Nowadays, in most system architectures, app servers are stateless and can be scaled horizontally. But something needs to hold the state, and that thing will be

Nowadays, in most system architectures, app servers are stateless and can be scaled horizontally. But something needs to hold the state, and that thing will be the bottleneck. Yes, you can horizontally scale some databases, but distributed state is very tricky to get right and will have performance trade-off.