Comment by dtornow | Hacker News Reader

Comment by dtornow | Hacker News Reader

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I was part of that debate, I remember a rather interesting point of discussion: Is the main operation "apply" or "dedup"

Apply seems to be the common notion of event sourcing: There is a function apply that takes a state an event and yields a new state. Then, starting in an init state and iteratively applying the entire event history, boom, latest state restored.

Dedup has a lot of charm though: Run and rerun your code, if that step of your code is executed for the first time (no corresponding event in the event history) execute the step and store its result as an event in the history, however, if that step of your code is executed for the second, third time (there is a corresponding event in the event history) do not execute the step and return its result from the event in the history. The Haskell Workflow Package (https://hackage.haskell.org/package/Workflow) is a good example

Temporal follows the second approach, so "proper" Event Sourcing? You be the judge :)

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!

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.

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