One simple question. I would like to containerize a postgresql RDBMS and use a volume for persistent storage. Say it is time to scale because the traffic is rising. Since postgresql does not support multi master, how this is going to work? Because one request might go to x pod and another to y pod.
Just like when you are running Postgres anywhere else, you scale it mostly by either manual sharding (i.e. breaking data into discrete silos) or by throwing more horsepower (read: money) at it. Kubernetes doesn't particularly help or hinder either approach, Postgres is still just Postgres no matter how you run it.
Shoutout to Citus though, which does allow some level of load-sharing, even if it's not a true distributed system.