Actix Web: Middleware, Scopes & Errors

Middleware

App::new()
    .wrap(Logger::default())
    .wrap(Cors::permissive())
    .service(get_user)

// .wrap() chains middleware into the request-processing
// pipeline for all routes registered on that App/scope

Organizing Routes With Scopes

App::new().service(
    web::scope("/api/v1")
        .wrap(AuthMiddleware)
        .service(get_user)
        .service(create_user)
)

// Shared prefix + shared middleware for a related group of routes

Custom Error Handling

impl ResponseError for AppError {
    fn error_response(&self) -> HttpResponse {
        match self {
            AppError::NotFound => HttpResponse::NotFound().finish(),
            AppError::Validation(msg) => HttpResponse::BadRequest().body(msg.clone()),
        }
    }
}

// Maps different failure conditions to appropriate HTTP
// responses, instead of collapsing everything into a 500

Testing

Actix Web's test module (test::TestRequest) builds mock requests and invokes handlers/services directly in async test functions, without needing a fully running server.

Why Rust/Actix Web for Performance-Critical Services

No garbage collector means no unpredictable GC pause times -- valuable for services where consistent, predictable low latency and raw throughput are priorities, at the cost of Rust's steeper learning curve versus a more permissive, dynamically-typed framework.