Tokio (software)

Tokio
Original author	Carl Lerche
Initial release	December 23, 2020 (2020-12-23)
Stable release	1.52.31
Written in	Rust
Operating system	Windows, Linux, macOS, FreeBSD, WebAssembly
Type	Asynchronous runtime
License	MIT License
Website	tokio.rs
Repository	github.com/tokio-rs/tokio

Tokio is a software library for the Rust programming language. It provides a runtime and functions that enable the use of asynchronous I/O, allowing for concurrency in regards to task completion.²³⁴

Tokio was released in August 2016 for Rust, a general-purpose programming language. Developed by Carl Lerche, Tokio began as a network application framework and supports features such as socket listening and broadcasting, allowing messages to be transferred between computers.

Tokio began in August 2016 by Carl Lerche as a network application framework for Rust built on futures, allowing for network-based middleware and a non-blocking, or asynchronous, implementation of readiness interest to the reactor. Tokio was inspired by Finagle, a Scala-based asynchronous remote procedure call (RPC) system developed at Twitter for Java virtual machines (JVM), allowing distributed systems to communicate within a JVM. Tokio utilizes the lower-level Rust crate mio, itself using system calls such as epoll (Linux), kqueue (FreeBSD), and the input/output completion port (IOCP) API (Windows). For Linux it can also use io_uring via tokio-uring.⁵⁶⁷ The name "Tokio" is derived from Tokyo and mio, and the Tokio logo vaguely resembles the city emblem of Tokyo.⁸ The preliminary version of Tokio was released in January 2017,⁹ followed by a full release in December 2020.¹⁰¹¹ In 2017, Tokio received a grant from the Mozilla Open Source Support fund.¹² In April 2021, Tokio funded its first paid contributor, Alice Ryhl, for her work both developing the project and assisting its users.¹³¹⁴

While Rust has supported asynchronous functions since version 1.39, released in November 2019,¹⁵ it provides no facilities to execute them, requiring an external runtime for that purpose.¹⁶ Tokio provides a runtime that uses a multi-threaded work stealing scheduler.¹⁰ Rust's futures are lazily evaluated, requiring functions to call .await before they do any work.¹⁷ 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.¹⁸

Users of Tokio include the development teams behind Discord and AWS Lambda.¹⁰ The JavaScript and TypeScript runtime Deno uses Tokio under the hood, in comparison to the JavaScript runtime Node.js, which uses the libuv library.¹⁹

Tokio allows for the execution of asynchronous functions in Rust through its built-in runtime, which may be initialized via the #[tokio::main] macro.¹⁸ For example:

use std::error::Error;

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

Here, the reqwest crate is used to request the HyperText Markup Language (HTML) for English Wikipedia. After reqwest::get is called to initialize the asynchronous request, .await will hand over control to the runtime, which then drives all the I/O operations of the request to completion before resuming the main function after the .await.

A simple example of a TCP echo server is as follows:

use std::error::Error;
use tokio::io::{AsyncBufReadExt, AsyncWriteExt, BufReader};
use tokio::net::TcpListener;

#[tokio::main]
async fn main() -> Result<(), Box<dyn Error>> {
    // Run a server on port 8080.
    let listener = TcpListener::bind("localhost:8080").await?;

    loop {
        // Wait for a new connection from a client.
        let (mut stream, _remote_addr) = listener.accept().await?;

        // Spawn a new asynchronous task to handle the connection.
        tokio::spawn(async move {
            let (reader, mut writer) = stream.split();
            let mut reader = BufReader::new(reader);

            // While there is data to be read from the stream…
            while !reader.fill_buf().await.unwrap().is_empty() {
                // Write the data back.
                writer.write_all(reader.buffer()).await.unwrap();
            }
        });
    }
}

This code makes use of the tokio::spawn function to create an asynchronous task (implemented as a stackless coroutine), allowing each connection to be handled separately in the same process, as the runtime ensures that tasks run in the background automatically.²⁰ Importantly however, the runtime multiplexes the tasks’ execution on a single thread pool (whose size is by default equal to the number of processors on the system), and so in comparison to the approach of spawning a separate thread for each task, fewer resources are consumed.

Tokio provides several I/O and timing primitives that work natively inside its runtime. The TcpListener structure used above contains a Transmission Control Protocol (TCP) socket listener that is registered with the runtime, allowing it to be used asynchronously; similarly, the tokio::time::sleep function can be used to suspend a task's execution for a certain duration of time, and again this is implemented by registration with the runtime.²¹

Tokio also provides several generic synchronization primitives suitable for use in an asynchronous context, including locks, semaphores, barriers and channels.²² Unlike the I/O and timer primitives, these work even outside of the runtime context.²³

To facilitate interopability with traditional synchronous code, Tokio provides as part of its runtime a thread pool on which synchronous I/O operations may run.²⁴ In particular, tokio::task::spawn_blocking creates a task which runs in this pool, and is allowed to perform blocking operations—this is unlike tokio::spawn, which may only run asynchronous code.²⁵ For example, this is used to implement filesystem operations, as many platforms do not provide native asynchronous filesystem APIs (an exception to this is Linux's io_uring, however support for this exists only in the external tokio_uring library and is not yet built in).²⁶