Swagger Descriptor takes a page from design-driven development. Here, we design our endpoints' behavior by describing them in a YML/YAML file or a JSON file. (Of course, as a developer I am too lazy to even write this file and I would prefer this to be autogenerated, which we will look at in a further section.) This is the most important section as it is the contextual information about the service.
Check out Chapter 8/hello_world_swagger.yaml to get an idea of the descriptor file.