Some background information

Relevant for:

  • High level introduction to concurrency in Rust
  • Knowing what Rust provides and not when working with async
  • Understanding why we need runtimes
  • Knowing that Rust has Futures 1.0 and Futures 3.0, and how to deal with them
  • Getting pointers to further reading on concurrency in general

Before we start implementing our Futures , we'll go through some background information that will help demystify some of the concepts we encounter.

Actually, after going through these concepts, implementing futures will seem pretty simple. I promise.

Async in Rust

Let's get some of the common roadblocks out of the way first.

Async in Rust is different from most other languages in the sense that Rust has a very lightweight runtime.

In languages like C#, JavaScript, Java and GO, already includes a runtime for handling concurrency. So if you come from one of those languages this will seem a bit strange to you.

What Rust's standard library takes care of

  1. The definition of an interruptible task
  2. An efficient technique to start, suspend, resume and store tasks which are executed concurrently.
  3. A defined way to wake up a suspended task

That's really what Rusts standard library does. As you see there is no definition of non-blocking I/O, how these tasks are created or how they're run.

What you need to find elsewhere

A runtime. Well, in Rust we normally divide the runtime into two parts:

  • The Reactor
  • The Executor

Reactors create leaf Futures, and provides things like non-blocking sockets, an event queue and so on.

Executors, accepts one or more asynchronous tasks called Futures and takes care of actually running the code we write, suspend the tasks when they're waiting for I/O and resumes them.

In theory, we could choose one Reactor and one Executor that have nothing to do with each other besides one creates leaf Futures and one runs them, but in reality today you'll most often get both in a Runtime.

There are mainly two such runtimes today async_std and tokio.

Quite a bit of complexity attributed to Futures are actually complexity rooted in runtimes. Creating an efficient runtime is hard. Learning how to use one correctly can be hard as well, but both are excellent and it's just like learning any new library.

The difference between Rust and other languages is that you have to make an active choice when it comes to picking a runtime. Most often you'll just use the one provided for you.

Futures 1.0 and Futures 3.0

I'll not spend too much time on this, but it feels wrong to not mention that there have been several iterations on how async should work in Rust.

Futures 3.0 works with the relatively new async/await syntax in Rust and it's what we'll learn.

Now, since this is rather recent, you can encounter creates that use Futures 1.0 still. This will get resolved in time, but unfortunately it's not always easy to know in advance.

A good sign is that if you're required to use combinators like and_then then you're using Futures 1.0.

While not directly compatible, there is a tool that let's you relatively easily convert a Future 1.0 to a Future 3.0 and vice a versa. You can find all you need in the futures-rs crate and all information you need here.

First things first

If you find the concepts of concurrency and async programming confusing in general, I know where you're coming from and I have written some resources to try to give a high level overview that will make it easier to learn Rusts Futures afterwards:

Now learning these concepts by studying futures is making it much harder than it needs to be, so go on and read these chapters. I'll be right here when you're back.

However, if you feel that you have the basics covered, then go right on.

Let's get moving!