Module tokio::runtime

The Tokio runtime.

Unlike other Rust programs, asynchronous applications require runtime support. In particular, the following runtime services are necessary:

• An I/O event loop, called the driver, which drives I/O resources and dispatches I/O events to tasks that depend on them.
• A scheduler to execute tasks that use these I/O resources.
• A timer for scheduling work to run after a set period of time.

Tokio's Runtime bundles all of these services as a single type, allowing them to be started, shut down, and configured together. However, often it is not required to configure a Runtime manually, and a user may just use the tokio::main attribute macro, which creates a Runtime under the hood.

Usage

When no fine tuning is required, the tokio::main attribute macro can be used.

use tokio::net::TcpListener;
use tokio::io::{AsyncReadExt, AsyncWriteExt};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let listener = TcpListener::bind("127.0.0.1:8080").await?;

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

        tokio::spawn(async move {
            let mut buf = [0; 1024];

            // In a loop, read data from the socket and write the data back.
            loop {
                let n = match socket.read(&mut buf).await {
                    // socket closed
                    Ok(n) if n == 0 => return,
                    Ok(n) => n,
                    Err(e) => {
                        println!("failed to read from socket; err = {:?}", e);
                        return;
                    }
                };

                // Write the data back
                if let Err(e) = socket.write_all(&buf[0..n]).await {
                    println!("failed to write to socket; err = {:?}", e);
                    return;
                }
            }
        });
    }
}

From within the context of the runtime, additional tasks are spawned using the tokio::spawn function. Futures spawned using this function will be executed on the same thread pool used by the Runtime.

A Runtime instance can also be used directly.

use tokio::net::TcpListener;
use tokio::io::{AsyncReadExt, AsyncWriteExt};
use tokio::runtime::Runtime;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Create the runtime
    let rt  = Runtime::new()?;

    // Spawn the root task
    rt.block_on(async {
        let listener = TcpListener::bind("127.0.0.1:8080").await?;

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

            tokio::spawn(async move {
                let mut buf = [0; 1024];

                // In a loop, read data from the socket and write the data back.
                loop {
                    let n = match socket.read(&mut buf).await {
                        // socket closed
                        Ok(n) if n == 0 => return,
                        Ok(n) => n,
                        Err(e) => {
                            println!("failed to read from socket; err = {:?}", e);
                            return;
                        }
                    };

                    // Write the data back
                    if let Err(e) = socket.write_all(&buf[0..n]).await {
                        println!("failed to write to socket; err = {:?}", e);
                        return;
                    }
                }
            });
        }
    })
}

Runtime Configurations

Tokio provides multiple task scheduling strategies, suitable for different applications. The runtime builder or #[tokio::main] attribute may be used to select which scheduler to use.

Multi-Thread Scheduler

The multi-thread scheduler executes futures on a thread pool, using a work-stealing strategy. By default, it will start a worker thread for each CPU core available on the system. This tends to be the ideal configuration for most applications. The multi-thread scheduler requires the rt-multi-thread feature flag, and is selected by default:

use tokio::runtime;

let threaded_rt = runtime::Runtime::new()?;

Most applications should use the multi-thread scheduler, except in some niche use-cases, such as when running only a single thread is required.

Current-Thread Scheduler

The current-thread scheduler provides a single-threaded future executor. All tasks will be created and executed on the current thread. This requires the rt feature flag.

use tokio::runtime;

let basic_rt = runtime::Builder::new_current_thread()
    .build()?;

Resource drivers

When configuring a runtime by hand, no resource drivers are enabled by default. In this case, attempting to use networking types or time types will fail. In order to enable these types, the resource drivers must be enabled. This is done with Builder::enable_io and Builder::enable_time. As a shorthand, Builder::enable_all enables both resource drivers.

Lifetime of spawned threads

The runtime may spawn threads depending on its configuration and usage. The multi-thread scheduler spawns threads to schedule tasks and for spawn_blocking calls.

While the Runtime is active, threads may shutdown after periods of being idle. Once Runtime is dropped, all runtime threads are forcibly shutdown. Any tasks that have not yet completed will be dropped.

Structs

Builder	Builds Tokio Runtime with custom configuration values.
EnterGuard	Runtime context guard.
Handle	Handle to the runtime.
Runtime	The Tokio runtime.