A mobile app calls three backend microservices directly — Orders, Users, and Payments — each managing their own auth validation and rate limiting. As the service count grows, clients must track every endpoint and cross-cutting concerns are duplicated across services. Introduce a single entry point.
An API gateway is a single entry point that centralizes authentication, rate limiting, and routing for a set of microservices — clients talk only to the gateway and need no knowledge of internal services. The payoff is decoupling: the backend can be restructured without changing the URLs clients use, and cross-cutting concerns like auth live in one place instead of being duplicated across every service. Use it when you have multiple microservices with external clients; the trade-off is that the gateway becomes a single point of failure if not deployed with high availability.
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