Async in Rust: what you need to know

There are two things that make asynchronous programming attractive.

First, it allows you to do more with less. You can use a single OS-level thread to field any number of simultaneous interactions; a single-threaded async server can scale to handle millions of connections.

Now, some operating systems make it possible to use a large number of OS threads relatively cheaply. But there is still overhead. And this leads to the second benefit of async programming: by making "tasks" essentially free, it enables highly expressive programming patterns that would be impractical in a synchronous setting.

In other words, the efficiency gains are so stark that they unlock a powerful new style of programming.

So what's the catch?

Threads are treated in a first class way at the OS level, but if you want to juggle different activities within the same thread, it's all on you. Fortunately, Rust is expressive enough that we can build shared, zero-cost abstractions that make "task-level" programming a first-class concept as well.

That said, there remain some important differences between sync and async programming in Rust—especially on stable Rust. The purpose of this book is, in part, to help guide you through these differences, teaching you a set of patterns for effective async programming.

Finally, it's worth remembering that traditional synchronous programming remains quite effective, outside of the highest-scale servers. In particular, Rust's advantages around memory footprint and predictability mean that you can get much farther with synchronous services than in many other languages. As with any other architectural choice, it's important to consider whether your application would be better served by using the simpler synchronous model.

Where async Rust is today, and where it's headed

Rust has a strong foundation for async programming with the futures and tokio crates, which cover the core abstractions for async, and primitives for async I/O, respectively. On top of these crates there's an ecosystem for interacting with various protocols and services, including HTTP, SSL, DNS, WebSockets, and more. This book covers much of this ecosystem, recommending production-quality libraries in each space.

In addition, there is ongoing work to improve the ergonomics via async/await notation. This work is currently only available on nightly Rust, and is expected to provide more flexible borrowing support in the future. Nevertheless, if you're willing to work with nightly today, the library itself is stable and helpful. This book also covers its usage.

As we'll see shortly, there are easy ways to "bridge" between async and sync code. In the long run, as async becomes a more first-class part of the language itself, the expectation is that core libraries will be written in an async fashion, but be easily consumed in either style. We'll see examples of that later in the book.

Who this book is for

This book aims to be a comprehensive, up-to-date guide on the async story in Rust, appropriate for beginners and old hands alike:

  • The early chapters provide a gentle introduction to async programming in general, and to Rust's particular take on it.

  • The middle chapters provide more powerful tools, best practices, and larger examples to help you work with async in a more serious way.

  • The later chapters cover a cross section of the broader async ecosystem and the most advanced features of the core libraries, and more extensive case studies.

Let's dive in!