API Gateway Patterns: Rate Limiting, Authentication, and Traffic Management at Scale

An API gateway is the single entry point for all client requests to your backend services. Done well, it simplifies clients, centralises cross-cutting concerns, and enables robust traffic management. Done poorly, it becomes a bottleneck, a single point of failure, and a maintenance nightmare.

Core Gateway Responsibilities

Authentication and authorisation: Validate JWTs or API keys, enforce scopes. Never let invalid requests reach downstream services.

Rate limiting: Protect services from abuse and overload. Implement at multiple granularities: per API key, per user, per IP, per endpoint.

Request routing: Route requests to appropriate downstream services based on path, headers, or query parameters.

Request/response transformation: Translate between external API contracts and internal service interfaces.

Observability: Log all requests with correlation IDs. Emit metrics for rate, errors, and latency per route.

Rate Limiting Algorithms

Token bucket: Allows burst traffic up to bucket capacity, then enforces a steady rate. Most forgiving for legitimate clients with occasional spikes.

Sliding window: More precise than fixed window (prevents burst at window boundaries). Higher computational cost.

Fixed window: Simple to implement, but allows 2x the rate limit at window boundaries.

For distributed rate limiting (multiple gateway instances), use Redis as the shared counter store.

Authentication Patterns

API Key: Simple, appropriate for machine-to-machine. Keys must be rotatable without downtime.

OAuth 2.0 with JWT: For user-delegated access. Validate signature and claims at the gateway—never forward unvalidated tokens to services.

mTLS: For service-to-service. Client certificates provide strong identity without application-layer authentication overhead.

High Availability

The gateway must be more reliable than the services it protects. Deploy multiple instances behind a load balancer. Implement circuit breakers to prevent gateway exhaustion when downstream services degrade. Use active health checks and automatic unhealthy instance removal.