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Futures Explained in 200 Lines of Rust

The rendered version is found at:

You can find the main example in the repository examples-futures.

This book aims to explain Futures in Rust using an example driven approach, exploring why they're designed the way they are, and how they work. We'll also take a look at some of the alternatives we have when dealing with concurrency in programming.

Going into the level of detail I do in this book is not needed to use futures or async/await in Rust. It's for the curious out there that want to know how it all works.

What this book covers

This book will try to explain everything you might wonder about up until the topic of different types of executors and runtimes. We'll just implement a very simple runtime in this book introducing some concepts but it's enough to get started.

Stjepan Glavina has made an excellent series of articles about async runtimes and executors, and if the rumors are right there is more to come from him in the near future.

The way you should go about it is to read this book first, then continue reading the articles from stejpang to learn more about runtimes and how they work, especially:

  1. Build your own block_on()
  2. Build your own executor

Sorry for the archive links. The original site doesn't exist anymore.


All kinds of contributions are welcome. Spelling, wording or clarifications are very welcome as well as adding or suggesting changes to the content. I'd appreciate if you contribute through a PR.

The images in chapter 3 is created using Power Point. The power point itself is located in the "resources" folder.

Feedback, questions or discussion is welcome in the issue tracker.


2020-04-06: Final draft finished

2020-04-10: Rather substantial rewrite of the Reactor to better the readability and make it easier to reason about. In addition I fixed a mistake in the Poll method and a possible race condition. See #2 for more details.

2020-04-13: Added a "bonus section" to the Implementing Futures chapter where we avoid using thread::park and instead show how we can use a Condvar and a Mutex to create a proper Parker. Updated the Finished Example to reflect these changes. Unfortunately, this led us a few lines over my initial promise of keeping the example below 200 LOC but the I think the inclusion is worth it.

2020-12-21: Rewrote the "Runtimes" paragraph of chapter 2 adding a useful model to understand how runtimes work and included a lot of the suggested text from @ckaran in #25. Added a new chapter "3 A mental model of how Futures work" which tries to visualize and give a good mental model to build upon.


This book is MIT licensed.