Tokio (software)

While Rust has supported asynchronous functions since version 1.39 which was released in November 2019,[6] it requires an external runtime to execute them.[7] Tokio provides a runtime that uses a multi-threaded work stealing scheduler.[8] Rust's futures are lazily evaluated, requiring functions to call .await before they do any work.[9] When .await is invoked, Tokio's runtime may pause the original future until its I/O completes, and unpauses a different task that is ready for further processing.[10] A basic Tokio program that fetches a webpage looks like the following:

#[tokio::main]
async fn main() -> Result<()> {
    let url = "https://en.wikipedia.org/";
    let text = reqwest::get(url).await?.text().await?;
    println!("{}", text);
    Ok(())
}

The #[tokio::main] macro transparently creates a Tokio runtime to execute the program with.

Tokio further allows users to create tasks, which are green threads, using a tokio::spawn() function. Unlike futures, tasks do not need to use .await, as the task will be automatically executed when a thread is available.

Tokio also includes a version of the Rust standard library that is designed for being used asynchronously. For example, tokio::fs::read_to_string(), which reads the contents of a file, is the asynchronous version of std::fs::read_to_string().

Tokio supports io_uring, a Linux asynchronous I/O syscall interface, in a separate crate named tokio-uring.[8][11]

Tokio was first announced in 2016 as a framework for building fast network applications.[12] In 2017, Tokio received a grant from the Mozilla Open Source Support fund.[13]

Tokio 0.3 was released in October 2020, and treated as a beta release preceding an eventual 1.0 stable release. Tokio 1.0 was released in December 2020 and will be supported for at least five years.[8][14]

Notable users of Tokio include the development teams behind Discord and AWS Lambda.[8] In April 2021, Tokio funded its first paid contributor, Alice Ryhl.[15][16]

• Official website
• Tokio on GitHub
• Tokio on crates.io