半同步复制功能的目的是为了保证在master出问题的时候，至少有一台Slave的数据是完整的。在超时的情况下也可以临时转入异步复制，保障业务的正常使用，
直到一台salve追赶上之后，继续切换到半同步模式。
如果主备网络故障或者备库挂了，主库在事务提交后等待10秒（rpl_semi_sync_master_timeout的默认值）后，就会继续。这时，主备将会自动降级为异步模式。
半同步模式其实是作为的一个plugin实现的，master和slave使用不同的plugin。默认情况下没有安装该plugin，官方提供的MySQL Server RPM包默认安装后，
会在/usr/lib(64)/mysql/plugin/下面找到该插件。安装完插件之后，还需要手动设置系统参数以开启半同步复制模式。
详细的过程可参考下面步骤。
[Master]
首先先检查 mysql是否支持动态添加插件
mysql> show variables like 'have_dynamic_loading';
+----------------------+-------+
| Variable_name | Value |
+----------------------+-------+
| have_dynamic_loading | YES |
+----------------------+-------+
1 row in set (0.02 sec)
mysql> install plugin rpl_semi_sync_master soname 'semisync_master.so';
Query OK, 0 rows affected (0.04 sec)
mysql> show variables like '%sem%';
+------------------------------------+-------+
| 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 (0.00 sec)
mysql> set global rpl_semi_sync_master_enabled = ON;
Query OK, 0 rows affected (0.00 sec)
mysql> set global rpl_semi_sync_master_timeout=1000; #毫秒
Query OK, 0 rows affected (0.00 sec)
mysql> show status like '%sem%';
+--------------------------------------------+-------+
| Variable_name | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients | 0 | #表示有多少slave设置了半同步模式
| 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 | #表示没有成功接收slave回执的提交次数
| Rpl_semi_sync_master_status | ON | #表示当前是异步模式还是半同步模式。
| 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 | #表示成功接收slave回执的提交次数
+--------------------------------------------+-------+
14 rows in set (0.01 sec)[Slave]
mysql> install plugin rpl_semi_sync_slave soname 'semisync_slave.so';
Query OK, 0 rows affected (0.03 sec)
mysql> show variables like '%sem%';
+---------------------------------+-------+
| 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 = ON;
Query OK, 0 rows affected (0.00 sec)
然后重启replication(stop slave;start slave;)即可。
一定需要重启，否则master无法确认该slave是否开启了半同步。相同的操作可以在任意多个slave中进行设置。[slave]
mysql> show status like '%sem%';
+----------------------------+-------+
| Variable_name | Value |
+----------------------------+-------+
| Rpl_semi_sync_slave_status | ON |
+----------------------------+-------+
1 row in set (0.01 sec)
[master]
mysql> show status like '%sem%';
+--------------------------------------------+-------+
| Variable_name | Value |
+--------------------------------------------+-------+
| Rpl_semi_sync_master_clients | 1 |
| Rpl_semi_sync_master_net_avg_wait_time | 9978 |
| Rpl_semi_sync_master_net_wait_time | 19957 |
| Rpl_semi_sync_master_net_waits | 2 |
| Rpl_semi_sync_master_no_times | 1 |
| Rpl_semi_sync_master_no_tx | 2 |
| 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)整理自网络
Svoid
2014-08-05
