[lexicality]

> This makes it possible to write simple code that’s both concurrent and safe.

Yeah, great, my hello world program is deterministic.

What happens when you introduce I/O? Is every network call deterministic? Can you depend on reading a file taking the same amount of time and being woken up by the scheduler in the same order every time?

[PufPufPuf]

This is about durable execution -- being able to resume execution "from the middle", which is often done by executing from the beginning but skipping external calls. Second time around, the I/O is exactly replayed from stored values, and the "deterministic" part only refers to the async scheduler which behaves the same as long as the results are the same.

Coincidentally I have been experimenting with something very similar in JavaScript in the past and there the scheduler also has the same property.

[TeMPOraL]

No, but determinism reduces the number of stones you need to turn over when debugging hairy problems such as your program occasionally returning different results for the same inputs. You may not have control over the timing of I/O operations or order of external events (including OS scheduler), but at least you know that your side of the innovation/response is, in isoaltion, behaving predictably.

[nomel]

I go the opposite approach for this sort of thing, since I would much rather flip and remove the stones: I explicitly randomize order of containers during development and testing, and always in my unit tests, so depending on order can't be a problem. No luck required!

[TeMPOraL]

You want both. More specifically, you want to be in control of which one you're actually doing.

Randomization is great at avoiding erroneous dependencies on spurious cause-and-effect chains. Determinism is needed to ensure the cause-and-effect chains that are core to the problem actually work.

[nomel]

I don't understand.

Determinism isn't required unless it's required.

If it's not required, then you must plan for it NOT being deterministic, with any accidental determinism being ignored (to be safe, forcefully so with an intentional randomization/delays within the library). If it is required, then my random input should always (from the tests perspective) come out the same as I put it in.

If possible, force the corner case if the corner case is a concern. That's the purpose of testing. If there's a concern with timing, force bad timing with random delays. The alternative is relying on luck. I try to make my code as unlucky as possible, during development/testing.

[KraftyOne]

That's the cool thing about this behavior--it doesn't matter how complex your program is, your async functions start in the same order they're called (though after that, they may interleave and finish in any order).

[lexicality]

Only for tasks that are created in synchronous code. If you start two tasks that each make a web request and then start a new task with the result of that request you will immediately lose ordering.

[KraftyOne]

Yes, this only applies for tasks created from the same (sync or async) function. If tasks are creating other tasks, anything is possible.