tokio

Tokio is the de facto async runtime for Rust network services. It provides an executor, timers, async IO, and synchronization primitives built for async/await.

Recipe

[dependencies]
tokio = { version = "1", features = ["full"] }

#[tokio::main]
async fn main() {
    let handle = tokio::spawn(async {
        tokio::time::sleep(std::time::Duration::from_millis(10)).await;
        42
    });
    let n = handle.await.unwrap();
    assert_eq!(n, 42);
}

When to reach for this:

• HTTP servers (axum, hyper)
• Database pools (sqlx)
• Long-lived daemons with concurrent connections

Working Example

use tokio::net::TcpListener;
use tokio::io::{AsyncReadExt, AsyncWriteExt};
 
#[tokio::main]
async fn main() -> std::io::Result<()> {
    let listener = TcpListener::bind("127.0.0.1:0").await?;
    let addr = listener.local_addr()?;
 
    tokio::spawn(async move {
        let (mut socket, _) = listener.accept().await.unwrap();
        let mut buf = [0u8; 4];
        socket.read_exact(&mut buf).await.unwrap();
        socket.write_all(b"pong").await.unwrap();
    });
 
    let mut client = tokio::net::TcpStream::connect(addr).await?;
    client.write_all(b"ping").await?;
    let mut out = [0u8; 4];
    client.read_exact(&mut out).await?;
    assert_eq!(&out, b"pong");
    Ok(())
}

What this demonstrates:

• #[tokio::main] sets up the multi-thread runtime
• spawn runs concurrent tasks
• Async IO without blocking threads

Deep Dive

Runtime Features

Cross-Link

See the Tokio basics section for channels, select!, graceful shutdown, and tracing integration.

Gotchas

• Blocking in async - std::fs::read blocks the worker thread. Fix: tokio::task::spawn_blocking or tokio::fs.
• CPU-heavy work on runtime - starves IO. Fix: dedicated thread pool or rayon.
• current_thread runtime surprises - tasks only run when polled on one thread. Fix: use multi-thread for servers.
• Unbounded channels - memory grows under load. Fix: bounded channels with backpressure.
• Missing enable_io - custom runtime builder without IO feature fails at accept. Fix: mirror full features explicitly.

Alternatives

FAQs

Related

• Tokio basics - full section
• reqwest & sqlx - async HTTP and DB
• tracing - async-aware logging
• Async / Tokio Skill - agent review patterns

Stack versions: This page was written for Rust 1.97.0 (edition 2024), Tokio 1.x, Axum 0.8, serde 1.0, sqlx 0.8, clap 4, and Polars 0.46+.

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