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Creation: March 29 2021
Modified: March 29 2021

Asynchronous Rust


Futures need to be polled to completion. This is the job of an executor.

Some futures additionally need to wait for events from the kernel for example to know when there might be data ready to read from a file.

A reactor handles this (by using mio or polling) to register for events from the kernel and know when things might be able to progress.

STD Future

std::future is the core futures functionality needed to implement async/await syntax.

Future trait

std::future::Future is a trait representing an asyncronous computation.

The core method of future, poll, attemps to resolve the future into a final value. This method does not block if the value is not ready. Instead, the current task is scheduled to be woken up when it's possible to make further progress by polling again.

The poll function returns:

When the future has finished, clients should not poll again.

When using a future, you generally won't poll directy, but instead .await the value.


The context passed to the poll method can provide a Waker, which is a handle for waking up the current task if it is not ready.

When future is not ready yet, poll returns Poll::Pending and stores a clone of the Waker copied from the current Context. This Waker is then woken once the future can make progress.


Futures alone are inert; they must be actively polled to make progress, meaning that each time the current task is woken up, it should actively re-poll pending futures that it still has interest in.

External Crates

Futures crate

futures-rs is a crate which adds utility and abstraction over futures: FutureExt, TryFutureExt, Stream, StreamExt, TryStreamExt, Sink, SinkExt.

Tokio crate

Tokio brings an async runtime and some additional utility to interact with environment in async way: IO, time, unix signals, sync primitives.

Tokio is built over futures-rs.

Has an executor and a reactor bundled within it.

Futures that rely on the tokio::io/fs need to be run inside the context of a tokio runtime (which makes the tokio reactor available to them and allows spawning).


Brings an API very similar to standard lib, but for async programming. Somehow like futures-rs and tokio merged together.

Uses a different executor than tokio.


A small and fast async runtime implemented around async-executor.

Doesn't come with a reactor itself.

Has a compatibility layer to interact withh tokio-based libraries.

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