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 |

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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

| 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()
    }
}

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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 |

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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 |

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Related Skills

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