Replication : Check if an instance can replicate from another before a switchover

[laurent-indermuehle]

I want to write a playbook that performs a manual switchover, exchanging the primary role with a replica, in a MariaDB GTID replica-set.
I need a method to verify if the replica has successfully read all the binary logs and if the current primary will be able to replicate from it.

Previously I was using the Golang app named Orchestrator. It was performing a series of tests here. In plain text this translate as follow:

• Is the hostname different : instance cannot replicate from itself
• Is replication updates enabled : I'm not sure why this test exists
• Is smaller Major Version : new replica should use a newer version than the new primary
• Is log bin enabled : cannot replicate if binlog are disabled
• Is there a filter on the replication : if the replica don't have all the data, it can't become a primary
• Is Server ID different : replication will fail if both instances uses the same ID
• Is Server UUID different : replication will fail if both instances uses the same UUID
• Is SQL thread lag acceptable : allow switchover only if the replica is below a threshold delay behind the primary

Also I think this test is missing:

• Is replica having errant GTID : if true, won't accept the switchover

How could I implement this in c.mysql? I need to compare 2 instances. Not only facts, but also the returned values of the community.mysql.mysql_replication and mysql_info modules.

It could be something like:

- name: Check if replica and primary are ready to switchover
  community.mysql.mysql_replication:
    mode: check_before_switchover
    primary: db_srv1
    replica: db_srv2

This should "run_once" if the playbook target both servers. Also I think it MUST target both instances to work.

I have very few experience writing Ansible modules. Any advice would be appreciated.