延迟复制的适用场景：
在大多数情况下默认的主从复制 延迟为零，但是有些情况需要从库的数据比主库的数据延迟。
1.主库误删除数据，可以延迟几个小时或者更久在从库查看到。
2.可以作为延迟测试。
3.可以查询历史数据，对准实时同步要求不高的情景。
MariaDB 10.2.3开始提供延迟复制的功能支持。
在从库设置：
查看正常的主动同步：
MariaDB [(none)]> show slave status\G
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 172.16.1.92
Master_User: repl
Master_Port: 4000
Connect_Retry: 60
Master_Log_File: node4_bin.000001
Read_Master_Log_Pos: 1534
Relay_Log_File: node5_relay.000002
Relay_Log_Pos: 1761
Relay_Master_Log_File: node4_bin.000001
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: 1534
Relay_Log_Space: 2066
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: 40
Master_SSL_Crl:
Master_SSL_Crlpath:
Using_Gtid: No
Gtid_IO_Pos:
Replicate_Do_Domain_Ids:
Replicate_Ignore_Domain_Ids:
Parallel_Mode: conservative
SQL_Delay: 0
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Slave has read all relay log; waiting for the slave I/O thread to update it
Slave_DDL_Groups: 6
Slave_Non_Transactional_Groups: 0
Slave_Transactional_Groups: 1
1 row in set (0.001 sec)
可以看到SQL_Delay的值为0.
在从库进行设置：
MariaDB [(none)]> stop slave;
Query OK, 0 rows affected (0.192 sec)
MariaDB [(none)]> CHANGE MASTER TO master_delay=60;
Query OK, 0 rows affected (0.003 sec)
MariaDB [(none)]> start slave;
Query OK, 0 rows affected (0.001 sec)
MariaDB [(none)]> show slave status\G
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 172.16.1.92
Master_User: repl
Master_Port: 4000
Connect_Retry: 60
Master_Log_File: node4_bin.000001
Read_Master_Log_Pos: 1534
Relay_Log_File: node5_relay.000002
Relay_Log_Pos: 555
Relay_Master_Log_File: node4_bin.000001
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: 1534
Relay_Log_Space: 860
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: 40
Master_SSL_Crl:
Master_SSL_Crlpath:
Using_Gtid: No
Gtid_IO_Pos:
Replicate_Do_Domain_Ids:
Replicate_Ignore_Domain_Ids:
Parallel_Mode: conservative
SQL_Delay: 60
SQL_Remaining_Delay: NULL
Slave_SQL_Running_State: Slave has read all relay log; waiting for the slave I/O thread to update it
Slave_DDL_Groups: 6
Slave_Non_Transactional_Groups: 0
Slave_Transactional_Groups: 1
1 row in set (0.000 sec)
延迟的单位是秒(s).设置之后 SQL_Delay: 60。
--验证：
在主库插入数据：
MariaDB [(none)]> insert into wuhan.city(cityname)values('shenzhen !');
Query OK, 1 row affected (0.022 sec)
MariaDB [(none)]> select * from wuhan.city;
+--------+------------+
| cityid | cityname |
+--------+------------+
| 1 | wuhan! |
| 2 | shenzhen ! |
+--------+------------+
2 rows in set (0.000 sec)
MariaDB [(none)]> select now();
+---------------------+
| now() |
+---------------------+
| 2018-07-18 17:12:30 |
+---------------------+
1 row in set (0.000 sec)
--在从库验证：
MariaDB [(none)]> select * from wuhan.city;
+--------+----------+
| cityid | cityname |
+--------+----------+
| 1 | wuhan! |
+--------+----------+
1 row in set (0.000 sec)
MariaDB [(none)]> select now();
+---------------------+
| now() |
+---------------------+
| 2018-07-18 17:12:41 |
+---------------------+
1 row in set (0.000 sec)
此时主库有2条记录，从库只有1条记录。
等待60s之后查询则有相同的数据：
MariaDB [(none)]> select now();
+---------------------+
| now() |
+---------------------+
| 2018-07-18 17:14:24 |
+---------------------+
1 row in set (0.000 sec)
MariaDB [(none)]> select * from wuhan.city;
+--------+------------+
| cityid | cityname |
+--------+------------+
| 1 | wuhan! |
| 2 | shenzhen ! |
+--------+------------+
2 rows in set (0.000 sec)延迟复制的过程中查看从库的状态：
MariaDB [(none)]> show slave status\G
*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 172.16.1.92
Master_User: repl
Master_Port: 4000
Connect_Retry: 60
Master_Log_File: node4_bin.000002
Read_Master_Log_Pos: 777
Relay_Log_File: node5_relay.000005
Relay_Log_Pos: 903
Relay_Master_Log_File: node4_bin.000002
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: 604
Relay_Log_Space: 1733
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: 4
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: 40
Master_SSL_Crl:
Master_SSL_Crlpath:
Using_Gtid: No
Gtid_IO_Pos:
Replicate_Do_Domain_Ids:
Replicate_Ignore_Domain_Ids:
Parallel_Mode: conservative
SQL_Delay: 60
SQL_Remaining_Delay: 56
Slave_SQL_Running_State: Waiting until MASTER_DELAY seconds after master executed event
Slave_DDL_Groups: 6
Slave_Non_Transactional_Groups: 0
Slave_Transactional_Groups: 3
1 row in set (0.000 sec)延迟复制我们关注4个参数选项即可：
Seconds_Behind_Master: 29
SQL_Delay: 60
SQL_Remaining_Delay: 31
Slave_SQL_Running_State: Waiting until MASTER_DELAY seconds after master executed event
说明：
Seconds_Behind_Master表示从库落后于主库的时间，单位为妙。不过此参数不够严谨，仅做参考。
SQL_Delay：设置的延迟时间，单位妙。
SQL_Remaining_Delay：若主库在insert、delete、update操作从库剩余延时的时间。若没有延时的时候此选项的值为NULL。
SQL_Remaining_Delay+Seconds_Behind_Master=SQL_Delay。
Slave_SQL_Running_State：当延时复制的时候出现Waiting until MASTER_DELAY seconds after master executed event。当延时复制完毕之后显示为Slave_SQL_Running_State: Slave has read all relay log; waiting for the slave I/O thread to update it
这里出现的状态可以通过命令 show processlist查看的一样：
MariaDB [(none)]> show processlist;
+----+-----------------+-----------+------+-----------+-------+----------------------------------------------------------------+------------------+----------+
| Id | User | Host | db | Command | Time | State | Info | Progress |
+----+-----------------+-----------+------+-----------+-------+----------------------------------------------------------------+------------------+----------+
| 1 | system user | | NULL | Daemon | NULL | InnoDB purge coordinator | NULL | 0.000 |
| 2 | system user | | NULL | Daemon | NULL | InnoDB purge worker | NULL | 0.000 |
| 4 | system user | | NULL | Daemon | NULL | InnoDB purge worker | NULL | 0.000 |
| 3 | system user | | NULL | Daemon | NULL | InnoDB purge worker | NULL | 0.000 |
| 5 | system user | | NULL | Daemon | NULL | InnoDB shutdown handler | NULL | 0.000 |
| 7 | event_scheduler | localhost | NULL | Daemon | 70173 | Waiting on empty queue | NULL | 0.000 |
| 20 | system user | | NULL | Slave_IO | 65817 | Waiting for master to send event | NULL | 0.000 |
| 21 | system user | | NULL | Slave_SQL | 11 | Waiting until MASTER_DELAY seconds after master executed event | NULL | 0.000 |