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Asynchrony is not concurrency

(kristoff.it)
292 points kristoff_it | 2 comments | 18 Jul 25 19:21 UTC | HN request time: 0.001s | source
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dvt ◴[18 Jul 25 22:44 UTC] No.44610616[source]▶
"Asynchrony" is a very bad word for this and we already have a very well-defined mathematical one: commutativity. Some operations are commutative (order does not matter: addition, multiplication, etc.), while others are non-commutative (order does matter: subtraction, division, etc.).

    try io.asyncConcurrent(Server.accept, .{server, io});
    io.async(Cient.connect, .{client, io});
Usually, ordering of operations in code is indicated by the line number (first line happens before the second line, and so on), but I understand that this might fly out the window in async code. So, my gut tells me this would be better achieved with the (shudder) `.then(...)` paradigm. It sucks, but better the devil you know than the devil you don't.

As written, `asyncConcurrent(...)` is confusing as shit, and unless you memorize this blog post, you'll have no idea what this code means. I get that Zig (like Rust, which I really like fwiw) is trying all kinds of new hipster things, but half the time they just end up being unintuitive and confusing. Either implement (async-based) commutativity/operation ordering somehow (like Rust's lifetimes maybe?) or just use what people are already used to.

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dooglius ◴[18 Jul 25 23:24 UTC] No.44610939[source]▶
Commutativity is a much weaker claim because one is totally before or after the other. e.g. AB may commute with C so ABC=CAB but it is not necessarily the case that this equals ACB. With asynchrony you are guaranteed ABC=ACB=CAB. (There may be an exisiting mathematical term for this but I don't know it)
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dvt ◴[18 Jul 25 23:37 UTC] No.44611038[source]▶
You can prove three-term commutativity from two-term (I did it years ago, I think it looked something like this[1]), so the ordering doesn't matter.

[1] https://math.stackexchange.com/questions/785576/prove-the-co...

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1. dooglius ◴[19 Jul 25 03:51 UTC] No.44612439[source]▶
I'm not talking about a universe where all elements commute, I'm talking about a situation in which A, B, and C do not necessarily commute but (AB) and C do. For a rigorous definition: given X and Y from some semigroup G, say X and Y are asynchronous if for any finite decompositions X=Z_{a_1}Z_{a_2}...Z_{a_n} and Y=Z_{b_1}Z_{b_2}...Z_{b_m} (with Z's in G) then for any permutation c_1,...,c_{n+m} of a_1,...,a_n,b_1,...,b_m that preserves the ordering of a's and the ordering of the b's has XY=Z_{c_1}Z_{c_2}...Z_{c_{n+m}}. I make the following claim: if G is commutative then all elements are asynchronous, but for a noncommutative G there can exist elements X and Y that commute (i.e. XY=YX) but X and Y are not asynchronous.
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2. JW_00000 ◴[19 Jul 25 07:43 UTC] No.44613418[source]▶
To give a concrete example, matrix multiplication is not commutative in general (AB ≠ BA), but e.g. multiplication with the identity matrix is (AI = IA). So AIB = ABI ≠ BAI.

Or applied to the programming example, the statements:

    1. Server.accept
    2. Client.connect
    3. File.write  # write to completely unrelated file
123 = 312 ≠ 321.