Async Programming in Rust

Async Programming in Rust

Asynchronous programming is a style of programming that allows multiple tasks to make progress concurrently, instead of having to wait for each one to finish before starting the next one. This is useful for tasks that have independent pieces and can potentially save a lot of time. In this tutorial, we will be looking at how to do asynchronous programming in the Rust language.

Table of Contents

  1. What is Async Programming
  2. Why Use Rust for Async Programming
  3. Returning Futures from Functions
  4. Using async/await Syntax in Rust
  5. Common Pitfalls in Rust Async Programming
  6. Further Reading

What is Async Programming

Asynchronous, or async, programming is a way to write code which allows multiple tasks to be run at the same time, without waiting for each task to complete before starting the next. Traditionally, a program would run task A to completion, then run task B, then task C, and so on. With asynchronous programming, once task A starts, if there is a spot where it has to wait (such as waiting for data from a network), instead of idly sitting there, the program can go off and start task B. This can lead to increased efficiency and better performance.

Why Use Rust for Async Programming

Rust is an ideal language for async programming due to its focus on safety and concurrency. In Rust, unlike most other languages, shared mutable state is safe by default due to its ownership system. This comes in handy when dealing with async programming because often tasks will need to share state, and in other languages such as JavaScript, this can lead to unexpected and hard to debug issues.

Moreover, Rust's performance is comparable to that of C and C++, which makes it perfectly suited for high-performance systems and application development where asynchronous programming is crucial.

Returning Futures from Functions

In Rust, when you want to make a function asynchronous, you return a Future. This Future is a value that can produce another value in the future. Here's how it looks in code:

use futures::future::Future;

fn compute() -> impl Future<Item = i32, Error = ()> {
    // Do something async

Any function that is marked with async keyword returns a Future. Inside an async function, you can use .await on futures to get their result.

Using async/await Syntax in Rust

The async and await keywords in Rust are used for defining and working with asynchronous functions.

async fn foo() -> usize {

async fn main() {
    let result = foo().await;
    println!("{}", result); // prints: 100

In the above example, foo is an asynchronous function that returns a usize. We can use the .await operator to wait for the future to complete and get its resolved value.

Common Pitfalls in Rust Async Programming

Async programming in Rust can be challenging, especially for beginners. Here are some common pitfalls:

  • Mistaking a future for a task: In Rust, futures and tasks are distinct concepts. A Future is a value that might not have finished computing yet, while a task is a future that is scheduled to run on an executor.

  • Forgetting to .await a future: Just creating a future doesn't do anything. You have to actually .await it to start the computation.

  • Blocking the executor: If you accidentally do a blocking operation in an async function, you can end up blocking the entire executor, which can lead to performance problems.

Tokio vs. Standard Library Async in Rust

Rust's async ecosystem offers various options for asynchronous programming. Two prominent choices are Tokio and the standard library's async features. Understanding their differences and use cases is crucial for efficient Rust development.


  • Comprehensive Runtime: Tokio provides a full-featured runtime for asynchronous applications. It includes a multi-threaded scheduler, I/O reactors, and a timer.
  • Rich Ecosystem: Tokio has a wide range of libraries and integrations specifically designed for its runtime.
  • High Performance: It's optimized for performance, particularly suitable for high-throughput and low-latency network applications.
  • Specialized Utilities: Offers utilities like tokio::stream, tokio::sync, and others, which are tailored for its runtime.

Standard Library Async

  • Built-in Support: Rust’s standard library includes built-in async/await support, offering a foundational layer for asynchronous programming.
  • Flexibility with Executors: While it provides the basics, it relies on external executors (like Tokio, async-std) for runtime implementation.
  • Simpler Use Cases: Ideal for simpler or less demanding asynchronous operations.
  • Compatibility: Standard library async can be used with various executors, making it versatile for different environments.

Choosing Between Them

  • Complexity and Performance Needs: For complex, high-performance network applications, Tokio is often the preferred choice.
  • Simplicity and Flexibility: For simpler applications or when you need flexibility in choosing different executors, standard library async may be more appropriate.
  • Library and Ecosystem Dependencies: Consider the libraries and frameworks you plan to use. Some are specifically designed for Tokio or other runtimes.

In summary, the choice between Tokio and standard library async in Rust depends on the complexity, performance requirements, and the specific libraries and frameworks in your project.

Further Reading

  • The Rust Programming Language (Book by Steve Klabnik and Carol Nichols)
  • Async programming in Rust with async-std (
  • Rust async programming tutorial (

Stay tuned and keep coding in Rust! Please, if you have any question or any suggestion don't hesitate to get in touch!