前提是主從異步複製環境要提前搭建好，然後再開啓mysql增強半同步
環境：mysql5.7.26 主從異步複製早已部署好。
1.加載plugin插件
建議master和slave上全部執行（考慮到MHA的主從自動切換的環境）
在主庫安裝semisync_master.so和semisync_slave.so插件：
mysql> INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
mysql> INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';linux服務器上的master庫執行：
root@localhost [(none)]>INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
Query OK, 0 rows affected (4.52 sec)
root@localhost [(none)]>INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';
Query OK, 0 rows affected (0.07 sec)linux服務器上的slave庫執行：
root@localhost [(none)]>INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
ERROR 1125 (HY000): Function 'rpl_semi_sync_master' already exists
root@localhost [(none)]>INSTALL PLUGIN rpl_semi_sync_slave SONAME 'semisync_slave.so';
ERROR 1125 (HY000): Function 'rpl_semi_sync_slave' already exists
root@localhost [(none)]>提示:slave上增強半同步的插件已經存在，所以報錯。
原因：master上安裝增強半同步的插件後自動同步到了slave庫上
2.slave和master開啓增強半同步參數
生產環境上建議先在slave庫上的開啓 增強半同步參數
set global rpl_semi_sync_slave_enabled =1；
stop slave io_thread;start slave io_thread;此刻觀察master的錯誤日誌：提示開啓了Semi-sync replication 複製
2019-06-16T20:31:28.923731+08:00 718 [Note] While initializing dump thread for slave with UUID <7659cf56-8e81-11e9-bcbd-842b2b5999d9>, found a zombie dump thread with the same UUID. Master is killing the zombie dump thread(717).
2019-06-16T20:31:28.923838+08:00 717 [Note] Stop asynchronous binlog_dump to slave (server_id: 113306)
2019-06-16T20:31:28.923951+08:00 718 [Note] Start binlog_dump to master_thread_id(718) slave_server(113306), pos(, 4)
2019-06-16T20:31:28.923984+08:00 718 [Note] Start semi-sync binlog_dump to slave (server_id: 113306), pos(, 4)
2019-06-16T20:31:29.189007+08:00 718 [Note] Semi-sync replication switched ON at (mysql-bin.000005, 852045921)然後master上再開啓增強半同步參數
set global rpl_semi_sync_master_enabled =1
set global rpl_semi_sync_master_timeout =10003.MySQL官網配置方法
官方要求master和slave的/etc/my.cnf要開啓下面的參數：
rpl_semi_sync_master_enabled =1 # 0
rpl_semi_sync_slave_enabled =1 # 0
rpl_semi_sync_master_timeout =1000 # 1000(1 second) 同步複製中由於網絡原因導致複製時間超過1s後，增強半同步複製就變成了異步複製了
rpl_semi_sync_master_wait_for_slave_count=N
plugin_load_add =semisync_master.so #
plugin_load_add =semisync_slave.so #重啓master和slave數據庫。
4.實際生產環境配置方法
當然實際操作中還是不建議把參數rpl_semi_sync_master_enabled 和rpl_semi_sync_slave_enabled 直接寫入配置文件的。
原因：把參數加入my.cnf配置文件時，在slave庫掛掉，重新開啓slave庫或者是slave庫重啓後，會自動開啓增強半同步複製。
如果在slave庫斷開master庫時間較長時，最好是先開啓mysql異步複製，讓slave庫追趕上master庫後，然後再開啓增強半同步複製。這樣不會拖垮master庫。要是直接開啓增強半同步複製是會拖垮主庫的。
rpl_semi_sync_master_timeout =1000 ##1s 這個轉化爲異步複製的超時參數是可以寫入配置文件的。
5.演示增強半同步超時被關閉
演示增強半同步複製超過設定的時間被關閉：
在master上設置超時時間
mysql> set global rpl_semi_sync_master_timeout=20000; ##爲20s在slave上停止io_thread 線程
mysql> stop slave io_thread;然後在master庫上刪除一個測試庫來演示增強半同步關閉：
mysql> drop database test02; ##刪除test02庫卡盾超時20s 才執行成功。
Query OK, 0 rows affected (20.00 sec)在卡盾的20s期間，在master庫上執行show proccesslist,發現這期間，master庫一直在等待slave的 semi-sync 的ack的響應：
mysql> show processlist;
+-----+-----------------+-----------+------+---------+--------+--------------------------------------+----------------------+
| Id | User | Host | db | Command | Time | State | Info |
+-----+-----------------+-----------+------+---------+--------+--------------------------------------+----------------------+
| 1 | event_scheduler | localhost | NULL | Daemon | 185099 | Waiting on empty queue | NULL |
| 700 | root | localhost | NULL | Query | 3 | Waiting for semi-sync ACK from slave | drop database test02 |
| 703 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 709 | root | localhost | NULL | Sleep | 999 | | NULL |
+-----+-----------------+-----------+------+---------+--------+--------------------------------------+----------------------+
4 rows in set (0.00 sec)此時查看master庫的錯誤日誌關閉了增強半同步複製：
2019-06-16T18:58:38.249020+08:00 0 [ERROR] ./mysqld: Got an error reading communication packets ###這個報錯是slave庫 stop slave io_thread導致的
2019-06-16T18:59:14.870329+08:00 711 [ERROR] Semi-sync master failed on net_flush() before waiting for slave reply
2019-06-16T18:59:14.870409+08:00 711 [Note] Stop semi-sync binlog_dump to slave (server_id: 113306)
2019-06-16T18:59:14.870469+08:00 711 [Note] Aborted connection 711 to db: 'unconnected' user: 'rept' host: '192.168.0.11' (Found net error)
2019-06-16T18:59:34.870253+08:00 703 [Warning] Timeout waiting for reply of binlog (file: mysql-bin.000004, pos: 422036), semi-sync up to file mysql-bin.000004, position 421882.
2019-06-16T18:59:34.870298+08:00 703 [Note] Semi-sync replication switched OFF.當超過master庫上設定的20s超時時間後，增強半同步複製被關閉了。Waiting for semi-sync ACK from slave 消失
mysql> show processlist;
+-----+-----------------+-----------+------+---------+--------+------------------------+------------------+
| Id | User | Host | db | Command | Time | State | Info |
+-----+-----------------+-----------+------+---------+--------+------------------------+------------------+
| 1 | event_scheduler | localhost | NULL | Daemon | 185120 | Waiting on empty queue | NULL |
| 700 | root | localhost | NULL | Sleep | 4 | | NULL |
| 703 | root | localhost | NULL | Query | 0 | starting | show processlist |
| 709 | root | localhost | NULL | Sleep | 1020 | | NULL |
+-----+-----------------+-----------+------+---------+--------+------------------------+------------------+
4 rows in set (0.00 sec)接着在master上刪除test01庫時不再出現卡盾，說明增強半同步複製已經被關閉了
mysql> drop database test01;
Query OK, 2 rows affected (0.00 sec)
此時在maser查看當前增強半同步複製中有幾個slave client，發現已沒有client鏈接
mysql> show global status like "Rpl_semi_sync_master_clients";
+------------------------------+-------+
| Variable_name | Value |
+------------------------------+-------+
| Rpl_semi_sync_master_clients | 0 |
+------------------------------+-------+
1 row in set (0.01 sec)在slave上開啓 io_thread
mysql> start slave io_thread;
Query OK, 0 rows affected (0.00 sec)查看master庫上的錯誤日誌：
2019-06-16T19:10:46.656964+08:00 713 [Note] Start binlog_dump to master_thread_id(713) slave_server(113306), pos(, 4)
2019-06-16T19:10:46.657003+08:00 713 [Note] Start semi-sync binlog_dump to slave (server_id: 113306), pos(, 4)
2019-06-16T19:10:46.670872+08:00 0 [Note] Semi-sync replication switched ON at (mysql-bin.000004, 422366)提示增強半同步有開啓了。
master 查看半同步鏈接狀態新增了1個：
mysql> show global status like "Rpl_semi_sync_master_clients";
+------------------------------+-------+
| Variable_name | Value |
+------------------------------+-------+
| Rpl_semi_sync_master_clients | 1 |
+------------------------------+-------+
1 row in set (0.00 sec)
mysql> 6.觀察master庫錯誤日誌的變化
6.1 slave庫關閉io 線程，再關閉半同步複製，然後再開啓 slave 的io_thread線程
mysql> stop slave io_thread;
Query OK, 0 rows affected (0.00 sec)
mysql> set global rpl_semi_sync_slave_enabled=0 ;
Query OK, 0 rows affected (0.00 sec)
mysql> start slave io_thread;
Query OK, 0 rows affected (0.00 sec)
mysql> 此刻觀察master庫的錯誤日誌：
2019-06-16T20:35:14.463978+08:00 0 [ERROR] ./mysqld: Got an error reading communication packets
2019-06-16T20:35:44.309080+08:00 719 [Note] While initializing dump thread for slave with UUID <7659cf56-8e81-11e9-bcbd-842b2b5999d9>, found a zombie dump thread with the same UUID. Master is killing the zombie dump thread(718).
2019-06-16T20:35:44.309252+08:00 718 [Note] Stop semi-sync binlog_dump to slave (server_id: 113306)
2019-06-16T20:35:44.309724+08:00 719 [Note] Start binlog_dump to master_thread_id(719) slave_server(113306), pos(, 4)
2019-06-16T20:35:44.309740+08:00 719 [Note] Start asynchronous binlog_dump to slave (server_id: 113306), pos(, 4)6.2 slave庫關閉io 線程，等待20s後然後再關閉半同步複製，然後再開啓 slave 的io_thread線程
此刻觀察master庫的錯誤日誌：
2019-06-16T20:39:18.806715+08:00 0 [ERROR] ./mysqld: Got an error reading communication packets
2019-06-16T20:39:58.854194+08:00 720 [Note] Stop semi-sync binlog_dump to slave (server_id: 113306)
2019-06-16T20:39:58.854305+08:00 720 [Note] Aborted connection 720 to db: 'unconnected' user: 'rept' host: '192.168.0.11' (failed on flush_net())
2019-06-16T20:41:06.636868+08:00 721 [Note] Start binlog_dump to master_thread_id(721) slave_server(113306), pos(, 4)
2019-06-16T20:41:06.636902+08:00 721 [Note] Start asynchronous binlog_dump to slave (server_id: 113306), pos(, 4)發現6.1和6.2的錯誤日誌還是有不同的地方的
7.案例
當線上運行的MySQL增強半同步複製架構中，當其中的一臺slave庫掛掉了應該如何正確重新添加到原先的增強半同步複製架構？？
正確的做法如下：
1. 此2個參數rpl_semi_sync_master_enabled 和rpl_semi_sync_slave_enabled 不要直接寫入到my.cnf配置文件開啓。
2.在slave庫上先 stop slave io_thread ;set global rpl_semi_sync_slave_enabled=0 關閉此參數。然後start slave io_thread 或者start slave 開啓異步複製，讓slave庫追趕上master庫。
3.然後在slave庫 set global rpl_semi_sync_slave_enabled=1 ；stop slave io_thread;start slave io_thread;演示到此處，歡迎留言一起交流學習。