

For linux only, lan only shared drive NFS is probably the easiest you’ll get, it’s made for that usecase.
If you want more of a dropbox/onedrive/google drive experience, Syncthing is really cool too, but that’s a whole other architecture qhere you have an actual copy on all machines.
I manage instances of both mongo and postgres at work.
I’ll say Mongo OpsManager is pretty sweet, and HA is way easier on Mongo.