Using Pt-online schema change on multi master MYSQL We are using two MYSQL servers in ring replication where master is also a slave of the other master. Both have slaves connected to each other.
We want to run pt-online-schema-change to add couple of columns but we are not sure if it may break the replication due to ring replication in place. Any advice would be great.
If you use a dsn table as a recursion method, then pt-online-schema-change can do the job for you. This is documented here:

https://www.percona.com/doc/percona-toolkit/LATEST/pt-online-schema-change.html#cmdoption-pt-online-schema-change-recursion-method

If you use the hosts method, though, pt-osc will detect that SHOW SLAVE STATUS returns more than 1 row, but it considers the topology is using replication channels. You would get this error:

This server returned more than one row for SHOW SLAVE STATUS but "channel"
was not specified on the command line at bin/pt-online-schema-change line 4494.


For setting up topologies to test different circumstances, dbdeployer is a useful tool https://github.com/datacharmer/dbdeployer#replication-topologies.

Disclosure: I work for Percona