Re: Wasn't Split Brain solved 30 years ago?
Yes, VMS cluster, Solaris cluster, Veritas cluster, they all do that for local HA, and it works. Disk vendors like EMC and HDS can also do it for their distributed storage.
It gets much harder once you bring geographical distances into the picture. Fully synchronized updates so that both sites always have identical copies of data gets too slow once you go much past a few mS latency, so you have to live with the fact that each site has a different view.
There are solutions, usually requiring more than three sites with a mix of local/remote and sync/async replication. That gets expensive.
Oracle DataGuard (yeah, I know it's the evil empire, but the DB is still good) has a useful feature called "far sync" where the data exists only at the two main sites, but there are additional copies of the redo logs maintained synchronously at nearby sites. If a main site is lost, the survivor will automatically fetch & replay the logs to make sure it has caught up before becoming active. Still needs multiple sites, but the intermediate ones only need small systems & storage for logs, not the big servers & TB disks it would require to keep another copy of the whole DB.