[btown]

This may be how Temporal works, but I imagine the way you ask "is there a corresponding event in the event history" is that you tag every "derived event" in the system with a hash of both the state and code used to create it? So if you change the code, you (eventually?) invalidate all events derived from it?

This of course would be incredibly reliant on the code not being able to access anything except the state, and have no side effects - so if you want to say "if X, fetch Y" you need to have something looking at derived state from the "if X" part and putting the results of "fetch Y" into the event stream, thus causing the state relevant to the code to change, and invalidating the need for something to "fetch Y" a second time? There's something incredibly clean and enticing about this, but also incredibly scary to implement correctly at scale!

[mfateev]

Temporal takes much simpler approach to updating code while a program is running. For any code update, it keeps both old and the new version of the code. Then it uses the old code to replay already recorded events which allows to reconstruct the program state and takes the new code path when it is executed for the first time. Eventually the old code is removed once all the executions that used it are completed.