alt Hacker NewsInteresting approach. I often get to the same goal by using the replicated state machine pattern. Where all inputs to a system are recorded. Both methods seem to rely on designing your application in a very specific way to be able to replay inputs and deterministically get the same outputs.
Replies
Another approach is to record at a lower level and then reconstruct the series of events, eg.g. https://engineering.fb.com/2021/04/27/developer-tools/revers...
How do you structure your program to do this?
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.
In some senses, that is true of every scheme; you need to ensure you capture all non-determinism and that can be done with either more capturing or less non-determinism.
However, the restrictions for generic replay-based time-travel debugging is mostly just not using shared memory and, as a corollary, not using multiple threads in a process (multiple processes is okay). Deliberately architecting your system in the way described in the article is largely unnecessary as the overhead of these generic schemes is low, much less work, applies to most codebases that could even attempt deliberate re-architecture, and integrates well with existing tooling and visualizers.
You can even lower these restrictions further to include explicit shared memory if you record those accesses. And you can do everything if you just record all accesses. The overhead of each of these schemes increasing as the amount of recording needed to capture these forms of non-determinism increases.