building-api-gateway

What this skill does

# Building API Gateway

## Overview

Create an API gateway that provides unified entry point routing, load balancing, authentication enforcement, rate limiting, request transformation, and response aggregation across multiple backend microservices. Support path-based and header-based routing, circuit breaker protection for downstream services, and centralized cross-cutting concern management.

## Prerequisites

- Multiple backend API services with known endpoints, health check URLs, and authentication requirements
- Gateway framework: Express Gateway, Kong (declarative config), KrakenD, or custom Node.js/Go implementation
- Service registry or static upstream configuration for backend service discovery
- TLS certificates for gateway termination and optional mTLS for backend communication
- Centralized logging and metrics collection for gateway-level observability

## Instructions

1. Inventory all backend services using Read and Grep, documenting their base URLs, endpoint paths, authentication requirements, and health check endpoints.
2. Define routing rules that map public-facing URL patterns to backend service endpoints: path-based (`/users/*` -> user-service), header-based (`X-API-Version: 2` -> v2-service), or method-based routing.
3. Implement authentication at the gateway layer: validate JWT tokens, API keys, or OAuth2 tokens once at the gateway and forward authenticated user context to backend services via headers (`X-User-ID`, `X-User-Roles`).
4. Add rate limiting at the gateway level with per-consumer quotas, applying limits before requests reach backend services to protect all downstream services uniformly.
5. Configure request transformation: strip internal headers from incoming requests, add correlation IDs, rewrite URL paths for backend routing, and inject service-specific headers.
6. Implement response aggregation for composite endpoints that fan out to multiple backend services, merge responses, and return a unified payload to the client.
7. Add circuit breaker protection per backend service: open the circuit after configurable failure thresholds, return 503 with the failed service identified, and auto-recover after health check success.
8. Configure health check aggregation: gateway `/health` endpoint reports overall status based on individual backend service health, with degraded state support for non-critical service failures.
9. Write integration tests covering routing correctness, auth enforcement, rate limiting, circuit breaker behavior, and response aggregation.

See `${CLAUDE_SKILL_DIR}/references/implementation.md` for the full implementation guide.

## Output

- `${CLAUDE_SKILL_DIR}/gateway/config/routes.yaml` - Route mapping definitions (path -> service)
- `${CLAUDE_SKILL_DIR}/gateway/middleware/auth.js` - Gateway-level authentication enforcement
- `${CLAUDE_SKILL_DIR}/gateway/middleware/rate-limiter.js` - Centralized rate limiting
- `${CLAUDE_SKILL_DIR}/gateway/middleware/circuit-breaker.js` - Per-service circuit breaker
- `${CLAUDE_SKILL_DIR}/gateway/middleware/transform.js` - Request/response transformation logic
- `${CLAUDE_SKILL_DIR}/gateway/aggregators/` - Response aggregation for composite endpoints
- `${CLAUDE_SKILL_DIR}/gateway/health.js` - Aggregated health check endpoint
- `${CLAUDE_SKILL_DIR}/tests/gateway/` - Gateway integration test suite

## Error Handling

| Error | Cause | Solution |
|-------|-------|----------|
| 502 Bad Gateway | Backend service returned invalid response or connection refused | Return descriptive error identifying the failed backend; trigger circuit breaker if threshold met |
| 503 Circuit Open | Backend service circuit breaker is open due to repeated failures | Return `Retry-After` header; serve cached response if available; route to fallback service if configured |
| 504 Gateway Timeout | Backend service response exceeded gateway timeout threshold | Configure per-route timeout limits; implement timeout cascading shorter than client timeout |
| Routing miss | Request path does not match any configured route | Return 404 with list of available API paths; log unmatched routes for route configuration review |
| Auth header stripping | Proxy strips Authorization header before forwarding to backend | Configure gateway to preserve or transform auth headers; verify proxy `proxy_pass_header` settings |

Refer to `${CLAUDE_SKILL_DIR}/references/errors.md` for comprehensive error patterns.

## Examples

**Microservices gateway**: Route `/users/*` to user-service (port 3001), `/orders/*` to order-service (port 3002), and `/products/*` to product-service (port 3003), with unified JWT validation and per-service circuit breakers.

**BFF (Backend for Frontend)**: Gateway aggregates data from user-service, preferences-service, and notification-service into a single `/dashboard` response, reducing frontend API calls from 3 to 1.

**API versioning gateway**: Route requests to different backend deployments based on `Accept-Version` header, enabling blue-green deployments and gradual version migration without client-side URL changes.

See `${CLAUDE_SKILL_DIR}/references/examples.md` for additional examples.

## Resources

- Kong API Gateway: https://konghq.com/
- Express Gateway: https://www.express-gateway.io/
- KrakenD: https://www.krakend.io/
- API Gateway pattern: Microservices.io