> > One with lots of persistent mutable state.

> You mean like a database?

No, I mean like a routing switcher for a TV station. You have a set of inputs and a set of outputs, and you have various sources of control, and you have commands to switch outputs to different inputs. And when one source of control makes a change, you have to update all the other sources of control about the change, so that they have a current view of the world. The state of the current connections is the fundamental thing in the program - more even than controlling the hardware is.