl_semi_sync_master_status                | ON    |
| Rpl_semi_sync_master_timefunc_failures     | 0     |
| Rpl_semi_sync_master_tx_avg_wait_time      | 588   |
| Rpl_semi_sync_master_tx_wait_time          | 3528  |
| Rpl_semi_sync_master_tx_waits              | 6     |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0     |
| Rpl_semi_sync_master_wait_sessions         | 0     |
| Rpl_semi_sync_master_yes_tx                | 6     |
+--------------------------------------------+-------+
14 rows in set (0.00 sec)#slave中mysql> install plugin rpl_semi_sync_slave soname 'semisync_slave.so';
Query OK, 0 rows affected (0.00 sec)mysql> show global variables like '%semi%';
+---------------------------------+-------+
| Variable_name                   | Value |
+---------------------------------+-------+
| rpl_semi_sync_slave_enabled     | OFF   |
| rpl_semi_sync_slave_trace_level | 32    |
+---------------------------------+-------+
2 rows in set (0.00 sec)
mysql> set global rpl_semi_sync_slave_enabled = 1;
Query OK, 0 rows affected (0.00 sec)
mysql> stop slave IO_THREAD;start slave IO_THREAD;#重启slave上的IO线程
Query OK, 0 rows affected (0.09 sec)Query OK, 0 rows affected (0.00 sec)
mysql> show status like '%Rpl_semi_sync%';
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | ON    |
+----------------------------+-------+
1 row in set (0.00 sec)

这样简单的semisync就配置好了，这里要注意的是如果master超过超时时间没有接收到slave的消息时会自动转为异步复制，当然如果不需要semisync可以直接uninstall相应的插件

mysql> uninstall plugin rpl_semi_sync_master;
Query OK, 0 rows affected (0.00 sec)
mysql> uninstall plugin rpl_semi_sync_slave;
Query OK, 0 rows affected (0.00 sec)