An API gateway is a service that’s the entry point into the application from the outside world. It’s responsible for request routing, API composition, and other functions, such as authentication. Like a facade, an API gateway encapsulates the application’s internal architecture and provides an API to its clients. All API requests from external clients first go to the API gateway, which routes some requests to the appropriate service whether that be an upstream API server, a third-party application, a database, or even a serverless.