There is an interesting feature in MySQL 5.7+: the global variable super_read_only.  MySQL 5.6 and before only had the read_only global variable which is not preventing a user with SUPER privilege to write to the database.  With super_read_only, those users cannot write anymore.  But this feature comes with a surprise.

TL&DR: changing super_read_only to ON also sets read_only to ON and setting

This blog we describe an issue with MySQL 5.7’s super_read_only feature when used alongside with GTID in chained slave instances.

Background

In MySQL 5.7.5 and onward introduced the gtid_executed table in the MySQL database to store every GTID. This allows slave instances to use the GTID feature regardless whether the binlog option is set or not. Here is an example of the rows in the gtid_executed table:

mysql> SELECT * FROM mysql.gtid_executed;
+--------------------------------------+----------------+--------------+
| source_uuid                          | interval_start | interval_end | …

This blog post will discuss how to use the MySQL super_read_only system variable.

It is well known that replica servers in a master/slave configuration, to avoid breaking replication due to duplicate keys, missing rows or other similar issues, should not receive write queries. It’s a good practice to set

read_only=1

 on slave servers to prevent any (accidental) writes. Servers acting as replicas will NOT be in read-only mode automatically by default.

Sadly, 

read_only

 has an historical issue: users with the SUPER privilege can override the setting and could still run DML queries. Since Percona Server 5.6.21 and MySQL 5.7.8, however, you can use the

super_read_only

 feature to extend the

read_only

Percona Server 5.6.21+ and MySQL 5.7.8+ offer the super_read_only option that was first implemented in WebscaleSQL. Unlike read_only, this option prevents all users from running writes (even those with the SUPER privilege). Sure enough, this is a great feature, but what’s the relation with GTID? Read on!

TL;DR

Enabling super_read_only on all slaves when using GTID replication makes your topology far less sensitive to errant transactions. Failover is then easier and safer because creating errant transactions is much harder.

GTID replication is awesome…

For years, all MySQL DBAs in the world have been fighting with positioning when working with replication. Each time you move a slave from one master to another, you must be very careful to start …