[adityaathalye]

I feel the Expression Problem neatly frames the "diagonal scaling" proposition; what system design choices will allow the architecture to scale vertically in what fashion, while also being able to scale what horizontally, without losing strict serialisability.

If we add a "vertical" capability, it cannot be at the cost of any existing "horizontal" capability, nor should doing so forfend any future "horizontal" capability. And vice-versa (adding horizontal capability should not mess with vertical ones). The point at which one will break the other is the theoretical design limit of the system.

[convolvatron]

in general these aren't in conflict. in particular once I have a system which can distribute work among faulty nodes and maintain serializability, exploiting parallelism _within_ a fault domain just falls out.