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Heads up This blog post series has been updated and published as an eBook by FP Complete. I'd recommend reading that version instead of these posts. If you're interested, please check out the Rust Crash Course eBook.

In the previous lesson in the crash course, we covered the new async/.await syntax stabilized in Rust 1.39, and the Future trait which lives underneath it. This information greatly supercedes the now-defunct lesson 7 from last year, which covered the older Future approach.

Now it's time to update the second half of lesson 7, and teach the hot-off-the-presses Tokio 0.2 release. For those not familiar with it, let me quote the project's overview:

Tokio is an event-driven, non-blocking I/O platform for writing asynchronous applications with the Rust programming language.

If you want to write an efficient, concurrent network service in Rust, you'll want to use something like Tokio. That's not to say that this is the only use case for Tokio; you can do lots of great things with an event driven scheduler outside of network services. It's also not to say that Tokio is the only solution; the async-std library provides similar functionality.

However, network services are likely the most common domain agitating for a non-blocking I/O system. And Tokio is the most popular and established of these systems today. So this combination is where we're going to get started.

And as a side note, if you have some other topic you'd like me to cover around this, please let me know on Twitter.

Exercise solutions will be included at the end of the blog post. Yes, I keep changing the rules, sue me.

This post is part of a series based on teaching Rust at FP Complete. If you're reading this post outside of the blog, you can find links to all posts in the series at the top of the introduction post. You can also subscribe to the RSS feed.

Hello Tokio!

Let's kick this off. Go ahead and create a new Rust project for experimenting:

$ cargo new --bin usetokio

If you want to make sure you're using the same compiler version as me, set up your rust-toolchain correctly:

$ echo 1.39.0 > rust-toolchain

And then set up Tokio as a dependency. For simplicity, we'll install all the bells and whistles. In your Cargo.toml:

[dependencies]
tokio = { version = "0.2", features = ["full"] }

PROTIP You can run cargo build now to kick off the download and build of crates while you keep reading...

And now we're going to write an asynchronous hello world application. Type this into your src/main.rs:

use tokio::io;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let mut stdout = io::stdout();
    let mut hello: &[u8] = b"Hello, world!\n";
    io::copy(&mut hello, &mut stdout).await?;
    Ok(())
}

NOTE I specifically said "type this in" instead of "copy and paste." For getting comfortable with this stuff, I recommend manually typing in the code.

A lot of this should look familiar from our previous lesson. To recap:

  • Since we'll be awaiting something and generating a Future, our main function is async.
  • Since main is async, we need to use an executor to run it. That's why we use the #[tokio::main] attribute.
  • Since performing I/O can fail, we return a Result.

The first really new thing since last lesson is this little bit of syntax:

.await?

I mentioned it last time, but now we're seeing it in real life. This is just the combination of our two pieces of prior art: .await for chaining together Futures, and ? for error handling. The fact that these work together so nicely is really awesome. I'll probably mention this a few more times, because I love it that much.

The next thing to note is that we use tokio::io::stdout() to get access to some value that lets us interact with standard output. If you're familiar with it, this looks really similar to std::io::stdout(). That's by design: a large part of the tokio API is simply async-ifying things from std.

And finally, we can look at the actual tokio::io::copy call. As you may have guessed, and as stated in the API docs:

This is an asynchronous version of std::io::copy.

However, instead of working with the Read and Write traits, this works with their async cousins: AsyncRead and AsyncWrite. A byte slice (&[u8]) is a valid AsyncRead, so we're able to store our input there. And as you may have guessed, Stdout is an AsyncWrite.

EXERCISE 1 Modify this application so that instead of printing "Hello, world!", it copies the entire contents of standard input to standard output.

NOTE You can simplify this code using stdout.write_all after useing tokio::io::AsyncWriteExt, but we'll stick to tokio::io::copy, since we'll be using it throughout. But if you're curious:

use tokio::io::{self, AsyncWriteExt};

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let mut stdout = io::stdout();
    stdout.write_all(b"Hello, world!\n").await?;
    Ok(())
}

Spawning processes

Tokio provides a tokio::process module which resembles the std::process module. We can use this to implement Hello World once again:

use tokio::process::Command;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    Command::new("echo").arg("Hello, world!").spawn()?.await?;
    Ok(())
}

Notice how the ? and .await bits can go in whatever order they are needed. You can read this line as:

  • Create a new Command to run echo
  • Give it the argument "Hello, world!"
  • Spawn this, which may fail
  • Using the first ?: if it fails, return the error. Otherwise, return a Future
  • Using the .await: wait until that Future completes, and capture its Result
  • Using the second ?: if that Result is Err, return that error.

Pretty nice for a single line!

One of the great advantages of async/.await versus the previous way of doing async with callbacks is how easily it works with looping.

EXERCISE 2 Extend this example so that it prints Hello, world! 10 times.

Take a break

So far we've only really done a single bit of .awaiting. But it's easy enough to .await on multiple things. Let's use delay_for to pause for a bit.

use tokio::time;
use tokio::process::Command;
use std::time::Duration;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    Command::new("date").spawn()?.await?;
    time::delay_for(Duration::from_secs(1)).await;
    Command::new("date").spawn()?.await?;
    time::delay_for(Duration::from_secs(1)).await;
    Command::new("date").spawn()?.await?;
    Ok(())
}

We can also use the tokio::time::interval function to create a stream of "ticks" for each time a certain amount of time has passed. For example, this program will keep calling date once per second until it is killed:

use tokio::time;
use tokio::process::Command;
use std::time::Duration;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let mut interval = time::interval(Duration::from_secs(1));
    loop {
        interval.tick().await;
        Command::new("date").spawn()?.await?;
    }
}

EXERCISE 3 Why isn't there a Ok(()) after the loop?

Time to spawn

This is all well and good, but we're not really taking advantage of asynchronous programming at all. Let's fix that! We've seen two different interesting programs:

  1. Infinitely pausing 1 seconds and calling date
  2. Copying all input from stdin to stdout

It's time to introduce spawn so that we can combine these two into one program. First, let's demonstrate a trivial usage of spawn:

use std::time::Duration;
use tokio::process::Command;
use tokio::task;
use tokio::time;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    task::spawn(dating()).await??;
    Ok(())
}

async fn dating() -> Result<(), std::io::Error> {
    let mut interval = time::interval(Duration::from_secs(1));
    loop {
        interval.tick().await;
        Command::new("date").spawn()?.await?;
    }
}

You may be wondering: what's up with that ?? operator? Is that some special super-error handler? No, it's just the normal error handling ? applied twice. Let's look at some type signatures to help us out here:

pub fn spawn<T>(task: T) -> JoinHandle<T::Output>;

impl<T> Future for JoinHandle<T> {
    type Output = Result<T, JoinError>;
}

Calling spawn gives us back a JoinHandle<T::Output>. In our case, the Future we provide as input is dating(), which has an output of type Result<(), std::io::Error>. So that means the type of task::spawn(dating()) is JoinHandle<Result<(), std::io::Error>>.

We also see that JoinHandle implements Future. So when we apply .await to this value, we end up with whatever that type Output = Result<T, JoinError> thing is. Since we know that T is Result<(), std::io::Error>, this means we end up with Result<Result<(), std::io::Error>, JoinError>.

The first ? deals with the outer Result, exiting with the JoinError on an Err, and giving us a Result<(), std::io::Error> value on Ok. The second ? deals with the std::io::Error, giving us a () on Ok. Whew!

EXERCISE 4 Now that we've seen spawn, you should modify the program so that it calls both date in a loop, and copies stdin to stdout.

Synchronous code

You may not have the luxury of interacting exclusively with async-friendly code. Maybe you have some really nice library you want to leverage, but it performs blocking calls internally. Fortunately, Tokio's got you covered with the spawn_blocking function. Since the docs are so perfect, let me quote them:

The task::spawn_blocking function is similar to the task::spawn function discussed in the previous section, but rather than spawning an non-blocking future on the Tokio runtime, it instead spawns a blocking function on a dedicated thread pool for blocking tasks.

EXERCISE 5 Rewrite the dating() function to use spawn_blocking and std::thread::sleep so that it calls date approximately once per second.

Let's network!

I could keep stepping through the other cools functions in the Tokio library. I encourage you to poke around at them yourself. But I promised some networking, and by golly, I'm gonna deliver!

I'm going to slightly extend the example from the TcpListener docs to (1) make it compile and (2) implement an echo server. This program has a pretty major flaw in it though, I recommend trying to find it.

use tokio::io;
use tokio::net::{TcpListener, TcpStream};

#[tokio::main]
async fn main() -> io::Result<()> {
    let mut listener = TcpListener::bind("127.0.0.1:8080").await?;

    loop {
        let (socket, _) = listener.accept().await?;
        echo(socket).await?;
    }
}

async fn echo(socket: TcpStream) -> io::Result<()> {
    let (mut recv, mut send) = io::split(socket);
    io::copy(&mut recv, &mut send).await?;
    Ok(())
}

We use TcpListener to bind a socket. The binding itself is asynchronous, so we use .await to wait for the listening socket to be available. And we use ? to deal with any errors while binding the listening socket.

Next, we loop forever. Inside the loop, we accept new connections, using .await? like before. We capture the socket (ignoring the address as the second part of the tuple). Then we call our echo function and .await it.

Within echo, we use tokio::io::split to split up our TcpStream into its constituent read and write halves, and then pass those into tokio::io::copy, as we've done before.

Awesome! Where's the bug? Let me ask you a question: what should the behavior be if a second connection comes in while the first connection is still active? Ideally, it would be handled. However, our program has just one task. And that task .awaits on each call to echo. So our second connection won't be serviced until the first one closes.

EXERCISE 6 Modify the program above so that it handles concurrent connections correctly.

TCP client and ownership

Let's write a poor man's HTTP client. It will establish a connection to a hard-coded server, copy all of stdin to the server, and then copy all data from the server to stdout. To use this, you'll manually type in the HTTP request and then hit Ctrl-D for end-of-file.

use tokio::io;
use tokio::net::TcpStream;

#[tokio::main]
async fn main() -> io::Result<()> {
    let stream = TcpStream::connect("127.0.0.1:8080").await?;
    let (mut recv, mut send) = io::split(stream);
    let mut stdin = io::stdin();
    let mut stdout = io::stdout();

    io::copy(&mut stdin, &mut send).await?;
    io::copy(&mut recv, &mut stdout).await?;

    Ok(())
}

That's all well and good, but it's limited. It only handles half-duplex protocols like HTTP, and doesn't actually support keep-alive in any way. We'd like to use spawn to run the two copys in different tasks. Seems easy enough:

let send = spawn(io::copy(&mut stdin, &mut send));
let recv = spawn(io::copy(&mut recv, &mut stdout));

send.await??;
recv.await??;

Unfortunately, this doesn't compile. We get four nearly-identical error messages. Let's look at the first:

error[E0597]: `stdin` does not live long enough
  --> src/main.rs:12:31
   |
12 |     let send = spawn(io::copy(&mut stdin, &mut send));
   |                      ---------^^^^^^^^^^------------
   |                      |        |
   |                      |        borrowed value does not live long enough
   |                      argument requires that `stdin` is borrowed for `'static`
...
19 | }
   | - `stdin` dropped here while still borrowed

Here's the issue: our copy Future does not own the stdin value (or the send value, for that matter). Instead, it has a (mutable) reference to it. That value remains in the main function's Future. Ignoring error cases, we know that the main function will wait for send to complete (thanks to send.await), and therefore the lifetimes appear to be correct. However, Rust doesn't recognize this lifetime information. (Also, and I haven't thought this through completely, I'm fairly certain that send may be dropped earlier than the Future using it in the case of panics.)

In order to fix this, we need to convince the compiler to make a Future that owns stdin. And the easiest way to do that here is to use an async move block.

Exercise 7 Make the code above compile using two async move blocks.

Playing with lines

This section will have a series of modifications to a program. I recommend you solve each challenge before looking at the solution. However, unlike the other exercises, I'm going to show the solutions inline since they build on each other.

Let's build an async program that counts the number of lines on standard input. You'll want to use the lines method for this. Read the docs and try to figure out what uses and wrappers will be necessary to make the types line up.

use tokio::prelude::*;
use tokio::io::AsyncBufReadExt;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let stdin = io::stdin();
    let stdin = io::BufReader::new(stdin);
    let mut count = 0u32;
    let mut lines = stdin.lines();
    while let Some(_) = lines.next_line().await? {
        count += 1;
    }
    println!("Lines on stdin: {}", count);
    Ok(())
}

OK, bumping this up one more level. Instead of standard input, let's take a list of file names as command line arguments, and count up the total number of lines in all the files. Initially, it's OK to read the files one at a time. In other words: don't bother calling spawn. Give it a shot, and then come back here:

use tokio::prelude::*;
use tokio::io::AsyncBufReadExt;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let mut args = std::env::args();
    let _me = args.next(); // ignore command name
    let mut count = 0u32;

    for filename in args {
        let file = tokio::fs::File::open(filename).await?;
        let file = io::BufReader::new(file);
        let mut lines = file.lines();
        while let Some(_) = lines.next_line().await? {
            count += 1;
        }
    }

    println!("Total lines: {}", count);
    Ok(())
}

But now it's time to make this properly asynchronous, and process the files in separate spawned tasks. In order to make this work, we need to spawn all of the tasks, and then .await each of them. I used a Vec of Future<Output=Result<u32, std::io::Error>>s for this. Give it a shot!

use tokio::prelude::*;
use tokio::io::AsyncBufReadExt;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let mut args = std::env::args();
    let _me = args.next(); // ignore command name
    let mut tasks = vec![];

    for filename in args {
        tasks.push(tokio::spawn(async {
            let file = tokio::fs::File::open(filename).await?;
            let file = io::BufReader::new(file);
            let mut lines = file.lines();
            let mut count = 0u32;
            while let Some(_) = lines.next_line().await? {
                count += 1;
            }
            Ok(count) as Result<u32, std::io::Error>
        }));
    }

    let mut count = 0;
    for task in tasks {
        count += task.await??;
    }

    println!("Total lines: {}", count);
    Ok(())
}

And finally in this progression: let's change how we handle the count. Instead of .awaiting the count in the second for loop, let's have each individual task update a shared mutable variable. You should use an Arc<Mutex<u32>> for that. You'll still need to keep a Vec of the tasks though to ensure you wait for all files to be read.

use tokio::prelude::*;
use tokio::io::AsyncBufReadExt;
use std::sync::Arc;

// avoid thread blocking by using Tokio's mutex
use tokio::sync::Mutex;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let mut args = std::env::args();
    let _me = args.next(); // ignore command name
    let mut tasks = vec![];
    let count = Arc::new(Mutex::new(0u32));

    for filename in args {
        let count = count.clone();
        tasks.push(tokio::spawn(async move {
            let file = tokio::fs::File::open(filename).await?;
            let file = io::BufReader::new(file);
            let mut lines = file.lines();
            let mut local_count = 0u32;
            while let Some(_) = lines.next_line().await? {
                local_count += 1;
            }

            let mut count = count.lock().await;
            *count += local_count;
            Ok(()) as Result<(), std::io::Error>
        }));
    }

    for task in tasks {
        task.await??;
    }

    let count = count.lock().await;
    println!("Total lines: {}", *count);
    Ok(())
}

LocalSet and !Send

Thanks to @xudehseng for the inspiration on this section.

OK, did that last exercise seem a bit contrived? It was! In my opinion, the previous approach of .awaiting the counts and summing in the main function itself was superior. However, I wanted to teach you something else.

What happens if you replace the Arc<Mutex<u32>> with a Rc<RefCell<u32>>? With this code:

use tokio::prelude::*;
use tokio::io::AsyncBufReadExt;
use std::rc::Rc;
use std::cell::RefCell;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let mut args = std::env::args();
    let _me = args.next(); // ignore command name
    let mut tasks = vec![];
    let count = Rc::new(RefCell::new(0u32));

    for filename in args {
        let count = count.clone();
        tasks.push(tokio::spawn(async {
            let file = tokio::fs::File::open(filename).await?;
            let file = io::BufReader::new(file);
            let mut lines = file.lines();
            let mut local_count = 0u32;
            while let Some(_) = lines.next_line().await? {
                local_count += 1;
            }

            *count.borrow_mut() += local_count;
            Ok(()) as Result<(), std::io::Error>
        }));
    }

    for task in tasks {
        task.await??;
    }

    println!("Total lines: {}", count.borrow());
    Ok(())
}

You get an error:

error[E0277]: `std::rc::Rc<std::cell::RefCell<u32>>` cannot be shared between threads safely
  --> src/main.rs:15:20
   |
15 |         tasks.push(tokio::spawn(async {
   |                    ^^^^^^^^^^^^ `std::rc::Rc<std::cell::RefCell<u32>>` cannot be shared between threads safely
   |
  ::: /Users/michael/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-0.2.2/src/task/spawn.rs:49:17
   |
49 |     T: Future + Send + 'static,
   |                 ---- required by this bound in `tokio::task::spawn::spawn`

Tasks can be scheduled to multiple different threads. Therefore, your Future must be Send. And Rc<RefCell<u32>> is definitely !Send. However, in our use case, using multiple OS threads is unlikely to speed up our program; we're going to be doing lots of blocking I/O. It would be nice if we could insist on spawning all our tasks on the same OS thread and avoid the need for Send. And sure enough, Tokio provides such a function: tokio::task::spawn_local. Using it (and adding back in async move instead of async), our program compiles, but breaks at runtime:

thread 'main' panicked at '`spawn_local` called from outside of a local::LocalSet!', src/libcore/option.rs:1190:5

Uh-oh! Now I'm personally not a big fan of this detect-it-at-runtime stuff, but the concept is simple enough: if you want to spawn onto the current thread, you need to set up your runtime to support that. And the way we do that is with LocalSet. In order to use this, you'll need to ditch the #[tokio::main] attribute.

EXERCISE 8 Follow the documentation for LocalSet to make the program above work with Rc<RefCell<u32>>.

Conclusion

That lesson felt short. Definitely compared to the previous Tokio lesson which seemed to go on forever. I think this is a testament to how easy to use the new async/.await` syntax is.

There's obviously a lot more that can be covered in asynchronous programming, but hopefully this establishes the largest foundations you need to understand to work with the async/.await syntax and the Tokio library itself.

If we have future lessons, I believe they'll cover additional libraries like Hyper as they move over to Tokio 0.2, as well as specific use cases people raise. If you want something covered, mention it to me on Twitter or in the comments below.

Solutions

Solution 1

use tokio::io;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let mut stdin = io::stdin();
    let mut stdout = io::stdout();
    io::copy(&mut stdin, &mut stdout).await?;
    Ok(())
}

Solution 2

use tokio::process::Command;

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    for _ in 1..=10 {
        Command::new("echo").arg("Hello, world!").spawn()?.await?;
    }
    Ok(())
}

Solution 3

Since the loop will either run forever or be short circuited by an error, any code following loop will never actually be called. Therefore, code placed there will generate a warning.

Solution 4

use std::time::Duration;
use tokio::process::Command;
use tokio::{io, task, time};

#[tokio::main]
async fn main() -> Result<(), std::io::Error> {
    let dating = task::spawn(dating());
    let copying = task::spawn(copying());

    dating.await??;
    copying.await??;

    Ok(())
}

async fn dating() -> Result<(), std::io::Error> {
    let mut interval = time::interval(Duration::from_secs(1));
    loop {
        interval.tick().await;
        Command::new("date").spawn()?.await?;
    }
}

async fn copying() -> Result<(), std::io::Error> {
    let mut stdin = io::stdin();
    let mut stdout = io::stdout();
    io::copy(&mut stdin, &mut stdout).await?;
    Ok(())
}

Solution 5

async fn dating() -> Result<(), std::io::Error> {
    loop {
        task::spawn_blocking(|| { std::thread::sleep(Duration::from_secs(1)) }).await?;
        Command::new("date").spawn()?.await?;
    }
}

Solution 6

The simplest tweak is to wrap the echo call with tokio::spawn:

loop {
    let (socket, _) = listener.accept().await?;
    tokio::spawn(echo(socket));
}

There is a downside to this worth noting, however: we're ignoring the errors produced by the spawned tasks. Likely the best behavior in this case is to handle the errors inside the spawned task:

#[tokio::main]
async fn main() -> io::Result<()> {
    let mut listener = TcpListener::bind("127.0.0.1:8080").await?;

    let mut counter = 1u32;
    loop {
        let (socket, _) = listener.accept().await?;
        println!("Accepted connection #{}", counter);
        tokio::spawn(async move {
            match echo(socket).await {
                Ok(()) => println!("Connection #{} completed successfully", counter),
                Err(e) => println!("Connection #{} errored: {:?}", counter, e),
            }
        });
        counter += 1;
    }
}

Exericse 7

use tokio::io;
use tokio::spawn;
use tokio::net::TcpStream;

#[tokio::main]
async fn main() -> io::Result<()> {
    let stream = TcpStream::connect("127.0.0.1:8080").await?;
    let (mut recv, mut send) = io::split(stream);
    let mut stdin = io::stdin();
    let mut stdout = io::stdout();

    let send = spawn(async move {
        io::copy(&mut stdin, &mut send).await
    });
    let recv = spawn(async move {
        io::copy(&mut recv, &mut stdout).await
    });

    send.await??;
    recv.await??;

    Ok(())
}

Solution 8

use tokio::prelude::*;
use tokio::io::AsyncBufReadExt;
use std::rc::Rc;
use std::cell::RefCell;

fn main() -> Result<(), std::io::Error> {
    let mut rt = tokio::runtime::Runtime::new()?;
    let local = tokio::task::LocalSet::new();
    local.block_on(&mut rt, main_inner())
}

async fn main_inner() -> Result<(), std::io::Error> {
    let mut args = std::env::args();
    let _me = args.next(); // ignore command name
    let mut tasks = vec![];
    let count = Rc::new(RefCell::new(0u32));

    for filename in args {
        let count = count.clone();
        tasks.push(tokio::task::spawn_local(async move {
            let file = tokio::fs::File::open(filename).await?;
            let file = io::BufReader::new(file);
            let mut lines = file.lines();
            let mut local_count = 0u32;
            while let Some(_) = lines.next_line().await? {
                local_count += 1;
            }

            *count.borrow_mut() += local_count;
            Ok(()) as Result<(), std::io::Error>
        }));
    }

    for task in tasks {
        task.await??;
    }

    println!("Total lines: {}", count.borrow());
    Ok(())
}
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