Announcing the Async Interviews

22 November 2019

Hello all! I’m going to be trying something new, which I call the “Async Interviews”. These interviews are going to be a series of recorded video calls with various “luminaries” from Rust’s Async I/O effort. In each one, I’m going to be asking roughly the same question: Now that the async-await MVP is stable, what should we be doing next? After each call, I’ll post the recording from the interview, along with a blog post that leaves a brief summary.

My intention in these interviews is to really get into details. That is, I want to talk about what our big picture goals should be, but also what the specific concerns are around stabilizing particular traits or macros. What sorts of libraries do they enable? And so forth. (You can view my rough interview script, but I plan to tailor the meetings as I go.)

I view these interviews as serving a few purposes:

  • Help to survey what different folks are thinking and transmit that thinking out to the community.
  • Help me to understand better what some of the tradeoffs are, especially around discussions that occurred before I was following closely.
  • Experiment with a new form of Rust discussion, where we substitute 1-on-1 exploration and discussion for bigger discussion threads.

First video: Rust and WebAssembly

The first video in this series, which I expect to post next week, will be me chatting with Alex Crichton and Nick Fitzgerald about Async I/O and WebAssembly. This video is a bit different from the others, since it’s still early days in that area – as a result, we talked more about what role Async I/O (and Rust!) might eventually play, and less about immediate priorities for Rust. Along with the video, I’ll post a blog post summarizing the main points that came up in the conversation, so you don’t necessarily have to watch the video itself.

What videos will come after that?

My plan is to be posting a fresh async interview roughly once a week. I’m not sure how long I’ll keep doing this – I guess as long as it seems like I’m still learning things. I’ll announce the people I plan to speak to as I go, but I’m also very open to suggestions!

I’d like to talk to folks who are working on projects at all levels of the “async stack”, such as runtimes, web frameworks, protocols, and consumers thereof. If you can think of a project or a person that you think would provide a useful perspective, I’d love to hear about it. Drop me a line via e-mail or on Zulip or Discord.

Creating design notes

One thing that I have found in trying to get up to speed on the design of Async I/O is that the discussions are often quite distributed, spread amongst issues, RFCs, and the like. I’d like to do a better job of organizing this information.

Therefore, as part of this effort to talk to folks, one of the things I plan to be doing is to collect and catalog the concerns, issues, and unknowns that are being brought up. I’d love to find people to help in this effort! If that is something that interests you, come join the #wg-async-foundations stream on the rust-lang Zulip and say hi!

So what are the things we might do now that async-await is stable?

If you take a look at my rough interview script, you’ll see a long list of possibilities. But I think they break down into two big categories:

  • improving interoperability
  • extending expressive power, convenience, and ergonomics

Let’s look a bit more at those choices.

Improving interoperability

A long time back, Rust actually had a built-in green-threading library. It was removed in RFC #230, and a big part of the motivation was that we knew we were unlikely to find a single runtime design that was useful for all tasks. And, even if we could, we certainly knew we hadn’t found it yet. Therefore, we opted to pare back the stdlib to just expose the primitives that the O/S had to offer.

Learning from this, our current design is intentionally much more “open-ended” and permits runtimes to be added as simple crates on Right now, to my knowledge, we have at least five distinct async runtimes for Rust, and I wouldn’t be surprised if I’ve forgotten a few:1

  • fuschia’s runtime, used for the Fuschia work at Google;
  • tokio, a venerable, efficient runtime with a rich feature set;
  • async-std, a newer contender which aims to couple libstd-like APIs with highly efficient primitives;
  • bastion, exploring a resilient, Erlang-like model2;
  • embrio-rs, exploring the embedded space.

I think this is great: I love to see people experimenting with different tradeoffs and priorities. Not only do I think we’ll wind up with better APIs and more efficient implementations, this also means we can target ’exotic’ environments like the Fuschia operating system or smaller embedded platforms. Very cool stuff.

However, that flexibility does come with some real risks. Most notably, I want us to be sure that it is possible to “mix and match” libraries from the ecosystem. No matter what base runtime you are using, it should be possible to take a protocol implementation like quinn, combine it with “middleware” crates like async-compression, and starting sending payloads.

In my mind, the best way to ensure interoperability is to ensure that we offer standard traits that define the interfaces between libraries. Adding the std::Future trait was a huge step in this direction – it means that you can create all kinds of combinators and things that are fully portable between runtimes. But what are the next steps we can take to help improve things further?

One obvious set of things we can do improve interop is to try and stabilize additional traits. Currently, the futures crate contains a number of interfaces that have been evolving over time, such as Stream, AsyncRead, and AsyncWrite. Maybe some of these traits are good candidates to be moved to the standard library next?

Here are some of the main things I’d like to discuss around interop:

  • As a meta-point, should we be moving the crates to the standard library, or should we move try to promote the futures crate (or, more likely, some of its subcrates, such as futures-io) as the standard for interop? I’ve found from talking to folks that there is a fair amount of confusion on “how standard” the futures crates are and what the plan is there.
  • Regardless of how we signal stability, I also want to talk about the specific traits or other things we might stabilizing. For each such item, there are two things I’d like to drill into:
    • What kinds of interop would be enabled by stabilizing this item? What are some examples of the sorts of libraries that could now exist independently of a runtime because of the existence of this item?
    • What are the specific concerns that remain about the design of this item? The AsyncRead and AsyncWrite traits, for example, presently align quite closely with their synchronous counterparts Read and Write. However, this interface does require that the buffer used to store data must be zeroed. The tokio crate is considering altering its own local definition of AsyncRead for this reason, is that something we should consider as well? If so, how?
  • On a broader note, what are the sorts of things crates need to truly operate that are not covered by the existing traits? For example, the global executors that boats recently proposed would give people the ability to “spawn tasks” into some ambient context… is that a capability that would enable more interop? Perhaps access to task-local data? Inquiring minds want to know.

Improving expressive power, convenience, and ergonomics

Interoperability isn’t the only thing that we might try to improve. We might also focus on language extensions that either grow our expressive power or add convenience and ergonomics. Something like supporting async fn in traits or async closures, for example, could be a huge enabler, even if there are some real difficulties to making them work.

Here are some of the specific features we might discuss:

  • Async destructors. As boats described in this blog post, there is sometimes a need to “await” things when running destructors, and our current system can’t support that.
  • Async fn in traits. We support async fn in free functions and inherent methods, but not in traits. As I explained in this blog post, there are a lot of challenges to support async fn in traits properly (but consider using the async-trait crate).
  • Async closures. Currently, we support async blocks (async move { .. }), which evaluate to a future, and async functions (async fn foo()), which are a function that returns a future. But, at least on stable, we have no way to make a closure that returns a future. Presumably this would be something like async || { ... }. (In fact, on nightly, we do have support for async closures, but there are some issues in the design that we need to work out.)
  • Combinator methods like map, or macros like join! and select!. The futures crate offers a number of useful combinators and macros. Maybe we should move some of those to the standard library?


I think these interviews are going to be a lot of fun, and I expect to learn a lot. Stay tuned for the first blog post, coming next week, about Async I/O and WebAssembly.


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


  1. Indeed, shortly after I published this post, I was directed to the drone-os project. ↩︎

  2. Woohoo! I just want to say that I’ve been hoping to see something like OTP for Rust for…quite some time. ↩︎