Or one could simply try very hard not to mutate any state that is available to another process and corral all such code that requires doing so (which most of th

Or one could simply try very hard not to mutate any state that is available to another process and corral all such code that requires doing so (which most of the time ends up being only a small portion of the code; the rest can be designed functionally, just passing values in and back out with no other side-effects) into a thin I/O layer, which is (I believe) the Hexagonal Architecture and is explained quite well by Gary Bernhardt here: https://www.destroyallsoftware.com/talks/boundaries https://www.destroyallsoftware.com/talks/boundaries