Backups in such a system are quite pointless; if losing 10 seconds of data means you lost 4000 transactions then periodic backups are invalid if not instantly than close to instantly.

The system I work on has such a property and the only real infra style approach is sync replication before responding to a caller and a delayed replica for delete/drop protections (say with a 2hr or more window).

Should also defend for this in your code (be able to reply from your initiation systems also etc)

Maybe? If you assume a cell can just disappear at a moment's notice, then I'm guessing you don't even try backing it up. Whatever goes into and out of the cell (request logs and results) gets backed up, and no doubt that's more complicated than a monolithic system, but it may not be so bad assuming the replay systems and global transaction router do their thing?