Tokio Patterns

This skill provides common patterns and idioms for building robust async applications with Tokio.

Worker Pool Pattern

Limit concurrent task execution using a semaphore:

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

pub struct WorkerPool {
    semaphore: Arc<Semaphore>,
}

impl WorkerPool {
    pub fn new(size: usize) -> Self {
        Self {
            semaphore: Arc::new(Semaphore::new(size)),
        }
    }

    pub async fn execute<F, T>(&self, f: F) -> T
    where
        F: Future<Output = T>,
    {
        let _permit = self.semaphore.acquire().await.unwrap();
        f.await
    }
}

// Usage
let pool = WorkerPool::new(10);
let results = futures::future::join_all(
    (0..100).map(|i| pool.execute(process_item(i)))
).await;

Request-Response Pattern

Use oneshot channels for request-response communication:

use tokio::sync::{mpsc, oneshot};

pub enum Command {
    Get { key: String, respond_to: oneshot::Sender<Option<String>> },
    Set { key: String, value: String },
}

pub async fn actor(mut rx: mpsc::Receiver<Command>) {
    let mut store = HashMap::new();

    while let Some(cmd) = rx.recv().await {
        match cmd {
            Command::Get { key, respond_to } => {
                let value = store.get(&key).cloned();
                let _ = respond_to.send(value);
            }
            Command::Set { key, value } => {
                store.insert(key, value);
            }
        }
    }
}

// Client usage
let (tx, rx) = mpsc::channel(32);
tokio::spawn(actor(rx));

let (respond_to, response) = oneshot::channel();
tx.send(Command::Get { key: "foo".into(), respond_to }).await.unwrap();
let value = response.await.unwrap();

Pub/Sub with Channels

Use broadcast channels for pub/sub messaging:

use tokio::sync::broadcast;

pub struct PubSub<T: Clone> {
    tx: broadcast::Sender<T>,
}

impl<T: Clone> PubSub<T> {
    pub fn new(capacity: usize) -> Self {
        let (tx, _) = broadcast::channel(capacity);
        Self { tx }
    }

    pub fn subscribe(&self) -> broadcast::Receiver<T> {
        self.tx.subscribe()
    }

    pub fn publish(&self, message: T) -> Result<usize, broadcast::error::SendError<T>> {
        self.tx.send(message)
    }
}

// Usage
let pubsub = PubSub::new(100);

// Subscriber 1
let mut rx1 = pubsub.subscribe();
tokio::spawn(async move {
    while let Ok(msg) = rx1.recv().await {
        println!("Subscriber 1: {:?}", msg);
    }
});

// Subscriber 2
let mut rx2 = pubsub.subscribe();
tokio::spawn(async move {
    while let Ok(msg) = rx2.recv().await {
        println!("Subscriber 2: {:?}", msg);
    }
});

// Publisher
pubsub.publish("Hello".to_string()).unwrap();

Timeout Pattern

Wrap operations with timeouts:

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

pub async fn with_timeout<F, T>(duration: Duration, future: F) -> Result<T, TimeoutError>
where
    F: Future<Output = Result<T, Error>>,
{
    match timeout(duration, future).await {
        Ok(Ok(result)) => Ok(result),
        Ok(Err(e)) => Err(TimeoutError::Inner(e)),
        Err(_) => Err(TimeoutError::Elapsed),
    }
}

// Usage
let result = with_timeout(
    Duration::from_secs(5),
    fetch_data()
).await?;

Retry with Exponential Backoff

Retry failed operations with backoff:

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

pub async fn retry_with_backoff<F, T, E>(
    mut operation: F,
    max_retries: u32,
    initial_backoff: Duration,
) -> Result<T, E>
where
    F: FnMut() -> Pin<Box<dyn Future<Output = Result<T, E>>>>,
{
    let mut retries = 0;
    let mut backoff = initial_backoff;

    loop {
        match operation().await {
            Ok(result) => return Ok(result),
            Err(e) if retries < max_retries => {
                retries += 1;
                sleep(backoff).await;
                backoff *= 2; // Exponential backoff
            }
            Err(e) => return Err(e),
        }
    }
}

// Usage
let result = retry_with_backoff(
    || Box::pin(fetch_data()),
    3,
    Duration::from_millis(100)
).await?;

Graceful Shutdown

Coordinate graceful shutdown across components:

use tokio::sync::broadcast;
use tokio::select;

pub struct ShutdownCoordinator {
    tx: broadcast::Sender<()>,
}

impl ShutdownCoordinator {
    pub fn new() -> Self {
        let (tx, _) = broadcast::channel(1);
        Self { tx }
    }

    pub fn subscribe(&self) -> broadcast::Receiver<()> {
        self.tx.subscribe()
    }

    pub fn shutdown(&self) {
        let _ = self.tx.send(());
    }
}

// Worker pattern
pub async fn worker(mut shutdown: broadcast::Receiver<()>) {
    loop {
        select! {
            _ = shutdown.recv() => {
                // Cleanup
                break;
            }
            result = do_work() => {
                // Process result
            }
        }
    }
}

// Main
let coordinator = ShutdownCoordinator::new();

let shutdown_rx1 = coordinator.subscribe();
let h1 = tokio::spawn(worker(shutdown_rx1));

let shutdown_rx2 = coordinator.subscribe();
let h2 = tokio::spawn(worker(shutdown_rx2));

// Wait for signal
tokio::signal::ctrl_c().await.unwrap();
coordinator.shutdown();

// Wait for workers
let _ = tokio::join!(h1, h2);

Async Initialization

Lazy async initialization with OnceCell:

use tokio::sync::OnceCell;

pub struct Service {
    connection: OnceCell<Connection>,
}

impl Service {
    pub fn new() -> Self {
        Self {
            connection: OnceCell::new(),
        }
    }

    async fn get_connection(&self) -> &Connection {
        self.connection
            .get_or_init(|| async {
                Connection::connect().await.unwrap()
            })
            .await
    }

    pub async fn query(&self, sql: &str) -> Result<Vec<Row>> {
        let conn = self.get_connection().await;
        conn.query(sql).await
    }
}

Resource Cleanup with Drop

Ensure cleanup even on task cancellation:

pub struct Resource {
    handle: SomeHandle,
}

impl Resource {
    pub async fn new() -> Self {
        Self {
            handle: acquire_resource().await,
        }
    }

    pub async fn use_resource(&self) -> Result<()> {
        // Use the resource
        Ok(())
    }
}

impl Drop for Resource {
    fn drop(&mut self) {
        // Synchronous cleanup
        // For async cleanup, use a separate shutdown method
        self.handle.close();
    }
}

// For async cleanup
impl Resource {
    pub async fn shutdown(self) {
        // Async cleanup
        self.handle.close_async().await;
    }
}

Select Multiple Futures

Use select! to race multiple operations:

use tokio::select;

pub async fn select_example() {
    let mut rx1 = channel1();
    let mut rx2 = channel2();

    loop {
        select! {
            msg = rx1.recv() => {
                if let Some(msg) = msg {
                    handle_channel1(msg).await;
                } else {
                    break;
                }
            }
            msg = rx2.recv() => {
                if let Some(msg) = msg {
                    handle_channel2(msg).await;
                } else {
                    break;
                }
            }
            _ = tokio::time::sleep(Duration::from_secs(60)) => {
                check_timeout().await;
            }
        }
    }
}

Cancellation Token Pattern

Use tokio_util::sync::CancellationToken for cooperative cancellation:

use tokio_util::sync::CancellationToken;

pub async fn worker(token: CancellationToken) {
    loop {
        tokio::select! {
            _ = token.cancelled() => {
                // Cleanup
                break;
            }
            _ = do_work() => {
                // Continue
            }
        }
    }
}

// Hierarchical cancellation
let parent_token = CancellationToken::new();
let child_token = parent_token.child_token();

tokio::spawn(worker(child_token));

// Cancel all
parent_token.cancel();

Best Practices

1. Use semaphores for limiting concurrent operations
2. Implement graceful shutdown in all long-running tasks
3. Add timeouts to external operations
4. Use channels for inter-task communication
5. Handle cancellation properly in all tasks
6. Clean up resources in Drop or explicit shutdown methods
7. Use appropriate channel types for different patterns
8. Implement retries for transient failures
9. Use select! for coordinating multiple async operations
10. Document lifetime and ownership patterns clearly