Time lord: building deterministic simulations

"It works on my machine" is a luxury we can’t afford when building a complex emergency room simulator. With dozens of independent actors (patients, doctors, equipment) and a ticking clock, bugs can be chaotic and hard to catch.

In this talk, I will share how we turned chaos into order by implementing a deterministic "Flight Recorder" system. We will explore how to treat time as a controlled event and why making your state transitions pure is the ultimate frontend superpower. I’ll show you how we can record a user’s session into a JSON file and replay it bit-by-bit to catch elusive bugs, perform time-travel debugging, and even run hours of simulations in seconds.