Async Python Is Secretly Deterministic
1–10 of 38 posts
Re: Async Python Is Secretly Deterministic
#2> 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?
Re: Async Python Is Secretly Deterministic
#3While not production ready, I’ve been happily surprised at this functionality when building with it. I love my interpreters to be deterministic, or when random to be explicitly seeded. It makes debugging much easier when I can rerun the same program multiple times and expect identical results.
Re: Async Python Is Secretly Deterministic
#4While not production ready, I’ve been happily surprised at this functionality when building with it. I love my interpreters to be deterministic, or when random to be explicitly seeded. It makes debugging much easier when I can rerun the same program multiple times and expect identical results.
Interestingly I think things that should not be deterministic should actually forced not to be.
Swift for instance will explicitly make iterating on a dictionary not deterministic (by randomizing the iteration), in order to catch weird bugs early if a client relies (knowingly or not) on the specific order the elements of the dictionary are ordered.
Re: Async Python Is Secretly Deterministic
#5> 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?
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).
Re: Async Python Is Secretly Deterministic
#6No, determinstic scheduling is not a property of async python.
Yes, the stdlib asyncio event loop does have deterministic scheduling, but that's an implementation detail and I would not rely on it for anything critical. Other event loops - for instance trio [1] - explicitly randomize startup order so that you won't accidentally write code that relies on it.
Re: Async Python Is Secretly Deterministic
#7Is this guaranteed by the async specification? Or is this just current behavior which could be changed in a future update. Feels like a brittle dependency if its not part of the spec.
Re: Async Python Is Secretly Deterministic
#8> 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?
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.
Re: Async Python Is Secretly Deterministic
#9> 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?
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).
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.
Re: Async Python Is Secretly Deterministic
#10While not production ready, I’ve been happily surprised at this functionality when building with it. I love my interpreters to be deterministic, or when random to be explicitly seeded. It makes debugging much easier when I can rerun the same program multiple times and expect identical results.
Interestingly I think things that should not be deterministic should actually forced not to be. Swift for instance will explicitly make iterating on a dictionary not deterministic (by randomizing the iteration), in order to catch weird bugs early if a client relies (knowingly or not) on the specific order the elements of the dictionary are ordered.
This claim sounds vaguely familiar to me (though the documentation on Dictionary does not state any reason for why the iteration order is unpredictable), though the more common reason for languages to have unstable hash table iteration orders is as a consequence of protection against hash flooding, malicious input causing all keys to hash to the same bucket (because iteration order is dependent on bucket order).