Async Interview #1: Alex and Nick talk about async I/O and WebAssembly

28 November 2019

Hello from Iceland! (I’m on vacation.) I’ve just uploaded [the first of the Async Interviews][video] to YouTube. It is a conversation with Alex Crichton (alexcrichton) and Nick Fitzgerald (fitzgen) about how WebAssembly and Rust’s Async I/O system interact. When you watch it, you will probably notice two things:

  • First, I spent a lot of time looking off to the side! This is because I had the joint Dropbox paper document open on my side monitor and I forgot how strange that would look. I’ll have to remember that for the future. =)
  • Second, we recorded this on October 3rd1, which was before async-await had landed on stable. So at various points we talk about async-await being on beta or not yet being stable. Don’t be confused. =)


You can view the [video][video] on YouTube, but it is also embedded here if that’s easier for you.

Rust futures, meet JavaScript promises

The first part of the chat focused on the interaction of Rust futures with JavaScript’s promises. As fitzgen points out early on, on the web platform, there is no notion of synchronous I/O – only async2. So if you want to execute Rust in the browser, you need to be using async I/O. Fortunately, the [current tooling makes this pretty easy][rw]!

The fetch example from the wasm-bindgen site kind of shows off some of what is possible. The example shows a Rust function that downloads content from the web by invoking the fetch function. To start, it shows you can write a Rust async fn that gets exported to JavaScript:

pub async fn run() -> Result<JsValue, JsValue> {

When users invoke this run function from JS, it acts just like a JavaScript asynchronous function, which means that it returns a JS promise. This is possible because wasm-bindgen includes the ability to interconvert between JS and Rust promises. You can see this at play also within the run function, which invokes fetch and then converts the result into a Rust future:

let window = web_sys::window().unwrap();
let resp_value = JsFuture::from(window.fetch_with_request(&request)).await?;

Note the JsFuture::from call in particular, which converts the JavaScript promise into a Rust future that can then be awaited. (You can go the other way using future_to_promise, which converts a Rust future into a JavaScript promise.)

As you can see from this example, a lot of the basic tooling for interoperating with JavaScript futures exists, but it’s also still at a fairly low-level. The web-sys crate used in the fetch example exports all the basic APIs, but does so in an untyped fashion, which means that using them from Rust can be error-prone. The gloo crate is an attempt to build a more idiomatic layer atop, but that is still fairly close to the JS APIs. Crates like surf offer a higher-level alternative. surf is an interface for fetching things off the web (think libcurl), and it can be compiled to use a number of backends, including the JS fetch API (which only works when compiling to webassembly, of course).

If you’re interested to learn more, here are some links to get you started:

WebAssembly outside the browser

Next we discussed what it mean to use Async I/O outside of the browser. This part of the space is much less developed. When you are running outside the browser, there aren’t standard APIs like fetch to build off of. This is where WASI comes in. It is an effort to build up a standardized set of APIs that WebAssembly apps can run against which will work both inside and outside the browser. (To learn more about WASI, I recommend Lin Clark’s excellent Mozilla Hacks blog post.)

At this point, the conversation turned much more speculative. WASI doesn’t yet include any asynchronous APIs, but it seems clear that they will be needed. Alex and Nick felt that some of the things we’ve learned in the Rust Async I/O effort would likely inform the design going forward. For example, WASI might export something vaguely epoll or mio-like, and let the languages supply the higher-level APIs that build on that. Still, there has also been talk of including direct support in WASI for protocols like HTTP, which might necessitate a different sort of interface altogether.

One of the interesting things we discussed is that WASI is very capability driven. Typically in programming languages, we aim to expose a small set of powerful, flexible primitives that can be used to do almost anything – but this has downsides, too. Offering higher-level options means you can build a more effective sandbox. For example, it might be useful to have some way to hand-off an open HTTP socket to a WASM3 program, which would mean that it can read and write through that connection, but that it cannot necessarily create new HTTP connections to other servers, or speak other protocols.

What does this mean for Rust async?

In the last few minutes, we turned our discussion to Rust async itself. What developments in Rust async would be most helpful to WASM? Since there are still so many unknowns, especially when it comes to WASI, this is a bit hard to say for sure, but Nick and Alex had a few things to say:

  • First, in terms of JS interop, building more crates like surf, which can be configured to target web APIs, so that people can write Rust programs that work equally well on the web or running natively.
  • Second, to support that effort, better support for async fn in traits (and perhaps other language primitives). The basic idea is that, on the web, async fn is just the only way to do I/O – this means we need it to work seamlessly across all of Rust’s language features. (As usual, I’ll refer folks to dtolnay’s async-trait crate, which is presently the best way to write crates that use async fn, for a host of reasons.)
  • Finally, support for async streams (and async generator functions), so as to interoperate with their JavaScript equivalents.


There is a thread on the Rust users forum for questions and discussion.


  1. Yeah… I’ve been planning to start these async interviews for a while! Just been busy. [video]: ↩︎

  2. We do not speak of [synchronous XHR] on this blog. Or ever. [synchronous XHR]: [rw]: ↩︎

  3. According to the spec, “WASM” is a contraction and hence should not be capitalized. I however maintain that it plainly fits the definition of an acronym. Moreover, if it were a contraction, it would be written w’asm’, and nobody wants that. So I’m going with WASM. Sue me. ↩︎