Web Domain

Layer 3: Domain Constraints

Domain Constraints → Design Implications

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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

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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

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Framework Comparison

Key Crates

Design Patterns

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()
    }
}

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Common Mistakes

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Trace to Layer 1

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Related Skills