An async story

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Katharina Fey


Software Engineer at Ferrous Systems

How to complain at me

@spacekookie on twitter

I make some attrocious puns

An async story

An async story

Rust is successful

How do you define success?

Rust is a young language

  • Development started in 2010
  • Stable since 2015

Rust communities are young

Yet, we have seen some amazing work from them

Lots of companies are adopting Rust

Finding Rust jobs is easier as ever


Every 6 weeks a new Rust.

Contribution pipeline is short

[Rust] is fundamentally about empowerment: no matter what kind of code you are writing now, Rust empowers you to reach farther, to program with confidence in a wider variety of domains than you did before.

– Rust book, foreword

Is the Rust community unique?


What is unique is how the language and tooling support the community in ways that other language ecosystems do not!

Rust is going places

  • Networking services
  • Embedded
  • Browser front-ends
  • General applications
  • Games
  • Spaaaaaaaaaaaaaace!

A community of developers empowered by their tools and each other

In my opinion: momentum is success

Bringing new people to new domains

People who are curious and adventerous.

Supported by their tools and the community,

who are encouraged to build exciting things!

This isn’t easy!

This isn’t perfect!

  • Problems scaling the descision process.
  • Problems bringing in new people

Rust 2018

Who here doesn’t know what Rust 2018 is?

Rust 2018

  • Accumulation of changes since 2015
  • Backwards compatible with Rust 2015, while still having some breaking changes
  • Focus on developer ergonomics and productivity

This is done via the edition flag

name = "foobar"
version = "1.0.0"
edition = "2018"

2015 is default to preserve backwards compatibility

The working groups

  • Commandline Applications
  • Embedded
  • WASM
  • Networking

The idea

The goal of these groups is to focus on the end-to-end user experience of using Rust in each domain.

– Announcing the 2018 Domain Working Groups

The Governance

  • Group leaders were assigned at first
  • People joined through collaboration
  • The longer time went on, the less hirarchical the group structures became

This meant a low barrier-to-entry for newcomers

[…] focusing on the end-to-end experience for both sync and async networking code

What is async I/O

Using available resources the most efficiently…

Doing things while waiting for long-running operations to finish…

Look mum, no threads!



Future = calculation that hasn’t happened yet

  • Is probably gonna happen at some point
  • Just keep asking

Event Loop = runtime for Futures

  • Keeps polling Future until it is ready
  • Runs your code whenever it can be run

This is a talk about Rust

It’s state machines all the way down

Let’s talk about some history

Late 2013


  • Rust had green threading support
  • Enabled non-blocking IO

Included a runtime in stdlib

  • This came with a lot of problems

Rust wanted to go in a different direction

Late 2014

libgreen is dead, long live libgreen


“Metal I/O” 🤘

  • Light, non-blocking IO library
  • Abstracts async over different platforms
  • Eventually developed an ecosystem around it

Mid 2016

  • zero cost abstraction for futures
  • Building async state-machines


  • Wraps around and
  • Event reactor

Fast foward to 2018

So what’s the problem?

async & await

Write code that looks synchronous/ blocking.

It actually does async under the hood!

The Future trait

The Future trait

Now provided by libcore/ libstd

New keywords for everybody

  • async - marks a function as … async
  • await - waits for something async to happen


A minimal tokio fork that uses async/await

Can I use this?


(on nightly)

  • RFC for async/await syntax
  • Future trait now in libcore/ libstd
  • futures-rs updated to the new APIs

Networking WG

Writing networking code has many problem domains

  • Pure async/await implementation
  • “I just want to write a web-app”
  • Low-level embedded networking

Working group mitosis

  • net-async-wg
  • net-web-wg
  • net-embedded-wg


  • Open RFC to stabilise Future trait
  • Working on the ecosystem & libraries

Why “an async story” ?

Kate, what hell was your point?

Rust is the async story!

async in Rust 2018 is still WIP.

But it’s on a good track,

because it’s the community that’s driving development!

Who makes Rust?

All of you! All of us!

And the language is better off because of it.

If you can remember one thing from this talk?

Collaboration takes time.

But the result will be amazing!

thank you