# Async Rust at a Glance — Rust

Source: https://www.skillbyai.com/en/rust/p-async

> Futures, executors and await.

## Many tasks, few threads

For I/O-heavy servers handling thousands of connections, **async** Rust uses `async fn` and `.await` to write non-blocking code. Futures do nothing until polled by an **executor** (runtime) such as **Tokio**, which schedules many tasks on a small pool of threads. Frameworks like Axum and Actix Web build on it. Async adds complexity (pinning, Send bounds across awaits), so use threads for CPU-bound work and async for high-concurrency I/O.

## An async HTTP handler with Tokio and Axum (sketch)

Needs the tokio and axum crates; not run here because these examples use only the standard library offline.

```rust
use axum::{routing::get, Router};

async fn health() -> &'static str {
    "ok"
}

#[tokio::main]
async fn main() {
    let app = Router::new().route("/health", get(health));
    let listener = tokio::net::TcpListener::bind("0.0.0.0:3000").await.unwrap();
    axum::serve(listener, app).await.unwrap();
}
```

## Do not block inside async tasks

CPU-heavy or blocking calls inside async code stall the runtime; move them to spawn_blocking or a thread.

**Quiz:** What runs async futures in Rust?

- [ ] Nothing; futures run when created
- [ ] The compiler at build time
- [ ] The operating system automatically
- [x] An executor (runtime) such as Tokio

*Answer:* An executor (runtime) such as Tokio. Futures are lazy until polled.