# API Gateway & Service Discovery — System Design

Source: https://www.geekswithgeeks.com/en/system-design/sd-api-gateway

> A single front door for clients, and how services find each other.

## The single front door

An **API gateway** sits in front of many backend services and gives clients one endpoint. It handles routing, auth, rate limiting, and response aggregation so each service doesn't reimplement them.

## Finding services

With services scaling up and down, hardcoded IPs break. **Service discovery** (Consul, Eureka, or Kubernetes DNS) keeps a live registry — services register on startup, and callers look up healthy instances at request time.

## The building receptionist

The gateway is a receptionist who checks your ID (auth), tells you which floor to go to (routing), and won't let too many visitors in at once (rate limiting) — so individual departments don't each need their own front desk.

## Don't let it become a monolith

Keep business logic out of the gateway. It should route and enforce cross-cutting policy, not know about order totals or inventory — otherwise it becomes the new bottleneck.