半同步复制架构在主库提交一个事务后，commit完成即反馈客户端，无需等待推送binlog完成，如图：
半同步复制在主库完成一个事务后，需等待事务信息写入binlog日志并且至少有一个从库写入relay log日志后才能commit完成反馈客户端。架构设计如图：
半同步复制在msyql5.5使用插件实现，主库和从库使用不同的插件。
半同步复制配置：
1）查看mysql服务器是否支持动态增加插件：
mysql> select @@have_dynamic_loading;
+------------------------+
| @@have_dynamic_loading |
+------------------------+
| YES |
+------------------------+
1 row in set (0.00 sec)
2）确认支持动态增加插件后，检查mysql安装目录下是否存在插件，一般默认在$MYSQL_HOME/lib/plugin目录下存在主库插件semisync_master.so和从库插件semisync_slave.so：
[root@chavinking plugin]# pwd
/usr/local/mysql/lib/plugin
[root@chavinking plugin]# ll *master.so *slave.so
-rwxr-xr-x 1 mysql mysql 416897 Mar 26 2015 semisync_master.so
-rwxr-xr-x 1 mysql mysql 250126 Mar 26 2015 semisync_slave.so
在主库上安装插件semisync_master.so：
mysql> install plugin rpl_semi_sync_master SONAME 'semisync_master.so';
Query OK, 0 rows affected (0.11 sec)
mysql> select * from mysql.plugin;
+----------------------+--------------------+
| name | dl |
+----------------------+--------------------+
| rpl_semi_sync_master | semisync_master.so |
+----------------------+--------------------+
1 row in set (0.00 sec)
在从库上安装插件semisync_slave.so：
mysql> install plugin rpl_semi_sync_slave SONAME 'semisync_slave.so';
Query OK, 0 rows affected (0.09 sec)
mysql> select * from mysql.plugin;
+---------------------+-------------------+
| name | dl |
+---------------------+-------------------+
| rpl_semi_sync_slave | semisync_slave.so |
+---------------------+-------------------+
1 row in set (0.00 sec)
3）需要分别在主库和从库上配置参数打开半同步semi_sync，默认半同步是不打开的，主库上配置全局参数：
MSYQL>set global rpl_semi_sync_master_enabled=1;
MYSQL>set global rpl_semi_sync_master_timeout=30000;
从库上配置全局参数：
MYSQL>set global rpl-semi_sync_slave_enabled=1;
重启从库上的io线程（异步复制->半同步复制，全新配置不需要）：
mysql> stop slave io_thread;
Query OK, 0 rows affected (0.01 sec)
mysql> start slave io_thread;
Query OK, 0 rows affected (0.00 sec)
至此，在mysql异步复制基础上配置mysql半同步复制配置完成。
检查复制状态：
mysql> show status like '%semi_sync%'; --主库
+--------------------------------------------+-------+
| 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 | 0 |
| Rpl_semi_sync_master_no_times | 0 |
| Rpl_semi_sync_master_no_tx | 0 |
| Rpl_semi_sync_master_status | ON |
| Rpl_semi_sync_master_timefunc_failures | 0 |
| Rpl_semi_sync_master_tx_avg_wait_time | 0 |
| Rpl_semi_sync_master_tx_wait_time | 0 |
| Rpl_semi_sync_master_tx_waits | 0 |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0 |
| Rpl_semi_sync_master_wait_sessions | 0 |
| Rpl_semi_sync_master_yes_tx | 0 |
+--------------------------------------------+-------+
14 rows in set (0.00 sec)
mysql> show status like '%semi_sync%' --从库
-> ;
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | ON |
+----------------------------+-------+
1 row in set (0.02 sec)
mysql> insert into t1 values(8),(9);
Query OK, 2 rows affected (0.05 sec)
Records: 2 Duplicates: 0 Warnings: 0
mysql> show status like '%semi_sync%';
+--------------------------------------------+-------+
| Variable_name | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients | 1 |
| Rpl_semi_sync_master_net_avg_wait_time | 20232 |
| Rpl_semi_sync_master_net_wait_time | 20232 |
| Rpl_semi_sync_master_net_waits | 1 |
| Rpl_semi_sync_master_no_times | 0 |
| Rpl_semi_sync_master_no_tx | 0 |
| Rpl_semi_sync_master_status | ON |
| Rpl_semi_sync_master_timefunc_failures | 0 |
| Rpl_semi_sync_master_tx_avg_wait_time | 0 |
| Rpl_semi_sync_master_tx_wait_time | 0 |
| Rpl_semi_sync_master_tx_waits | 0 |
| Rpl_semi_sync_master_wait_pos_backtraverse | 0 |
| Rpl_semi_sync_master_wait_sessions | 0 |
| Rpl_semi_sync_master_yes_tx | 1 |
+--------------------------------------------+-------+
14 rows in set (0.02 sec)