半同步複製功能的目的是爲了保證在master出問題的時候，至少有一臺Slave的數據是完整的。在超時的情況下也可以臨時轉入異步複製，保障業務的正常使用，
直到一臺salve追趕上之後，繼續切換到半同步模式。
如果主備網絡故障或者備庫掛了，主庫在事務提交後等待10秒（rpl_semi_sync_master_timeout的默認值）後，就會繼續。這時，主備將會自動降級爲異步模式。
半同步模式其實是作爲的一個plugin實現的，master和slave使用不同的plugin。默認情況下沒有安裝該plugin，官方提供的MySQL Server RPM包默認安裝後，
會在/usr/lib(64)/mysql/plugin/下面找到該插件。安裝完插件之後，還需要手動設置系統參數以開啓半同步複製模式。
詳細的過程可參考下面步驟。
[Master]
首先先檢查 mysql是否支持動態添加插件
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設置了半同步模式
| 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 | #表示沒有成功接收slave回執的提交次數
| Rpl_semi_sync_master_status | ON | #表示當前是異步模式還是半同步模式。
| 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回執的提交次數
+--------------------------------------------+-------+
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)
然後重啓replication(stop slave;start slave;)即可。
一定需要重啓，否則master無法確認該slave是否開啓了半同步。相同的操作可以在任意多個slave中進行設置。[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)整理自網絡
Svoid
2014-08-05
