MySQL rpl_semi

半同步

MySQL从 5.5 版本开始通过以插件的形式开始支持半同步的主从复制模式。

1 半同步复制模式参数

1.1 主库

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_semi_sync_slave_enabled               | OFF        |
| rpl_semi_sync_slave_trace_level           | 32         |
| rpl_stop_slave_timeout                    | 120        |
+-------------------------------------------+------------+
9 rows in set (0.00 sec)
-- 半同步复制模式开关
rpl_semi_sync_master_enabled
-- 半同步复制，超时时间，单位毫秒，当超过此时间后，自动切换为异步复制模式 
rpl_semi_sync_master_timeout
-- MySQL 5.7.3引入的，该变量设置主需要等待多少个slave应答，才能返回给客户端，默认为1。
rpl_semi_sync_master_wait_for_slave_count
-- 此值代表当前集群中的slave数量是否还能够满足当前配置的半同步复制模式，默认为ON，当不满足半同步复制模式后，全部Slave切换到异步复制，此值也会变为OFF
rpl_semi_sync_master_wait_no_slave
-- 代表半同步复制提交事务的方式，5.7.2之后，默认为AFTER_SYNC
rpl_semi_sync_master_wait_point

1.2 备库

mysql>  show variables like '%Rpl%';
+-------------------------------------------+------------+
| 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_for_slave_count | 1          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | ON         |
| rpl_semi_sync_slave_trace_level           | 32         |
| rpl_stop_slave_timeout                    | 120        |
+-------------------------------------------+------------+
9 rows in set (0.00 sec)

1.3 从库

mysql> show variables like '%Rpl%';
+-------------------------------------------+------------+
| 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_for_slave_count | 1          |
| rpl_semi_sync_master_wait_no_slave        | ON         |
| rpl_semi_sync_master_wait_point           | AFTER_SYNC |
| rpl_semi_sync_slave_enabled               | OFF        |
| rpl_semi_sync_slave_trace_level           | 32         |
| rpl_stop_slave_timeout                    | 120        |
+-------------------------------------------+------------+
9 rows in set (0.01 sec)

2 关闭半同步

set global rpl_semi_sync_master_enabled = 0;

var sql string
if activeSlaveCount == 0 {
	sql = "set global rpl_semi_sync_master_enabled = 0; set global rpl_semi_sync_slave_enabled = 0"
} else {
	sql = "set global rpl_semi_sync_master_wait_for_slave_count = 1"
}

3 线上实践

切换流程中不对此参数做修改，备节点自愈完成时对参数进行重新设置