I had huge success writing a trading system where everything went through the same `on_event(Inputs) -> Outputs` function of the core and a thin shell was translating everything to inputs and the outputs to actions. I actually had a handful of these components communicating via message passing.
This worked rather well as most of the input is async messages anyway, but building anything else this way feels very tiresome.
Usually two methods `onMessage(long timeNow, byte[] buf)` and `onTimer(long timeNow, int timerId)`.
All output sinks and a scheduler need to be passed in on construction of the application.
Then you can record all inputs to a file. Outputs don’t need to be recorded because the inputs can be replayed but it is useful for easier analysis when bugs happen.
I have even worked on systems where there were tools that you could paste recorded input and outputs to and they code generated the source code for a unit test. Super useful for reproducing issues quickly.
But you are spot on in that there is an overhead. For example, if you want to open a TCP socket and then read and write to it, you need to create a separate service and serialise all the inputs and outputs in a way that can be recorded and replayed.
How easy one can debug such a systems easily outweighs the costs, if you have a low number of separate services.