Lesson 08

Async Send and Sync boundaries

Use thread-safe shared state around async tasks instead of holding non-Send values across await points.

async
send
sync

Good Code

src/review_cache.rs

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

pub async fn remember_review(cache: Arc<RwLock<Vec<Review>>>, review: Review) {
    // Async-aware locking keeps shared state safe across spawned tasks.
    let mut reviews = cache.write().await;
    reviews.push(review);
}

Bad Code

review_cache.rs

use std::cell::RefCell;
use std::rc::Rc;

pub async fn remember_review(cache: Rc<RefCell<Vec<Review>>>, review: Review) {
    // Rc and RefCell are not safe to share across multi-threaded async tasks.
    cache.borrow_mut().push(review);
    send_audit_event().await;
}

Review Notes

What to review

Good Code

The good version uses Arc and an async-aware lock, making the shared cache suitable for task boundaries that may move between worker threads.

Bad Code

The bad version uses Rc<RefCell<_>>, then awaits after mutating shared state. That design breaks down when the future must be Send or when borrowing spans grow.

Takeaways

Rust async review should check which values cross await points and task boundaries.