Mysql半同步复制模式
Mysql复制默认是异步完成的，半同步方式是google为mysql开发的一个补丁，在mysql5.5或者更高的版本中已经集成了这个功能。
半同步的意思表示MASTER 只需要接收到其中一台SLAVE的返回信息，就会commit；否则需等待直至达到超时时间然后切换成异步再提交。
这个做可以使主从库的数据的延迟较小，可以在损失很小的性能的前提下提高数据的安全性。
半同步的开启比较简单，是需要在master和slave都安装半同步插件，并启用就可以了。
查询mysql复制有没有使用半同步，
mysql> show variables like 'rpl%';
+-------------------+-------+
| Variable_name | Value |
+-------------------+-------+
| rpl_recovery_rank | 0 |
+-------------------+-------+
1 row in set (0.00 sec)
或者使用show status like 'rpl%';
启用半同步模式：
在master执行
mysql> install plugin rpl_semi_sync_master soname 'semisync_master.so';
Query OK, 0 rows affected (0.03 sec)
mysql> set global rpl_semi_sync_master_enabled=0;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like 'rpl%';
+------------------------------------+-------+
| Variable_name | Value |
+------------------------------------+-------+
| rpl_recovery_rank | 0 |
| 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 |
+------------------------------------+-------+
5 rows in set (0.00 sec)
mysql> set global rpl_semi_sync_master_enabled=1;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like 'rpl%';
+------------------------------------+-------+
| Variable_name | Value |
+------------------------------------+-------+
| rpl_recovery_rank | 0 |
| rpl_semi_sync_master_enabled | ON |
| rpl_semi_sync_master_timeout | 10000 |
| rpl_semi_sync_master_trace_level | 32 |
| rpl_semi_sync_master_wait_no_slave | ON |
+------------------------------------+-------+
rpl_semi_sync_master_enabled 的值是0(on)或者1(off)，默认是1.
rpl_semi_sync_master_timeout的值默认是10000（10s）
5 rows in set (0.00 sec)
在slave执行如下操作：
mysql> install plugin rpl_semi_sync_slave soname 'semisync_slave.so';
Query OK, 0 rows affected (0.03 sec)
mysql> set global rpl_semi_sync_slave_enabled=1;
Query OK, 0 rows affected (0.00 sec)
mysql> show variables like 'rpl%';
+---------------------------------+-------+
| Variable_name | Value |
+---------------------------------+-------+
| rpl_recovery_rank | 0 |
| rpl_semi_sync_slave_enabled | ON |
| rpl_semi_sync_slave_trace_level | 32 |
+---------------------------------+-------+
3 rows in set (0.00 sec)
mysql> show status like 'rpl%';
+----------------------------+-------------+
| Variable_name | Value |
+----------------------------+-------------+
| Rpl_semi_sync_slave_status | ON |
| Rpl_status | AUTH_MASTER |
+----------------------------+-------------+
2 rows in set (0.01 sec)
mysql> stop slave io_thread;
Query OK, 0 rows affected (0.00 sec)
mysql> start slave io_thread;
Query OK, 0 rows affected (0.00 sec)
rpl_semi_sync_slave_enabled的值是0(on)或者1(off)，默认是1.
这些执行完成后，半同步就配置完了。
Master看到的状态：
mysql> show status like 'rpl%';
+--------------------------------------------+-------------+
| 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 |
| Rpl_status | AUTH_MASTER |
+--------------------------------------------+-------------+
15 rows in set (0.00 sec)
Slave看到的状态：
mysql> show status like 'rpl%';
+----------------------------+-------------+
| Variable_name | Value |
+----------------------------+-------------+
| Rpl_semi_sync_slave_status | ON |
| Rpl_status | AUTH_MASTER |
+----------------------------+-------------+
2 rows in set (0.01 sec)