Rust async

Async & Concurrency

Core Question

Is this I/O-bound (async) or CPU-bound (threads/spawn_blocking)?

Async is for waiting on external things (network, disk, timers). CPU-heavy work blocks the runtime — move it to spawn_blocking or a dedicated thread pool.

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Error → Design Question

Symptom	Don't Just Say	Ask Instead
Future is not Send	"Add Send bound"	Why does data cross a thread boundary? Can you restructure?
Deadlock with Mutex	"Use tokio::sync::Mutex"	Should you hold a lock across await at all?
Task hangs forever	"Add timeout"	Is there a cancellation path?
Channel fills up	"Make it unbounded"	What's the backpressure strategy?

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Quick Decisions

Situation	Reach For	Why
Run independent futures concurrently	tokio::join!	Runs all, returns all results
Run fallible futures, fail fast	tokio::try_join!	Returns first error, drops rest
Race futures, handle first completion	tokio::select!	Cancel losers automatically
Dynamic number of spawned tasks	JoinSet	Add/remove tasks, collect results
CPU-intensive work in async context	spawn_blocking	Moves to blocking thread pool
File I/O in async code	tokio::fs	Non-blocking file operations
Graceful shutdown	CancellationToken	Hierarchical cancellation
One producer, one consumer, one message	oneshot	Request-response pattern
Work queue (multiple producers)	mpsc (bounded)	Backpressure built in
All subscribers get all messages	broadcast	Pub/sub pattern
Share latest value, skip intermediate	watch	Config updates, state sharing
Shared read-only data across tasks	Arc<T>	Clone Arc, not the data
Shared mutable state across tasks	Arc<Mutex<T>>	Or Arc<RwLock<T>> if reads dominate
Stream of async values	tokio_stream + StreamExt	Async equivalent of Iterator
Paginated API consumption	stream::unfold	Lazy, backpressure-aware page fetching
CPU-bound data parallelism	rayon::par_iter	Automatic work-stealing across cores
CPU-bound work in async context	spawn_blocking + rayon	Keep the async runtime unblocked
Scoped parallel work (no Arc)	std::thread::scope	Borrows stack data safely across threads
Atomic flag or counter	AtomicBool / AtomicUsize	Lock-free, single-word synchronization
async fn in trait definition	native async fn in traits	No #[async_trait] needed since Rust 1.75

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Channel Selection

Channel	Pattern	Capacity	Receivers
oneshot	Request → Response	1 message	1
mpsc	Work queue	Bounded (set capacity)	1
broadcast	Pub/sub (all get all)	Bounded (ring buffer)	N (all messages)
watch	Latest value	1 (latest only)	N (skip to newest)

Always use bounded channels unless you have a specific reason not to. Unbounded channels grow without limit when producer outpaces consumer.

Buffer sizing: start with num_producers * 2 or expected burst size. Monitor with capacity() and len().

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The Lock-Across-Await Problem

Never hold a std::sync::Mutex guard across an .await:

// Bad: guard held across await — can deadlock
let mut guard = data.lock().unwrap();
*guard = fetch().await;

// Good: extract, await, then lock again
let current = data.lock().unwrap().clone();
let new_data = process(current).await;
*data.lock().unwrap() = new_data;

tokio::sync::Mutex is await-safe but has higher overhead. Prefer restructuring to avoid holding locks across await entirely.

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Usage Scenarios

Scenario 1: "I need to make 5 HTTP requests and combine the results" → Use tokio::try_join! for a fixed number, or JoinSet for a dynamic number. Don't await them sequentially — that's 5x slower.

Scenario 2: "My async task needs to do JSON parsing on large payloads" → JSON parsing is CPU-bound. Use spawn_blocking(move || serde_json::from_str(&data)) to avoid blocking the runtime.

Scenario 3: "I need to shut down gracefully when Ctrl+C is pressed" → Create a CancellationToken, pass child tokens to tasks, and use tokio::select! to race work against token.cancelled(). On signal, cancel the root token.

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Reference Files

File	Read When
references/tokio-patterns.md	Runtime setup, spawn_blocking, join/select patterns, JoinSet, cancellation
references/channels.md	Choosing and using mpsc/broadcast/watch/oneshot, backpressure, message patterns
references/safety.md	Lock safety across await, Send/Sync issues, clone-before-await patterns
references/streams.md	Stream trait, StreamExt, Pin, async fn in traits, paginated/WebSocket patterns
references/threads-and-parallelism.md	std::thread, rayon, crossbeam, atomics, async-vs-threads decision

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Cross-References

When	Check
Smart pointers for shared state (Arc, Mutex)	rust-ownership → Quick Decisions
Error handling in async (try_join, ?)	rust-errors → Quick Decisions
Async trait design and Send bounds	rust-types → Quick Decisions
Tokio runtime profile settings	rust-perf → Quick Decisions
Tracing spans in async code, .instrument()	rust-tracing → Quick Decisions
Rayon, parallel iterators	rust-perf → Quick Decisions