测试环境：mysql5.7.26 一主两从
查看主库状态：
mysql> show status like 'Rpl_semi_sync_master_status'; +-----------------------------+-------+ | Variable_name | Value | +-----------------------------+-------+ | Rpl_semi_sync_master_status | ON | +-----------------------------+-------+ 1 row in set (0.00 sec) mysql> show variables like 'rpl_semi_sync_master_timeout'; +------------------------------+-------+ | Variable_name | Value | +------------------------------+-------+ | rpl_semi_sync_master_timeout | 10000 | +------------------------------+-------+ 1 row in set (0.00 sec)
查看从库状态：
从库一
mysql> show status like 'Rpl_semi_sync_slave_status'; +----------------------------+-------+ | Variable_name | Value | +----------------------------+-------+ | Rpl_semi_sync_slave_status | ON | +----------------------------+-------+ 1 row in set (0.00 sec) mysql> show slave status \G *************************** 1. row *************************** Slave_IO_State: Waiting for master to send event Master_Host: 172.16.254.163 Master_User: repl Master_Port: 3307 Connect_Retry: 60 Master_Log_File: mysql-bin.000012 Read_Master_Log_Pos: 154 Relay_Log_File: 163-3308-relay-bin.000005 Relay_Log_Pos: 367 Relay_Master_Log_File: mysql-bin.000012 Slave_IO_Running: Yes Slave_SQL_Running: Yes Replicate_Do_DB: Replicate_Ignore_DB: Replicate_Do_Table: Replicate_Ignore_Table: Replicate_Wild_Do_Table: Replicate_Wild_Ignore_Table: Last_Errno: 0 Last_Error: Skip_Counter: 0 Exec_Master_Log_Pos: 154 Relay_Log_Space: 743 Until_Condition: None Until_Log_File: Until_Log_Pos: 0 Master_SSL_Allowed: No Master_SSL_CA_File: Master_SSL_CA_Path: Master_SSL_Cert: Master_SSL_Cipher: Master_SSL_Key: Seconds_Behind_Master: 0 Master_SSL_Verify_Server_Cert: No Last_IO_Errno: 0 Last_IO_Error: Last_SQL_Errno: 0 Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 1633307 Master_UUID: badf10a9-e13f-11ea-ac1b-1866daf6f710 Master_Info_File: /mysql/mysql3308/data/master.info SQL_Delay: 0 SQL_Remaining_Delay: NULL Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates Master_Retry_Count: 86400 Master_Bind: Last_IO_Error_Timestamp: Last_SQL_Error_Timestamp: Master_SSL_Crl: Master_SSL_Crlpath: Retrieved_Gtid_Set: Executed_Gtid_Set: Auto_Position: 0 Replicate_Rewrite_DB: Channel_Name: Master_TLS_Version: 1 row in set (0.00 sec)
从库二
show status like 'Rpl_semi_sync_slave_status'; +----------------------------+-------+ | Variable_name | Value | +----------------------------+-------+ | Rpl_semi_sync_slave_status | ON | +----------------------------+-------+ 1 row in set (0.00 sec) mysql> show slave status \G *************************** 1. row *************************** Slave_IO_State: Waiting for master to send event Master_Host: 172.16.254.163 Master_User: repl Master_Port: 3307 Connect_Retry: 60 Master_Log_File: mysql-bin.000012 Read_Master_Log_Pos: 154 Relay_Log_File: 163-3309-relay-bin.000005 Relay_Log_Pos: 367 Relay_Master_Log_File: mysql-bin.000012 Slave_IO_Running: Yes Slave_SQL_Running: Yes Replicate_Do_DB: Replicate_Ignore_DB: Replicate_Do_Table: Replicate_Ignore_Table: Replicate_Wild_Do_Table: Replicate_Wild_Ignore_Table: Last_Errno: 0 Last_Error: Skip_Counter: 0 Exec_Master_Log_Pos: 154 Relay_Log_Space: 743 Until_Condition: None Until_Log_File: Until_Log_Pos: 0 Master_SSL_Allowed: No Master_SSL_CA_File: Master_SSL_CA_Path: Master_SSL_Cert: Master_SSL_Cipher: Master_SSL_Key: Seconds_Behind_Master: 0 Master_SSL_Verify_Server_Cert: No Last_IO_Errno: 0 Last_IO_Error: Last_SQL_Errno: 0 Last_SQL_Error: Replicate_Ignore_Server_Ids: Master_Server_Id: 1633307 Master_UUID: badf10a9-e13f-11ea-ac1b-1866daf6f710 Master_Info_File: /mysql/mysql3309/data/master.info SQL_Delay: 0 SQL_Remaining_Delay: NULL Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates Master_Retry_Count: 86400 Master_Bind: Last_IO_Error_Timestamp: Last_SQL_Error_Timestamp: Master_SSL_Crl: Master_SSL_Crlpath: Retrieved_Gtid_Set: Executed_Gtid_Set: Auto_Position: 0 Replicate_Rewrite_DB: Channel_Name: Master_TLS_Version: 1 row in set (0.00 sec)
主库创建测试表：
mysql> use test; Reading table information for completion of table and column names You can turn off this feature to get a quicker startup with -A Database changed mysql> create table t1(id int); Query OK, 0 rows affected (0.01 sec)
可以看到很快就行创建完毕。
此时关闭从库一的复制
mysql> stop slave; Query OK, 0 rows affected (0.00 sec)
主库插入一条记录
mysql> insert into t1(id) values(1); Query OK, 1 row affected (0.00 sec)
依然很快就能插入成功。
此时把从库二的复制也关闭
mysql> stop slave; Query OK, 0 rows affected (0.00 sec)
再次往测试表t1插入数据
mysql> insert into t1(id) values(1); Query OK, 1 row affected (10.00 sec) mysql> insert into t1(id) values(1); Query OK, 1 row affected (0.00 sec) mysql> insert into t1(id) values(1); Query OK, 1 row affected (0.00 sec)
可以看到第一次insert用了10s，这和rpl_semi_sync_master_timeout=10000相吻合。
此时查看主库状态：
mysql> show status like 'Rpl_semi_sync_master_status'; +-----------------------------+-------+ | Variable_name | Value | +-----------------------------+-------+ | Rpl_semi_sync_master_status | OFF | +-----------------------------+-------+ 1 row in set (0.00 sec)
半同步复制已经关闭，降级为了异步复制。
主库在执行完客户端提交的事务后不是立刻返回给客户端，而是等待至少一个从库接收到并写到relay log中才返回给客户端。相对于异步复制，半同步复制提高了数据的安全性，同时它也造成了一定程度的延迟，这个延迟最少是一个TCP/IP往返的时间。所以， 半同步复制最好在低延时的网络中使用。