Express Middleware Chain

Auth, rate limit, validation

Note from the N-iX interview prep — backend HTTP layer.

Summary: Order matters. I structure from most broad / least expensive to most specific / expensive.

Correct order

Logging / Request ID – adds req.id for tracing.
CORS / Helmet – security headers first.
Body parsers – express.json(), express.urlencoded().
Rate limiter – express-rate-limit based on IP or user ID.
Authentication – validates JWT / OAuth token. Blocks early if invalid.
Authorization – checks permissions (e.g., req.user.role === 'admin').
Validation – Joi / zod to validate request body/query.
Route handler – the actual business logic.
Error handler – catches all errors and sends appropriate HTTP status.

app.use(logger);
app.use(helmet());
app.use(cors());
app.use(express.json());
app.use(rateLimiter);
app.use(authMiddleware);
app.use(authorizationMiddleware);
app.use(validateRequest);
app.use("/api/orders", orderRoutes);
app.use(errorHandler);

Why this order

Rate limiting before auth avoids wasting CPU on invalid tokens.
Auth before validation prevents validating requests from unauthenticated users.
Error handler is last to catch any thrown errors.

Real-world note: I also add circuit breakers on external API calls inside the route handler to prevent cascading failures.