A controller is one of the most important components of OpenWhisk. As the name implies, it mainly controls the invocation process of the cluster. Controllers can work without having the API gateway in front of them. A controller serves the HTTP protocol directly, in an insecure form, as the HTTPS part is the function of the API gateway. Basically, a controller source is a part of the OpenWhisk project. The configuration used in this chapter is the latest Docker image published by OpenWhisk.