Overview

Master asynchronous programming in Rust using async/await syntax, tokio runtime, and the futures ecosystem for concurrent I/O operations.

Async/Await Basics

Basic async function:

async fn fetch_data() -> String {
    String::from("data")
}

#[tokio::main]
async fn main() {
    let data = fetch_data().await;
    println!("{}", data);
}

Cargo.toml setup:

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

Tokio Runtime

Different runtime configurations:

// Multi-threaded runtime (default)
#[tokio::main]
async fn main() {
    // Code here
}

// Single-threaded runtime
#[tokio::main(flavor = "current_thread")]
async fn main() {
    // Code here
}

// Manual runtime creation
use tokio::runtime::Runtime;

fn main() {
    let rt = Runtime::new().unwrap();
    rt.block_on(async {
        println!("Running async code");
    });
}

Spawning Tasks

Creating concurrent tasks:

use tokio::task;

#[tokio::main]
async fn main() {
    let task1 = task::spawn(async {
        println!("Task 1");
        42
    });

    let task2 = task::spawn(async {
        println!("Task 2");
        100
    });

    let result1 = task1.await.unwrap();
    let result2 = task2.await.unwrap();

    println!("Results: {}, {}", result1, result2);
}

Spawning with move:

#[tokio::main]
async fn main() {
    let data = String::from("hello");

    let handle = task::spawn(async move {
        println!("{}", data);
    });

    handle.await.unwrap();
}

Async HTTP with reqwest

Install reqwest:

[dependencies]
reqwest = { version = "0.11", features = ["json"] }
serde = { version = "1.0", features = ["derive"] }

Making HTTP requests:

use reqwest;

#[tokio::main]
async fn main() -> Result<(), reqwest::Error> {
    let response = reqwest::get("https://api.github.com/users/rust-lang")
        .await?
        .text()
        .await?;

    println!("{}", response);
    Ok(())
}

Concurrent requests:

use reqwest;

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let urls = vec![
        "https://api.github.com/users/rust-lang",
        "https://api.github.com/users/tokio-rs",
    ];

    let mut handles = vec![];

    for url in urls {
        let handle = tokio::spawn(async move {
            reqwest::get(url).await?.text().await
        });
        handles.push(handle);
    }

    for handle in handles {
        let response = handle.await??;
        println!("{}", response);
    }

    Ok(())
}

Select and Join

tokio::select! for racing futures:

use tokio::time::{sleep, Duration};

#[tokio::main]
async fn main() {
    tokio::select! {
        _ = sleep(Duration::from_secs(1)) => {
            println!("Timer finished first");
        }
        _ = async_operation() => {
            println!("Operation finished first");
        }
    }
}

async fn async_operation() {
    sleep(Duration::from_secs(2)).await;
}

tokio::join! for concurrent execution:

use tokio::time::{sleep, Duration};

#[tokio::main]
async fn main() {
    let (r1, r2, r3) = tokio::join!(
        async { sleep(Duration::from_secs(1)).await; 1 },
        async { sleep(Duration::from_secs(1)).await; 2 },
        async { sleep(Duration::from_secs(1)).await; 3 },
    );

    println!("Results: {}, {}, {}", r1, r2, r3);
}

Channels

mpsc channel for message passing:

use tokio::sync::mpsc;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = mpsc::channel(32);

    tokio::spawn(async move {
        for i in 0..10 {
            tx.send(i).await.unwrap();
        }
    });

    while let Some(value) = rx.recv().await {
        println!("Received: {}", value);
    }
}

Multiple producers:

use tokio::sync::mpsc;

#[tokio::main]
async fn main() {
    let (tx, mut rx) = mpsc::channel(32);

    for i in 0..3 {
        let tx = tx.clone();
        tokio::spawn(async move {
            tx.send(format!("Message from {}", i)).await.unwrap();
        });
    }

    drop(tx); // Close channel when all senders dropped

    while let Some(msg) = rx.recv().await {
        println!("{}", msg);
    }
}

oneshot channel:

use tokio::sync::oneshot;

#[tokio::main]
async fn main() {
    let (tx, rx) = oneshot::channel();

    tokio::spawn(async move {
        tx.send("result").unwrap();
    });

    let result = rx.await.unwrap();
    println!("{}", result);
}

Synchronization Primitives

Mutex for shared state:

use tokio::sync::Mutex;
use std::sync::Arc;

#[tokio::main]
async fn main() {
    let counter = Arc::new(Mutex::new(0));
    let mut handles = vec![];

    for _ in 0..10 {
        let counter = Arc::clone(&counter);
        let handle = tokio::spawn(async move {
            let mut num = counter.lock().await;
            *num += 1;
        });
        handles.push(handle);
    }

    for handle in handles {
        handle.await.unwrap();
    }

    println!("Result: {}", *counter.lock().await);
}

RwLock for read-write access:

use tokio::sync::RwLock;
use std::sync::Arc;

#[tokio::main]
async fn main() {
    let data = Arc::new(RwLock::new(vec![1, 2, 3]));

    // Multiple readers
    let data1 = Arc::clone(&data);
    let data2 = Arc::clone(&data);

    let reader1 = tokio::spawn(async move {
        let d = data1.read().await;
        println!("Reader 1: {:?}", *d);
    });

    let reader2 = tokio::spawn(async move {
        let d = data2.read().await;
        println!("Reader 2: {:?}", *d);
    });

    // One writer
    let data3 = Arc::clone(&data);
    let writer = tokio::spawn(async move {
        let mut d = data3.write().await;
        d.push(4);
    });

    reader1.await.unwrap();
    reader2.await.unwrap();
    writer.await.unwrap();
}

Semaphore for limiting concurrency:

use tokio::sync::Semaphore;
use std::sync::Arc;

#[tokio::main]
async fn main() {
    let semaphore = Arc::new(Semaphore::new(3));
    let mut handles = vec![];

    for i in 0..10 {
        let permit = semaphore.clone();
        let handle = tokio::spawn(async move {
            let _permit = permit.acquire().await.unwrap();
            println!("Task {} acquired permit", i);
            tokio::time::sleep(tokio::time::Duration::from_secs(1)).await;
            println!("Task {} releasing permit", i);
        });
        handles.push(handle);
    }

    for handle in handles {
        handle.await.unwrap();
    }
}

Streams

Using async streams:

use tokio_stream::{self as stream, StreamExt};

#[tokio::main]
async fn main() {
    let mut stream = stream::iter(vec![1, 2, 3, 4, 5]);

    while let Some(value) = stream.next().await {
        println!("{}", value);
    }
}

Creating custom streams:

use tokio_stream::{Stream, StreamExt};
use std::pin::Pin;
use std::task::{Context, Poll};

struct Counter {
    count: usize,
    max: usize,
}

impl Stream for Counter {
    type Item = usize;

    fn poll_next(
        mut self: Pin<&mut Self>,
        _cx: &mut Context<'_>
    ) -> Poll<Option<Self::Item>> {
        if self.count < self.max {
            let current = self.count;
            self.count += 1;
            Poll::Ready(Some(current))
        } else {
            Poll::Ready(None)
        }
    }
}

#[tokio::main]
async fn main() {
    let mut counter = Counter { count: 0, max: 5 };

    while let Some(value) = counter.next().await {
        println!("{}", value);
    }
}

Timeouts and Intervals

Using timeout:

use tokio::time::{timeout, Duration};

async fn slow_operation() -> String {
    tokio::time::sleep(Duration::from_secs(5)).await;
    String::from("done")
}

#[tokio::main]
async fn main() {
    match timeout(Duration::from_secs(2), slow_operation()).await {
        Ok(result) => println!("Success: {}", result),
        Err(_) => println!("Operation timed out"),
    }
}

Using intervals:

use tokio::time::{interval, Duration};

#[tokio::main]
async fn main() {
    let mut interval = interval(Duration::from_secs(1));

    for _ in 0..5 {
        interval.tick().await;
        println!("Tick");
    }
}

Error Handling

Propagating errors with ?:

use reqwest;

async fn fetch_url(url: &str) -> Result<String, reqwest::Error> {
    let response = reqwest::get(url).await?;
    let body = response.text().await?;
    Ok(body)
}

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let content = fetch_url("https://example.com").await?;
    println!("{}", content);
    Ok(())
}

Blocking Operations

Running CPU-intensive tasks:

use tokio::task;

fn blocking_operation() -> u64 {
    // CPU-intensive work
    (0..1_000_000).sum()
}

#[tokio::main]
async fn main() {
    let result = task::spawn_blocking(|| {
        blocking_operation()
    }).await.unwrap();

    println!("Result: {}", result);
}

Async Traits

Using async-trait crate:

[dependencies]
async-trait = "0.1"

use async_trait::async_trait;

#[async_trait]
trait Repository {
    async fn find(&self, id: u64) -> Option<String>;
    async fn save(&self, data: String) -> Result<(), String>;
}

struct UserRepository;

#[async_trait]
impl Repository for UserRepository {
    async fn find(&self, id: u64) -> Option<String> {
        tokio::time::sleep(tokio::time::Duration::from_millis(100)).await;
        Some(format!("User {}", id))
    }

    async fn save(&self, data: String) -> Result<(), String> {
        println!("Saving: {}", data);
        Ok(())
    }
}

When to Use This Skill

Use rust-async-patterns when you need to:

Build async web servers or clients
Handle concurrent I/O operations efficiently
Make multiple HTTP requests concurrently
Implement producer-consumer patterns
Work with async streams of data
Manage shared state across async tasks
Control concurrency limits
Handle timeouts and cancellation
Build event-driven systems
Process data asynchronously

Best Practices

Use tokio::spawn for CPU-independent tasks
Use spawn_blocking for CPU-intensive work
Prefer channels over shared state with locks
Use select! for racing futures
Use join! for concurrent independent operations
Set appropriate timeouts for network operations
Use Semaphore to limit concurrent operations
Avoid holding locks across await points
Use Arc for shared ownership in async context
Handle errors properly with Result

Common Pitfalls

Holding std::sync::Mutex across await (use tokio::sync::Mutex)
Not using move with closures in spawn
Forgetting to await futures
Blocking the runtime with CPU-intensive work
Creating too many tasks without limits
Not handling cancellation properly
Using the wrong channel type
Deadlocks with improper lock ordering
Not configuring runtime appropriately
Ignoring errors in spawned tasks

📖 rust-async-patterns