3 node Group Replication cluster is accepting writes with single primary online upon 2 pod failure

MySQL & MariaDB
1

Objective: Test behaviour of cluster upon 2 consecutive pod failure
Cluster Detail: 3 node Group replication cluster
Rejoin disabled (for testing): SET PERSIST group_replication_start_on_boot=OFF;
Platform: Kubernetes
Image used: percona/percona-server:8.0.34-aarch64

Steps:

  1. On a 3 node healthy cluster setup, we disabled the rejoin: SET PERSIST group_replication_start_on_boot=OFF;

  2. Issued pod delete command: kubectl delete mysql-1 mysql-2

  3. Identified both the primary (mysql-1) and secondary (mysql-2) were shutdown via mysql logs and left the replica group.

  4. The only available node was mysql-0 as PRIMARY and ONLINE, accepting writes.
    PFA screenshot:

    Screenshot 2023-12-19 at 12.02.46 AM
    Screenshot 2023-12-19 at 12.02.46 AM3456×358 113 KB

  5. Same behaviour happens when we delete mysql-0 and mysql-1. mysql-2 eventually gets elected as PRIMARY in few seconds and is PRIMARY & ONLINE accepting writes.

Question:

  1. As per the behaviour, the cluster must be in RO mode when majority of the mysql nodes are down. Why is the above cluster still writable?
  2. Is there any mysql configuration we need to explicitly set to disable this behaviour?
2

Was able to identify the reason:

Reference: https://dev.mysql.com/doc/refman/8.0/en/group-replication-network-partitioning.html

On the other hand, if servers exit the group voluntarily, they instruct the 
group that it should reconfigure itself. In practice, this means that a server 
that is leaving tells others that it is going away. This means that other 
members can reconfigure the group properly, the consistency of the membership 
is maintained and the majority is recalculated. For example, in the above 
scenario of 5 servers where 3 leave at once, if the 3 leaving servers warn
the group that they are leaving, one by one, then the membership is able
to adjust itself from 5 to 2, and at the same time, securing quorum while 
that happens.
3

@matthewb / team

Is there any mysql setting / configuration using which if a cluster size gets reduced to 1, even after voluntary exit of members, the primary will get set with READ_ONLY = ON?
As i want to restrict writes to a single GR node as in case its down will cause data loss.