🤔 refer to The State of Async Rust

The State

graph LR;
A("Async madness")
A-.Try to cover Rust standard.->C("async-std<br/>abandoned 😱")
A-.Try to cover any runtime.->B("Tokio<br/>Multi threaded by default = complex 🫠")

Async vs Threads


use std::error::Error;
use std::fs::File;
use std::io::Read;
use std::path::Path;
use std::{thread, time};

// This fn could be used by both async and sync callers,
// eliminating the need for an asynchronous runtime.
fn read_contents<T: AsRef<Path>>(file: T) -> Result<String, Box<dyn Error>> {
    let mut file = File::open(file)?;
    let mut contents = String::new();
    file.read_to_string(&mut contents)?;
    return Ok(contents);

fn main() {
    thread::scope(|scope| {
        // worker thread 1
        scope.spawn(|| {
            let contents = read_contents("foo.txt");
            // do something with contents

        // worker thread 2
        scope.spawn(|| {
            let contents = read_contents("bar.txt");
            // ...

        // worker thread 3
        scope.spawn(|| {
            let contents = read_contents("baz.txt");
            // ...

    // Threads get joined automatically


async fn main() {
    // This will print a warning, but compile and do nothing at runtime
    // futures do nothing unless you `.await` or poll them

🤔 refer to Asynchronous Programming in Rust

If you don't need async for performance reasons, threads can often be the simpler alternative.

Best practices

  1. synchronous by default.
  2. opt into async as needed.
  3. less main with #[tokio::main].
fn index() -> impl Response {
    let users = db::get_users();
    Response::ok().body(render_template("index.html", users))

// A route, which profits from concurrent IO
// It sends multiple requests to an external API and aggregates the results
// Note how the function itself is does not need to be async
fn users(count: usize) -> impl Response {
    // Start a local, single-threaded runtime with smol's async-executor
    let rt = smol::LocalExecutor::new();

    // Run the async code on the runtime
    let results = {
        let mut results = Vec::new();
        for id in 0..count {
            let result = reqwest::get(format!("{}", id)).await?;

    Response::ok().body(render_template("users.html", results))

// This does not need to be async either!
// In the background, it might use a thread pool to handle multiple requests
fn main() -> Result<()> {
    let app = App::new()
        .mount("/", index)
        .mount("/users", users)

Read more 👉 Asynchronous Programming in Rust