I'm curious about emergent problems that are difficult to diagnose with many systems operating seemingly independently. Systems interacting in odd ways, and ordering of systems (dependency ie one system MUST run before another). Do these come up?
You’re right, they do. You add a layer on top that is in charge of worrying about those interactions, and it works pretty smoothly in my experience, but even then there can be some challenges from multiple pieces of code trying to use that controller code to do contradictory things. But I think you end up in the same situation with all big apps, and in this setup at least some of your code is still easy to reason about. It’s still several layers of abstractions. Command pattern with a good entry-in-progress-exit lifecycle is another system to add on top that helps things stay flexible and easy to work with later on
