What is an API Gateway?

Rather than every backend service reimplementing its own auth checks, throttling, and request logging, a gateway centralizes that work: it terminates incoming requests, validates credentials, applies rate limits, and then routes the request to whichever internal service actually handles it. This keeps individual services simpler and ensures consistent policy enforcement across the whole API surface. Gateways are especially important in microservice architectures, where dozens of small services would otherwise each need their own security and observability code. A gateway also gives a natural place to version APIs, apply request/response transformations, and cache frequent responses — all without touching the underlying service logic. For systems exposing APIs to external, less-trusted callers (partners, third-party developers, AI agents making tool calls), the gateway is also the natural place to enforce stricter scrutiny — deeper request validation, tighter rate limits, and more aggressive logging — than internal service-to-service calls need.