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 |