API Gateways

An API gateway sits between your clients and your backend services. It's the single entry point for all API traffic. It routes requests, enforces authentication, applies rate limits, and aggregates responses. In system design interviews, it appears whenever you have multiple backend services that clients need to interact with.

Underneath the label it is a server, running the same way your other servers run. Every request from the outside lands there first, gets inspected, and is forwarded to whichever internal service owns that path. The services stop being reachable from the internet: they sit on a private network and the gateway holds the only public address, so clients see one hostname, one TLS certificate, and one error format no matter how many services hide behind it.