Domain Web

# 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

```rust
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 |
|------|--