At its most basic, an API service accepts a remote request and returns a response. But real life is never that simple. Consider your various concerns when you host large-scale APIs.

• You want to protect your APIs from overuse and abuse, so you use an authentication service and rate limiting. 
• You want to understand how people use your APIs, so you’ve added analytics and monitoring tools.
• If you have monetized APIs, you’ll want to connect to a billing system.
• You may have adopted a microservices architecture, in which case a single request could require calls to dozens of distinct applications.
• Over time you’ll add some new API services and retire others, but your clients will still want to find all your services in the same place.

Your challenge is offering your clients a simple and dependable experience in the face of all this complexity. An API gateway is a way to decouple the client interface from your backend implementation. When a client makes a request, the API gateway breaks it into multiple requests, routes them to the right places, produces a response, and keeps track of everything.