That's great if your shards are truly independent of each other, but if not then you just invented a custom transaction layer living in your application code, w

That's great if your shards are truly independent of each other, but if not then you just invented a custom transaction layer living in your application code, which sounds way way worse than the original problem. And quite frankly, i think it is incredibly rare for the shards to both be fine grained and independent in typical oltp DB usecase.