异步复制的缺点在于主库只负责将数据库的增，改，查操作记录到 binary log中，而从库在通过日志对操作进行回放同步之后，主库对从库的同步内容不检测，不知道是否同步,这里引入半同步复制解决这一问题.
半同步复制在协议中添加了一个同步步骤。 这意味着主节点在提交时需要等待从
节点确认它已经接收到事务。只有这样,主节点才能继续提交操作。
这种模式下主节点只需要接收到其中一台从节点的返回信息，就会commit；否则需要等待直到超时时间然后切换成异步模式再提交；这样做的目的可以使主从数据库的数据延迟缩小，可以提高数据安全性，确保了事务提交后，binlog至少传输到了一个从节点上，不能保证从节点将此事务更新到db中。性能上会有一定的降低，响应时间会变长。

如下图所示：

1.在主库安装插件并激活

mysql> INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';		##安装插件
Query OK, 0 rows affected (0.04 sec)

mysql> SELECT PLUGIN_NAME, PLUGIN_STATUS FROM 	INFORMATION_SCHEMA.PLUGINS WHERE PLUGIN_NAME LIKE '%semi%';			## 查看插件
+----------------------+---------------+
| PLUGIN_NAME          | PLUGIN_STATUS |
+----------------------+---------------+
| rpl_semi_sync_master | ACTIVE        |
+----------------------+---------------+
1 row in set (0.00 sec)

mysql> SET GLOBAL rpl_semi_sync_master_enabled =1;			##激活
Query OK, 0 rows affected (0.00 sec)

2.在从库安装插件并激活

mysql> INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';
Query OK, 0 rows affected (0.04 sec)

mysql> SET GLOBAL rpl_semi_sync_slave_enabled =1;
Query OK, 0 rows affected (0.00 sec)

3.在主库查看信息

mysql> show status like '%rpl%';
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 0     |
| 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 variables like '%rpl%';
+-------------------------------------------+------------+
| Variable_name                             | Value      |
+-------------------------------------------+------------+
| rpl_semi_sync_master_enabled              | ON         |
| rpl_semi_sync_master_timeout              | 10000      |
| 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_stop_slave_timeout                    | 31536000   |
+-------------------------------------------+------------+
7 rows in set (0.01 sec)

4.在从库查看信息

mysql> show variables like '%rpl%';
+---------------------------------+----------+
| Variable_name                   | Value    |
+---------------------------------+----------+
| rpl_semi_sync_slave_enabled     | ON       |
| rpl_semi_sync_slave_trace_level | 32       |
| rpl_stop_slave_timeout          | 31536000 |
+---------------------------------+----------+
3 rows in set (0.00 sec)

5.测试

1.将从库的IO线程关闭，在主库添加信息,模拟网络卡顿

slave:

mysql> STOP SLAVE IO_THREAD;		#此时主库收不到从库发送的ack，模仿网络卡顿
Query OK, 0 rows affected (0.00 sec)

master:

mysql> use redhat;
Database changed
mysql> select * from usertb;
+----------+----------+
| username | password |
+----------+----------+
| user1    | 123      |
| user2    | 123      |
+----------+----------+
2 rows in set (0.01 sec)

mysql> insert into usertb values ('user3','123');
Query OK, 1 row affected (10.05 sec)

mysql> select * from usertb;
+----------+----------+
| username | password |
+----------+----------+
| user1    | 123      |
| user2    | 123      |
| user3    | 123      |
+----------+----------+
3 rows in set (0.00 sec)

在slaver上查看,可以看到数据不能同步

mysql> SELECT * FROM redhat.usertb;                             +----------+----------+
| username | password |
+----------+----------+
| user1    | 123      |
| user2    | 123      |
+----------+----------+
2 rows in set (0.00 sec)

2.在主库查看状态,打开从库IO线程，模拟网络通畅

master:

mysql> show status like '%rpl%';		#此时半同步已经关闭,变成了异步复制
+--------------------------------------------+-------+
| Variable_name                              | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients               | 0     |
| 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              | 1     |
| Rpl_semi_sync_master_no_tx                 | 1     |
| Rpl_semi_sync_master_status                | OFF   |
| 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)

slave:

mysql> show status like '%rpl%';			#半同步关闭，异步开启
+----------------------------+-------+
| Variable_name              | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | OFF   |
+----------------------------+-------+
1 row in set (0.00 sec)

mysql> START SLAVE IO_THREAD;		#打开io,模拟网络恢复通畅
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM redhat.usertb;		##查看数据,已经同步
+----------+----------+
| username | password |
+----------+----------+
| user1    | 123      |
| user2    | 123      |
| user3    | 123      |
+----------+----------+
3 rows in set (0.00 sec)