# Async Programming in Rust

Asynchronous programming in Rust allows you to write concurrent code that's both fast and safe.

## What is Async?

Async programming is about doing multiple things at once without blocking. Instead of waiting for one operation to complete before starting another, async code can switch between tasks.

## Key Concepts

### Futures

A `Future` represents a value that may not be available yet:

```rust
async fn fetch_data() -> String {
    // Simulate async work
    tokio::time::sleep(Duration::from_secs(1)).await;
    "Data fetched!".to_string()
}
```

### Async/Await

The `async` keyword makes a function asynchronous, and `await` pauses execution until a future is ready:

```rust
async fn process_data() {
    let data = fetch_data().await;
    println!("Got: {}", data);
}
```

### Runtimes

Rust doesn't have a built-in async runtime. Popular choices include:

- **Tokio**: Full-featured runtime
- **async-std**: Standard library approach
- **smol**: Minimal runtime

## Example: Concurrent HTTP Requests

```rust
use tokio::task;

async fn fetch_all() {
    let task1 = task::spawn(fetch_url("https://api1.example.com"));
    let task2 = task::spawn(fetch_url("https://api2.example.com"));
    let task3 = task::spawn(fetch_url("https://api3.example.com"));

    let (result1, result2, result3) = tokio::join!(task1, task2, task3);
}
```

## Benefits

- **Performance**: Handle thousands of connections efficiently
- **Resource Usage**: Lower memory and CPU usage
- **Scalability**: Build systems that scale

Async Rust might seem complex at first, but it's worth learning for building high-performance applications!