MySQL replication works great and indeed an great way to scale reads as well for high availability but what are pain points if you have not planned for a delayed slave ? What is delayed slave ? Before I answer this question, Have you ever thought about how an wrong transaction can corrupt MySQL master and all connected slaves ? The answer for this problem is a delayed slave replication, It’s actually you are intentionally lagging MySQL slave to a master by few minutes / hours so that you have always a MySQL instance far from damage. There are two ways to do this, 1. MySQL delayed slave replication , 2. Percona Toolkit pt-slave-delay (from Percona – https://www.percona.com/software/database-tools/percona-toolkit)

In this exercise I have used MySQL 5.7 on CentOS 7.4 with Percona Toolkit . I have written a blog on step-by-step installation of MySQL Master-Slave replication on CentOS here – https://minervadb.com/index.php/2018/01/24/step-by-step-mysql-master-slave-replication-on-centos/

MySQL delayed slave replication 

Step 1: Login with MySQL root user in Slave server

[root@localhost ~]# mysql -u root -p


Enter password:


mysql>

Step 2: Stop the slave

mysql> stop slave;


Query OK, 0 rows affected (0.01 sec)

mysql>

Step 3: Now set delay time for replication in MySQL slave server. Here in this example, I am setting for 60 minutes or 3600 seconds.

mysql> CHANGE MASTER TO MASTER_DELAY = 3600;

Step 4: Start the mysql slave

mysql> start slave;

Query OK, 0 rows affected (0.01 sec)

Step 5: Now check the SLAVE status

mysql> show slave status\G

 Master_Server_Id: 500

                  Master_UUID: 5ea8a1ea-f45e-11e7-bf7b-080027169869

             Master_Info_File: /var/lib/mysql/master.info

                    SQL_Delay: 3600

          SQL_Remaining_Delay: NULL

      Slave_SQL_Running_State: Slave has read all relay log; waiting for more updates

How to reset back to delay seconds Zero or default delay setting ?

You just need to stop and start MySQL slave

mysql> stop slave;

Query OK, 0 rows affected (0.00 sec)

mysql> start slave;

Query OK, 0 rows affected (0.01 sec)

Using  pt-slave-delay for implementing delayed slave 

pt-slave-delay is bundled with Percona Toolkit (https://www.percona.com/software/database-tools/percona-toolkit) . The steps here are quite direct and simple (here also we have delayed slave by 60 minutes), I have explained same below :

[root@localhost ~]# pt-slave-delay --delay 60m --user root --password PasswordComplex001

2018-01-29T16:07:28 slave running 0 seconds behind

2018-01-29T16:07:28 STOP SLAVE until 2018-01-29T17:07:28 at master position mysql-bin.000008/154

When you are building MySQL replication, please plan for delayed slaves to rollback if needed

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