Futures and the Async Syntax - The Rust Programming Language

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## Futures and the Async Syntax

The key elements of asynchronous programming in Rust are futures and Rust’s `async` and `await` keywords.

A future is a value that may not be ready now but will become ready at some point in the future. (This same concept shows up in many languages, sometimes under other names such as task or promise.) Rust provides a `Future` trait as a building block so that different async operations can be implemented with different data structures but with a common interface. In Rust, futures are types that implement the `Future` trait. Each future holds its own information about the progress that has been made and what “ready” means.

You can apply the `async` keyword to blocks and functions to specify that they can be interrupted and resumed. Within an async block or async function, you can use the `await` keyword to await a future (that is, wait for it to become ready). Any point where you await a future within an async block or function is a potential spot for that block or function to pause and resume. The process of checking with a future to see if its value is available yet is called polling.

Some other languages, such as C# and JavaScript, also use `async` and `await` keywords for async programming. If you’re familiar with those languages, you may notice some significant differences in how Rust handles the syntax. That’s for good reason, as we’ll see!

When writing async Rust, we use the `async` and `await` keywords most of the time. Rust compiles them into equivalent code using the `Future` trait, much as it compiles `for` loops into equivalent code using the `Iterator` trait. Because Rust provides the `Future` trait, though, you can also implement it for your own data types when you need to. Many of the functions we’ll see throughout this chapter return types with their own implementations of `Future`. We’ll return to the definition of the trait at the end of the chapter and dig into more of how it works, but this is enough detail to keep us moving forward.

This may all feel a bit abstract, so let’s write our first async program: a little web scraper. We’ll pass in two URLs from the command line, fetch both of them concurrently, and return the result of whichever one finishes first. This example will have a fair bit of new syntax, but don’t worry—we’ll explain everything you need to know as we go.

## Our First Async Program

To keep the focus of this chapter on learning async rather than juggling parts of the ecosystem, we’ve created the `trpl` crate (`trpl` is short for “The Rust Programming Language”). It re-exports all the types, traits, and functions you’ll need, primarily from the `futures` and `tokio` crates. The `futures` crate is an official home for Rust experimentation for async code, and it’s actually where the `Future` trait was originally designed. Tokio is the most widely used async runtime in Rust today, especially for web applications. There are other great runtimes out there, and they may be more suitable for your purposes. We use the `tokio` crate under the hood for `trpl` because it’s well tested and widely used.

In some cases, `trpl` also renames or wraps the original APIs to keep you focused on the details relevant to this chapter. If you want to understand what the crate does, we encourage you to check out its source code. You’ll be able to see what crate each re-export comes from, and we’ve left extensive comments explaining what the crate does.

Create a new binary project named `hello-async` and add the `trpl` crate as a dependency:

    $ cargo new hello-async
    $ cd hello-async
    $ cargo add trpl

Now we can use the various pieces provided by `trpl` to write our first async program. We’ll build a little command line tool that fetches two web pages, pulls the `<title>` element from each, and prints out the title of whichever page finishes that whole process first.

### Defining the page_title Function

Let’s start by writing a function that takes one page URL as a parameter, makes a request to it, and returns the text of the `<title>` element (see Listing 17-1).

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    fn main() {
        // TODO: we'll add this next!
    }

    use trpl::Html;

    async fn page_title(url: &str) -> Option<String> {
        let response = trpl::get(url).await;
        let response_text = response.text().await;
        Html::parse(&response_text)
            .select_first("title")
            .map(|title| title.inner_html())
    }

Listing 17-1: Defining an async function to get the title element from an HTML page

First, we define a function named `page_title` and mark it with the `async` keyword. Then we use the `trpl::get` function to fetch whatever URL is passed in and add the `await` keyword to await the response. To get the text of the `response`, we call its `text` method and once again await it with the `await` keyword. Both of these steps are asynchronous. For the `get` function, we have to wait for the server to send back the first part of its response, which will include HTTP headers, cookies, and so on and can be delivered separately from the response body. Especially if the body is very large, it can take some time for it all to arrive. Because we have to wait for the entirety of the response to arrive, the `text` method is also async.

We have to explicitly await both of these futures, because futures in Rust are lazy: they don’t do anything until you ask them to with the `await` keyword. (In fact, Rust will show a compiler warning if you don’t use a future.) This might remind you of the discussion of iterators in the “Processing a Series of Items with Iterators” section in Chapter 13. Iterators do nothing unless you call their `next` method—whether directly or by using `for` loops or methods such as `map` that use `next` under the hood. Likewise, futures do nothing unless you explicitly ask them to. This laziness allows Rust to avoid running async code until it’s actually needed.

Note: This is different from the behavior we saw when using `thread::spawn` in the “Creating a New Thread with spawn” section in Chapter 16, where the closure we passed to another thread started running immediately. It’s also different from how many other languages approach async. But it’s important for Rust to be able to provide its performance guarantees, just as it is with iterators.

Once we have `response_text`, we can parse it into an instance of the `Html` type using `Html::parse`. Instead of a raw string, we now have a data type we can use to work with the HTML as a richer data structure. In particular, we can use the `select_first` method to find the first instance of a given CSS selector. By passing the string `"title"`, we’ll get the first `<title>` element in the document, if there is one. Because there may not be any matching element, `select_first` returns an `Option<ElementRef>`. Finally, we use the `Option::map` method, which lets us work with the item in the `Option` if it’s present, and do nothing if it isn’t. (We could also use a `match` expression here, but `map` is more idiomatic.) In the body of the function we supply to `map`, we call `inner_html` on the `title` to get its content, which is a `String`. When all is said and done, we have an `Option<String>`.

Notice that Rust’s `await` keyword goes after the expression you’re awaiting, not before it. That is, it’s a postfix keyword. This may differ from what you’re used to if you’ve used `async` in other languages, but in Rust it makes chains of methods much nicer to work with. As a result, we could change the body of `page_title` to chain the `trpl::get` and `text` function calls together with `await` between them, as shown in Listing 17-2.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use trpl::Html;

    fn main() {
        // TODO: we'll add this next!
    }

    async fn page_title(url: &str) -> Option<String> {
        let response_text = trpl::get(url).await.text().await;
        Html::parse(&response_text)
            .select_first("title")
            .map(|title| title.inner_html())
    }

Listing 17-2: Chaining with the `await` keyword

With that, we have successfully written our first async function! Before we add some code in `main` to call it, let’s talk a little more about what we’ve written and what it means.

When Rust sees a block marked with the `async` keyword, it compiles it into a unique, anonymous data type that implements the `Future` trait. When Rust sees a function marked with `async`, it compiles it into a non-async function whose body is an async block. An async function’s return type is the type of the anonymous data type the compiler creates for that async block.

Thus, writing `async fn` is equivalent to writing a function that returns a future of the return type. To the compiler, a function definition such as the `async fn page_title` in Listing 17-1 is roughly equivalent to a non-async function defined like this:

    #![allow(unused)]
    fn main() {
    extern crate trpl; // required for mdbook test
    use std::future::Future;
    use trpl::Html;

    fn page_title(url: &str) -> impl Future<Output = Option<String>> {
        async move {
            let text = trpl::get(url).await.text().await;
            Html::parse(&text)
                .select_first("title")
                .map(|title| title.inner_html())
        }
    }
    }

Let’s walk through each part of the transformed version:

- It uses the `impl Trait` syntax we discussed back in Chapter 10 in the “Traits as Parameters” section.
- The returned value implements the `Future` trait with an associated type of `Output`. Notice that the `Output` type is `Option<String>`, which is the same as the original return type from the `async fn` version of `page_title`.
- All of the code called in the body of the original function is wrapped in an `async move` block. Remember that blocks are expressions. This whole block is the expression returned from the function.
- This async block produces a value with the type `Option<String>`, as just described. That value matches the `Output` type in the return type. This is just like other blocks you have seen.
- The new function body is an `async move` block because of how it uses the `url` parameter. (We’ll talk much more about `async` versus `async move` later in the chapter.)

Now we can call `page_title` in `main`.

### Executing an Async Function with a Runtime

To start, we’ll get the title for a single page, shown in Listing 17-3. Unfortunately, this code doesn’t compile yet.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use trpl::Html;

    async fn main() {
        let args: Vec<String> = std::env::args().collect();
        let url = &args[1];
        match page_title(url).await {
            Some(title) => println!("The title for {url} was {title}"),
            None => println!("{url} had no title"),
        }
    }

    async fn page_title(url: &str) -> Option<String> {
        let response_text = trpl::get(url).await.text().await;
        Html::parse(&response_text)
            .select_first("title")
            .map(|title| title.inner_html())
    }

Listing 17-3: Calling the `page_title` function from `main` with a user-supplied argument

We follow the same pattern we used to get command line arguments in the “Accepting Command Line Arguments” section in Chapter 12. Then we pass the URL argument to `page_title` and await the result. Because the value produced by the future is an `Option<String>`, we use a `match` expression to print different messages to account for whether the page had a `<title>`.

The only place we can use the `await` keyword is in async functions or blocks, and Rust won’t let us mark the special `main` function as `async`.

    error[E0752]: `main` function is not allowed to be `async`
     --> src/main.rs:6:1
      |
    6 | async fn main() {
      | ^^^^^^^^^^^^^^^ `main` function is not allowed to be `async`

The reason `main` can’t be marked `async` is that async code needs a runtime: a Rust crate that manages the details of executing asynchronous code. A program’s `main` function can initialize a runtime, but it’s not a runtime itself. (We’ll see more about why this is the case in a bit.) Every Rust program that executes async code has at least one place where it sets up a runtime that executes the futures.

Most languages that support async bundle a runtime, but Rust does not. Instead, there are many different async runtimes available, each of which makes different tradeoffs suitable to the use case it targets. For example, a high-throughput web server with many CPU cores and a large amount of RAM has very different needs than a microcontroller with a single core, a small amount of RAM, and no heap allocation ability. The crates that provide those runtimes also often supply async versions of common functionality such as file or network I/O.

Here, and throughout the rest of this chapter, we’ll use the `block_on` function from the `trpl` crate, which takes a future as an argument and blocks the current thread until this future runs to completion. Behind the scenes, calling `block_on` sets up a runtime using the `tokio` crate that’s used to run the future passed in (the `trpl` crate’s `block_on` behavior is similar to other runtime crates’ `block_on` functions). Once the future completes, `block_on` returns whatever value the future produced.

We could pass the future returned by `page_title` directly to `block_on` and, once it completed, we could match on the resulting `Option<String>` as we tried to do in Listing 17-3. However, for most of the examples in the chapter (and most async code in the real world), we’ll be doing more than just one async function call, so instead we’ll pass an `async` block and explicitly await the result of the `page_title` call, as in Listing 17-4.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use trpl::Html;

    fn main() {
        let args: Vec<String> = std::env::args().collect();

        trpl::block_on(async {
            let url = &args[1];
            match page_title(url).await {
                Some(title) => println!("The title for {url} was {title}"),
                None => println!("{url} had no title"),
            }
        })
    }

    async fn page_title(url: &str) -> Option<String> {
        let response_text = trpl::get(url).await.text().await;
        Html::parse(&response_text)
            .select_first("title")
            .map(|title| title.inner_html())
    }

Listing 17-4: Awaiting an async block with `trpl::block_on`

When we run this code, we get the behavior we expected initially:

    $ cargo run -- "https://www.rust-lang.org"
        Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.05s
         Running `target/debug/async_await 'https://www.rust-lang.org'`
    The title for https://www.rust-lang.org was
                Rust Programming Language

Phew—we finally have some working async code! But before we add the code to race two sites against each other, let’s briefly turn our attention back to how futures work.

Each await point—that is, every place where the code uses the `await` keyword—represents a place where control is handed back to the runtime. To make that work, Rust needs to keep track of the state involved in the async block so that the runtime could kick off some other work and then come back when it’s ready to try advancing the first one again. This is an invisible state machine, as if you’d written an enum like this to save the current state at each await point:

    #![allow(unused)]
    fn main() {
    extern crate trpl; // required for mdbook test

    enum PageTitleFuture<'a> {
        Initial { url: &'a str },
        GetAwaitPoint { url: &'a str },
        TextAwaitPoint { response: trpl::Response },
    }
    }

Writing the code to transition between each state by hand would be tedious and error-prone, however, especially when you need to add more functionality and more states to the code later. Fortunately, the Rust compiler creates and manages the state machine data structures for async code automatically. The normal borrowing and ownership rules around data structures all still apply, and happily, the compiler also handles checking those for us and provides useful error messages. We’ll work through a few of those later in the chapter.

Ultimately, something has to execute this state machine, and that something is a runtime. (This is why you may come across mentions of executors when looking into runtimes: an executor is the part of a runtime responsible for executing the async code.)

Now you can see why the compiler stopped us from making `main` itself an async function back in Listing 17-3. If `main` were an async function, something else would need to manage the state machine for whatever future `main` returned, but `main` is the starting point for the program! Instead, we called the `trpl::block_on` function in `main` to set up a runtime and run the future returned by the `async` block until it’s done.

Note: Some runtimes provide macros so you can write an async `main` function. Those macros rewrite `async fn main() { ... }` to be a normal `fn main`, which does the same thing we did by hand in Listing 17-4: call a function that runs a future to completion the way `trpl::block_on` does.

Now let’s put these pieces together and see how we can write concurrent code.

### Racing Two URLs Against Each Other Concurrently

In Listing 17-5, we call `page_title` with two different URLs passed in from the command line and race them by selecting whichever future finishes first.

Filename: src/main.rs

    extern crate trpl; // required for mdbook test

    use trpl::{Either, Html};

    fn main() {
        let args: Vec<String> = std::env::args().collect();

        trpl::block_on(async {
            let title_fut_1 = page_title(&args[1]);
            let title_fut_2 = page_title(&args[2]);

            let (url, maybe_title) =
                match trpl::select(title_fut_1, title_fut_2).await {
                    Either::Left(left) => left,
                    Either::Right(right) => right,
                };

            println!("{url} returned first");
            match maybe_title {
                Some(title) => println!("Its page title was: '{title}'"),
                None => println!("It had no title."),
            }
        })
    }

    async fn page_title(url: &str) -> (&str, Option<String>) {
        let response_text = trpl::get(url).await.text().await;
        let title = Html::parse(&response_text)
            .select_first("title")
            .map(|title| title.inner_html());
        (url, title)
    }

Listing 17-5: Calling `page_title` for two URLs to see which returns first

We begin by calling `page_title` for each of the user-supplied URLs. We save the resulting futures as `title_fut_1` and `title_fut_2`. Remember, these don’t do anything yet, because futures are lazy and we haven’t yet awaited them. Then we pass the futures to `trpl::select`, which returns a value to indicate which of the futures passed to it finishes first.

Note: Under the hood, `trpl::select` is built on a more general `select` function defined in the `futures` crate. The `futures` crate’s `select` function can do a lot of things that the `trpl::select` function can’t, but it also has some additional complexity that we can skip over for now.

Either future can legitimately “win,” so it doesn’t make sense to return a `Result`. Instead, `trpl::select` returns a type we haven’t seen before, `trpl::Either`. The `Either` type is somewhat similar to a `Result` in that it has two cases. Unlike `Result`, though, there is no notion of success or failure baked into `Either`. Instead, it uses `Left` and `Right` to indicate “one or the other”:

    #![allow(unused)]
    fn main() {
    enum Either<A, B> {
        Left(A),
        Right(B),
    }
    }

The `select` function returns `Left` with that future’s output if the first argument wins, and `Right` with the second future argument’s output if that one wins. This matches the order the arguments appear in when calling the function: the first argument is to the left of the second argument.

We also update `page_title` to return the same URL passed in. That way, if the page that returns first does not have a `<title>` we can resolve, we can still print a meaningful message. With that information available, we wrap up by updating our `println!` output to indicate both which URL finished first and what, if any, the `<title>` is for the web page at that URL.

You have built a small working web scraper now! Pick a couple URLs and run the command line tool. You may discover that some sites are consistently faster than others, while in other cases the faster site varies from run to run. More importantly, you’ve learned the basics of working with futures, so now we can dig deeper into what we can do with async.