Getting Started with Rust Async Runtimes

Rust’s async ecosystem has matured significantly. Choosing the right runtime is one of the first decisions you’ll make when building an async Rust application.

The Big Three

There are three major async runtimes in the Rust ecosystem today:

Tokio

Tokio is the most popular async runtime, with over 150 million downloads. It provides a multi-threaded, work-stealing scheduler.

#[tokio::main]
async fn main() {
    let listener = TcpListener::bind("127.0.0.1:8080").await.unwrap();
    loop {
        let (socket, _) = listener.accept().await.unwrap();
        tokio::spawn(async move {
            handle_connection(socket).await;
        });
    }
}

Strengths:

• Most mature ecosystem
• Best-in-class documentation
• Default choice for web frameworks (Axum, Actix)

async-std

async-std aims to provide async versions of the standard library APIs.

Think of async-std as “the async standard library” — it mirrors std’s API surface as closely as possible.

Strengths:

• Familiar API (mirrors std)
• Simpler mental model
• Good for learning async Rust

smol

smol is a small, fast async runtime that prioritizes simplicity.

Strengths:

• Minimal footprint
• Composable architecture
• Great for embedded and CLI tools

Decision Matrix

Runtime	Ecosystem	Performance	Learning Curve	Best For
Tokio	⭐⭐⭐⭐⭐	⭐⭐⭐⭐⭐	⭐⭐⭐	Web servers, microservices
async-std	⭐⭐⭐	⭐⭐⭐⭐	⭐⭐	Learning, prototyping
smol	⭐⭐	⭐⭐⭐⭐	⭐⭐⭐⭐	CLI tools, embedded

Recommendation

For most projects, start with Tokio. Its ecosystem advantage is overwhelming — every major async library is tested against Tokio first. Switch to smol only if you need minimal binary size or are building a CLI tool where startup time matters.