Concurrency

Layer 1: Language Mechanics

Core Question

Is this CPU-bound or I/O-bound, and what's the sharing model?

Before choosing concurrency primitives:

• What's the workload type?

• What data needs to be shared?

• What's the thread safety requirement?

Error → Design Question

Error Don't Just Say Ask Instead

E0277 Send "Add Send bound" Should this type cross threads?

E0277 Sync "Wrap in Mutex" Is shared access really needed?

Future not Send "Use spawn_local" Is async the right choice?

Deadlock "Reorder locks" Is the locking design correct?

Thinking Prompt

Before adding concurrency:

What's the workload?

• CPU-bound → threads (std::thread, rayon)

• I/O-bound → async (tokio, async-std)

• Mixed → hybrid approach

What's the sharing model?

• No sharing → message passing (channels)

• Immutable sharing → Arc

• Mutable sharing → Arc<Mutex> or Arc<RwLock>

What are the Send/Sync requirements?

• Cross-thread ownership → Send

• Cross-thread references → Sync

• Single-thread async → spawn_local

Trace Up ↑ (MANDATORY)

CRITICAL: Don't just fix the error. Trace UP to find domain constraints.

Domain Detection Table

Context Keywords Load Domain Skill Key Constraint

Web API, HTTP, axum, actix, handler domain-web Handlers run on any thread

交易, 支付, trading, payment domain-fintech Audit + thread safety

gRPC, kubernetes, microservice domain-cloud-native Distributed tracing

CLI, terminal, clap domain-cli Usually single-thread OK

Example: Web API + Rc Error

"Rc cannot be sent between threads" in Web API context ↑ DETECT: "Web API" → Load domain-web ↑ FIND: domain-web says "Shared state must be thread-safe" ↑ FIND: domain-web says "Rc in state" is Common Mistake ↓ DESIGN: Use Arc<T> with State extractor ↓ IMPL: axum::extract::State<Arc<AppConfig>>

Generic Trace

"Send not satisfied for my type" ↑ Ask: What domain is this? Load domain-* skill ↑ Ask: Does this type need to cross thread boundaries? ↑ Check: m09-domain (is the data model correct?)

Situation Trace To Question

Send/Sync in Web domain-web What's the state management pattern?

Send/Sync in CLI domain-cli Is multi-thread really needed?

Mutex vs channels m09-domain Shared state or message passing?

Async vs threads m10-performance What's the workload profile?

Trace Down ↓

From design to implementation:

"Need parallelism for CPU work" ↓ Use: std::thread or rayon

"Need concurrency for I/O" ↓ Use: async/await with tokio

"Need to share immutable data across threads" ↓ Use: Arc<T>

"Need to share mutable data across threads" ↓ Use: Arc<Mutex<T>> or Arc<RwLock<T>> ↓ Or: channels for message passing

"Need simple atomic operations" ↓ Use: AtomicBool, AtomicUsize, etc.

Send/Sync Markers

Marker Meaning Example

Send

Can transfer ownership between threads Most types

Sync

Can share references between threads Arc<T>

!Send

Must stay on one thread Rc<T>

!Sync

No shared refs across threads RefCell<T>

Quick Reference

Pattern Thread-Safe Blocking Use When

std::thread

Yes Yes CPU-bound parallelism

async/await

Yes No I/O-bound concurrency

Mutex<T>

Yes Yes Shared mutable state

RwLock<T>

Yes Yes Read-heavy shared state

mpsc::channel

Yes Optional Message passing

Arc<Mutex<T>>

Yes Yes Shared mutable across threads

Decision Flowchart

What type of work? ├─ CPU-bound → std::thread or rayon ├─ I/O-bound → async/await └─ Mixed → hybrid (spawn_blocking)

Need to share data? ├─ No → message passing (channels) ├─ Immutable → Arc<T> └─ Mutable → ├─ Read-heavy → Arc<RwLock<T>> └─ Write-heavy → Arc<Mutex<T>> └─ Simple counter → AtomicUsize

Async context? ├─ Type is Send → tokio::spawn ├─ Type is !Send → spawn_local └─ Blocking code → spawn_blocking

Common Errors

Error Cause Fix

E0277 Send not satisfied Non-Send in async Use Arc or spawn_local

E0277 Sync not satisfied Non-Sync shared Wrap with Mutex

Deadlock Lock ordering Consistent lock order

future is not Send

Non-Send across await Drop before await

MutexGuard across await Guard held during suspend Scope guard properly

Anti-Patterns

Anti-Pattern Why Bad Better

Arc<Mutex> everywhere Contention, complexity Message passing

thread::sleep in async Blocks executor tokio::time::sleep

Holding locks across await Blocks other tasks Scope locks tightly

Ignoring deadlock risk Hard to debug Lock ordering, try_lock

Async-Specific Patterns

Avoid MutexGuard Across Await

// Bad: guard held across await let guard = mutex.lock().await; do_async().await; // guard still held!

// Good: scope the lock { let guard = mutex.lock().await; // use guard } // guard dropped do_async().await;

Non-Send Types in Async

// Rc is !Send, can't cross await in spawned task // Option 1: use Arc instead // Option 2: use spawn_local (single-thread runtime) // Option 3: ensure Rc is dropped before .await

Related Skills

When See

Smart pointer choice m02-resource

Interior mutability m03-mutability

Performance tuning m10-performance

Domain concurrency needs domain-*