Web Domain

Layer 3: Domain Constraints

Domain Constraints → Design Implications

Domain Rule	Design Constraint	Rust Implication
Stateless HTTP	No request-local globals	State in extractors
Concurrency	Handle many connections	Async, Send + Sync
Latency SLA	Fast response	Efficient ownership
Security	Input validation	Type-safe extractors
Observability	Request tracing	tracing + tower layers

---

Critical Constraints

Async by Default

RULE: Web handlers must not block
WHY: Block one task = block many requests
RUST: async/await, spawn_blocking for CPU work

State Management

RULE: Shared state must be thread-safe
WHY: Handlers run on any thread
RUST: Arc<T>, Arc<RwLock<T>> for mutable

Request Lifecycle

RULE: Resources live only for request duration
WHY: Memory management, no leaks
RUST: Extractors, proper ownership

---

Trace Down ↓

From constraints to design (Layer 2):

"Need shared application state"
    ↓ m07-concurrency: Use Arc for thread-safe sharing
    ↓ m02-resource: Arc<RwLock<T>> for mutable state

"Need request validation"
    ↓ m05-type-driven: Validated extractors
    ↓ m06-error-handling: IntoResponse for errors

"Need middleware stack"
    ↓ m12-lifecycle: Tower layers
    ↓ m04-zero-cost: Trait-based composition

---

Framework Comparison

Framework	Style	Best For
axum	Functional, tower	Modern APIs
actix-web	Actor-based	High performance
warp	Filter composition	Composable APIs
rocket	Macro-driven	Rapid development

Key Crates

Purpose	Crate
HTTP server	axum, actix-web
HTTP client	reqwest
JSON	serde_json
Auth/JWT	jsonwebtoken
Session	tower-sessions
Database	sqlx, diesel
Middleware	tower

Design Patterns

Pattern	Purpose	Implementation
Extractors	Request parsing	State(db), Json(payload)
Error response	Unified errors	impl IntoResponse
Middleware	Cross-cutting	Tower layers
Shared state	App config	Arc<AppState>

Code Pattern: Axum Handler

async fn handler(
    State(db): State<Arc<DbPool>>,
    Json(payload): Json<CreateUser>,
) -> Result<Json<User>, AppError> {
    let user = db.create_user(&payload).await?;
    Ok(Json(user))
}

// Error handling
impl IntoResponse for AppError {
    fn into_response(self) -> Response {
        let (status, message) = match self {
            Self::NotFound => (StatusCode::NOT_FOUND, "Not found"),
            Self::Internal(_) => (StatusCode::INTERNAL_SERVER_ERROR, "Internal error"),
        };
        (status, Json(json!({"error": message}))).into_response()
    }
}

---

Common Mistakes

Mistake	Domain Violation	Fix
Blocking in handler	Latency spike	spawn_blocking
Rc in state	Not Send + Sync	Use Arc
No validation	Security risk	Type-safe extractors
No error response	Bad UX	IntoResponse impl

---

Trace to Layer 1

Constraint	Layer 2 Pattern	Layer 1 Implementation
Async handlers	Async/await	tokio runtime
Thread-safe state	Shared state	Arc<T>, Arc<RwLock<T>>
Request lifecycle	Extractors	Ownership via From<Request>
Middleware	Tower layers	Trait-based composition

---

Related Skills

When	See
Async patterns	m07-concurrency
State management	m02-resource
Error handling	m06-error-handling
Middleware design	m12-lifecycle