問題

設定半同步複製變數時報錯

mysql> set global rpl_semi_sync_master_enabled = 0;
ERROR 1193 (HY000): Unknown system variable 'rpl_semi_sync_master_enabled'

或者

mysql> set global rpl_semi_sync_slave_enabled = 0;
ERROR 1193 (HY000): Unknown system variable 'rpl_semi_sync_slave_enabled'

這是因為沒有安裝半同步外掛。
檢視plugins，沒有發現半同步複製外掛。

mysql> show plugins;
+----------------------------+----------+--------------------+-----------------+---------+
| Name                       | Status   | Type               | Library         | License |
+----------------------------+----------+--------------------+-----------------+---------+
| keyring_file               | ACTIVE   | KEYRING            | keyring_file.so | GPL     |
| binlog                     | ACTIVE   | STORAGE ENGINE     | NULL            | GPL     |
| mysql_native_password      | ACTIVE   | AUTHENTICATION     | NULL            | GPL     |
| sha256_password            | ACTIVE   | AUTHENTICATION     | NULL            | GPL     |
| CSV                        | ACTIVE   | STORAGE ENGINE     | NULL            | GPL     |
| MEMORY                     | ACTIVE   | STORAGE ENGINE     | NULL            | GPL     |
| InnoDB                     | ACTIVE   | STORAGE ENGINE     | NULL            | GPL     |
| INNODB_TRX                 | ACTIVE   | INFORMATION SCHEMA | NULL            | GPL     |
| INNODB_LOCKS               | ACTIVE   | INFORMATION SCHEMA | NULL            | GPL     |
| INNODB_LOCK_WAITS          | ACTIVE   | INFORMATION SCHEMA | NULL            | GPL     |
... ...
 

解決方案

首先，當前MySQL版本是5.7.23，已自帶半同步複製外掛安裝包。

mysql> select @@version;
+-----------+
| @@version |
+-----------+
| 5.7.23    |
+-----------+
1 row in set (0.00 sec)

安裝rpl_semi_sync_master plugin

mysql> install plugin rpl_semi_sync_master soname 'semisync_master.so';
Query OK, 0 rows affected (0.08 sec)
 

重新設定

mysql> set global rpl_semi_sync_master_enabled = 0;
Query OK, 0 rows affected (0.00 sec)

安裝rpl_semi_sync_slave plugin：

mysql> install plugin rpl_semi_sync_slave soname 'semisync_slave.so';
Query OK, 0 rows affected (0.02 sec)

重新設定：

mysql> set global rpl_semi_sync_slave_enabled = 0;
Query OK, 0 rows affected (0.00 sec)

在主庫例項和從庫例項上，都安裝兩個半同步複製外掛。
因為如果發生主從切換，從庫會成為主庫。

檢視半同步複製相關的plugins：

mysql> SELECT PLUGIN_NAME FROM INFORMATION_SCHEMA.PLUGINS WHERE PLUGIN_NAME LIKE 'rpl_semi_sync_%';
+----------------------+
| PLUGIN_NAME          |
+----------------------+
| rpl_semi_sync_master |
| rpl_semi_sync_slave  |
+----------------------+
2 rows in set (0.00 sec)

mysql>

可以看到，有兩個半同步複製外掛。