Building a Resilient API Gateway with TypeScript

API gateways sit at the edge of your system, handling cross-cutting concerns so your services can focus on business logic. In this post, we'll build a resilient gateway in TypeScript that's reliable, observable, and ready to scale.

Why API Gateways Matter

A well-designed gateway improves developer experience, reduces duplication, and gives you a single place to enforce security, quotas, and routing.

Designing for Reliability

We'll use a layered approach to keep concerns separated and testable. Each request flows through a predictable pipeline: client → gateway → auth & rate limiting → routing & transform → services.

A TypeScript Implementation

Here's a simplified example of a middleware that adds request IDs and structured logging.

import { Request, Response, NextFunction } from "express";
import { randomUUID } from "crypto";
 
export function requestContext(req: Request, res: Response, next: NextFunction) {
  const requestId = randomUUID();
  req.headers["x-request-id"] = requestId;
  console.log({ requestId, method: req.method, path: req.path });
  next();
}

Composing small, single-purpose middleware keeps each layer easy to reason about and unit-test.

Observability & Monitoring

Emit structured logs, propagate a trace ID across services, and export metrics for latency and error rates. You can't fix what you can't see.

Testing for Resilience

Simulate downstream failures, inject latency, and assert that your circuit breakers and timeouts behave. Resilience is a property you test, not one you hope for.

Key Takeaways

• Keep the gateway thin and the pipeline explicit.
• Make every request traceable end to end.
• Design failure modes deliberately, then test them.