Concurrency

"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>>

"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?)

"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.

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

// 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;

// 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