The OpenFaaS API gateway provides routing mechanisms to expose your functions to the external world.

When a function is invoked by an external request, the function metric will be collected and put into a Prometheus instance. The API gateway keeps monitoring a number of requests for each function, and scales it on demand by increasing the service replica via the Docker Swarm API. Basically, OpenFaaS fully utilizes the scheduling mechanism of Docker Swarm for its autoscaling. The API gateway also comes with a built-in user interface, called the UI portal. The UI allows us to define and invoke functions with a browser.