Announcing the Tokio runtime

08 March 2018

I’m happy to announce a new release of Tokio. This release includes the first iteration of the Tokio Runtime.


This is how a multi-threaded Tokio based server is now written:

extern crate tokio;

use tokio::net::TcpListener;
use tokio::prelude::*;

fn process(s: TcpStream)
  -> impl Future<Item = (), Error = ()> + Send
{ ... }

let addr = "".parse().unwrap();
let listener = TcpListener::bind(&addr).unwrap();

let server = listener.incoming()
    .map_err(|e| println!("error = {:?}", e))
    .for_each(|socket| {


where process represents a user defined function that takes a socket and returns a future that process it. In the case of an echo server, that might be reading all data from the socket and writing it back to the same socket.

The guides and examples have been updated to use the runtime.

What is the Tokio Runtime?

The Rust asynchronous stack is evolving to a set of loosely coupled components. To get a basic networking application running, you need at a minimum an asynchronous task executor and an instance of the Tokio reactor. Because everything is decoupled, there are multiple options for these various components, but this adds a bunch of boilerplate to all apps.

To help mitigate this, Tokio now provides the concept of a runtime. This is a pre-configured package of all the various components that are necessary for running the application.

This initial release of the runtime includes the reactor as well as a work-stealing based thread pool for scheduling and executing the application’s code. This provides a multi-threaded default for applications.

The work-stealing default is ideal for most applications. It uses a similar strategy as Go, Erlang, .NET, Java (the ForkJoin pool), etc… The implementation provided by Tokio is designed for use cases where many unrelated tasks are multiplexed on a single thread pool.

Using the Tokio Runtime

As illustrated in the example above, the easiest way to use the Tokio runtime is with two functions:

  • tokio::run
  • tokio::spawn.

The first function takes a future to seed the application and starts the runtime. Roughly, it does the following:

  1. Start the reactor.
  2. Start the thread pool.
  3. Spawn the future onto the thread pool.
  4. Blocks the thread until the runtime becomes idle.

The runtime becomes idle once all spawned futures have completed and all I/O resources bound to the reactor are dropped.

From within the context of a runtime. The application may spawn additional futures onto the thread pool using tokio::spawn.

Alternatively, the Runtime type can be used directly. This allows for more flexibility around setting up and using the runtime.

Future improvements

This is just the initial release of the Tokio runtime. Upcoming releases will include additional functionality that is useful for Tokio based applications. A blog post will be coming soon that goes into the roadmap in more detail.

The goal, as mentioned before, is to release early and often. Providing new features to enable the community to experiment with them. Sometime in the next few months, there will be a breaking release of the entire Tokio stack, so any changes in the API need to be discovered before then.


There has also been a new release of tokio-core. This release updates tokio-core to use tokio under the hood. This enables all existing applications and libraries that currently depend on tokio-core (like Hyper) to be able to use the improvements that come with the Tokio runtime without requiring a breaking change.

Given the amount of churn that is expected to happen in the next few months, we’re hoping to help ease the transition across releases.