Rust Async

Solution Patterns

Pattern 1: Stream Processing

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

async fn process_stream(stream: impl Stream<Item = Data>) {
    stream
        .chunks(100)           // Batch processing
        .for_each(|batch| async {
            process_batch(batch).await;
        })
        .await;
}

When to use: Processing continuous data flows (websockets, file streams, API pagination).

Key insight: Streams are async iterators - pull-based, lazy evaluation.

Pattern 2: Backpressure Control

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

let semaphore = Arc::new(Semaphore::new(10));  // Max 10 concurrent

let stream = tokio_stream::iter(0..1000)
    .map(|i| {
        let permit = semaphore.clone().acquire_owned();
        async move {
            let _permit = permit.await?;
            process(i).await
        }
    })
    .buffer_unordered(100);  // Max 100 buffered futures

When to use: Prevent overwhelming downstream systems or resource exhaustion.

Trade-offs: Adds latency but prevents overload.

Pattern 3: Select Multiplexing

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

async fn multiplex() {
    loop {
        select! {
            msg = receiver.recv() => {
                if let Some(msg) = msg {
                    handle(msg).await;
                } else {
                    break;  // Channel closed
                }
            }
            _ = sleep(Duration::from_secs(5)) => {
                // Timeout handling
                check_health().await;
            }
            else => break,  // All branches complete
        }
    }
}

When to use: Waiting on multiple async operations, first-to-complete wins.

Gotcha: All branches must be cancellation-safe.

Pattern 4: Task Cancellation

use tokio::time::timeout;
use std::time::Duration;

async fn with_timeout() -> Result<Value, TimeoutError> {
    timeout(Duration::from_secs(5), long_operation()).await
        .map_err(|_| TimeoutError)?
}

// Cooperative cancellation
let mut task = tokio::spawn(async move {
    loop {
        // Check cancellation
        tokio::task::yield_now().await;  // Yield point

        // Do work
        if let Err(_) = work().await {
            return;
        }
    }
});

// Cancel task
task.abort();
let _ = task.await;  // Will return JoinError::Cancelled

When to use: Operations with time limits or user-requested cancellation.

Key insight: Cancellation is cooperative - requires yield points.

Workflow

Step 1: Choose Stream vs Iterator

Sync data source?
  → Use Iterator (more efficient)

Async data source (network, DB)?
  → Use Stream

Need backpressure?
  → Definitely Stream

Step 2: Design Concurrency Strategy

Sequential processing?
  → for_each / fold

Limited concurrency?
  → buffer_unordered(N) + Semaphore

Unlimited (dangerous)?
  → Use with extreme caution

Step 3: Handle Cancellation

Long-running task?
  → Add timeout wrapper

User-initiated?
  → Implement abort signal

Resource cleanup?
  → Use Drop or explicit cleanup

Join vs Try_Join

Join - Wait for All

use tokio::join;

// All operations run concurrently, wait for all to complete
let (a, b, c) = join!(
    fetch_user(),
    fetch_posts(),
    fetch_comments()
);
// All values available, even if some operations failed

Use when: All results needed regardless of individual failures.

Try_Join - Fail Fast

use tokio::try_join;

// Stop on first error
let (a, b) = try_join!(
    async_op_a(),
    async_op_b()
)?;
// Both succeeded, or error from first failure

Use when: All operations must succeed, fail fast on errors.

Combined Pattern

async fn fetch_dashboard() -> Result<Dashboard, Error> {
    let (user, posts, comments) = try_join!(
        fetch_user(),
        fetch_posts(),
        fetch_comments()
    )?;

    Ok(Dashboard { user, posts, comments })
}

Common Errors & Solutions

Backpressure Strategies

Strategy 1: Semaphore-Based

let sem = Arc::new(Semaphore::new(10));

stream
    .map(|item| {
        let sem = sem.clone();
        async move {
            let _permit = sem.acquire().await?;
            process(item).await
        }
    })
    .buffer_unordered(10)

Pros: Precise control, easy to reason about Cons: Semaphore overhead

Strategy 2: Buffered Stream

stream
    .chunks(100)
    .for_each_concurrent(5, |batch| async move {
        process_batch(batch).await
    })
    .await

Pros: Simple, built-in to StreamExt Cons: Less fine-grained control

Strategy 3: Channel-Based

let (tx, mut rx) = mpsc::channel(100);  // Buffer size = backpressure

// Producer respects backpressure
tx.send(item).await?;

// Consumer pulls at own pace
while let Some(item) = rx.recv().await {
    process(item).await;
}

Pros: Natural backpressure from bounded channel Cons: Extra copy/move overhead

Performance Tips

Advanced: Future Trait

Implementing Future

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

struct Delay {
    when: Instant,
}

impl Future for Delay {
    type Output = ();

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        if Instant::now() >= self.when {
            Poll::Ready(())
        } else {
            // Wake me later
            cx.waker().wake_by_ref();
            Poll::Pending
        }
    }
}

When to implement: Custom async primitives, advanced control flow.

Gotcha: Must properly handle wakeup notifications.

Review Checklist

When reviewing async code:

• All async functions are properly .awaited
• Backpressure mechanisms in place for streams
• Cancellation handled cooperatively (yield points)
• No locks held across .await points
• Stream processing uses appropriate concurrency limits
• Error propagation uses ? or proper handling
• select! branches are cancellation-safe
• Long-running tasks have timeout protection
• Resource cleanup happens even on cancellation
• CPU-bound work uses spawn_blocking

Verification Commands

# Check async code compilation
cargo check

# Run async tests
cargo test

# Check for common async mistakes
cargo clippy -- -W clippy::await_holding_lock

# Test with tokio-console for debugging
RUSTFLAGS="--cfg tokio_unstable" cargo run

# Profile async runtime
cargo flamegraph --bin your-app

Common Pitfalls

1. Forgotten Await

Symptom: Future never executes, unexpected behavior

// ❌ Bad: future not awaited
async fn bad() {
    fetch_data();  // Returns Future, never runs!
}

// ✅ Good
async fn good() {
    fetch_data().await;  // Actually runs
}

2. Unbounded Concurrency

Symptom: Resource exhaustion, system overload

// ❌ Bad: all operations run concurrently
let futures: Vec<_> = urls.iter()
    .map(|url| fetch(url))
    .collect();
let results = join_all(futures).await;

// ✅ Good: limited concurrency
use futures::stream::{self, StreamExt};

let results = stream::iter(urls)
    .map(|url| fetch(url))
    .buffer_unordered(10)  // Max 10 concurrent
    .collect::<Vec<_>>()
    .await;

3. Lock Across Await

Symptom: Deadlock, "future cannot be sent between threads safely"

// ❌ Bad: lock held during await
let guard = mutex.lock().await;
some_async_op().await;  // DANGER
drop(guard);

// ✅ Good: drop lock before await
let value = {
    let guard = mutex.lock().await;
    guard.clone()
};  // lock dropped
some_async_op().await;

4. Async Drop

Symptom: Cannot await in Drop impl

// ❌ Bad: async operation in Drop
impl Drop for Resource {
    fn drop(&mut self) {
        // Cannot await here!
        self.cleanup().await;  // Won't compile
    }
}

// ✅ Good: explicit async cleanup
impl Resource {
    async fn cleanup(self) {
        // Async cleanup logic
    }
}

// Or spawn cleanup task
impl Drop for Resource {
    fn drop(&mut self) {
        let handle = self.handle.take();
        tokio::spawn(async move {
            if let Some(h) = handle {
                h.cleanup().await;
            }
        });
    }
}

Related Skills

• rust-concurrency - Thread safety, Send/Sync basics
• rust-async-pattern - Async architecture patterns
• rust-ownership - Lifetime issues in async contexts
• rust-pin - Pin and self-referential types
• rust-performance - Async performance optimization
• rust-web - Async web frameworks (axum, actix)

Localized Reference

• Chinese version: SKILL_ZH.md - 完整中文版本，包含所有内容