Public APIs exposed on the web should be well documented, otherwise it would be difficult for developers to use them in their applications. While WADL definitions might be considered a source of documentation, they address a different problem—the discovery of the service. They serve metadata for the services to machines, not to humans. The Swagger project (https://swagger.io/) addresses the need for neat documentation of RESTful APIs. It defines a meta description of an API from an almost human-readable JSON format. The following is a sample swagger.json file, partially describing the catalog service:

{
  "swagger": "2.0",
  "info": {
    "title": "Catalog API Documentation",
    "version": "v1"
  },
  "paths": {
    "/categories/{id}" : {
      "get": {
        "operationId": "getCategoryV1",
        "summary": "Get a specific category ",
        "produces": [
          "application/json"
        ],
        "responses": {
          "200": {
            "description": "200 OK",
            "examples": 
              {"application/json": {                
                "id": 1,
                "name": "Watches",
                "itemsCount": 550
                }                
              } 
          },
          "404": {"description" : "404 Not Found"},
          "500": {"description": "500 Internal Server Error"}
        }
      }
    }
  },
  "consumes": ["application/json"]
}

The swagger.json file is really straightforward: it defines a name and version of your API and gives a brief description of each operation it exposes, nicely coupled with a sample payload. But the real benefit from it comes in another subproject of Swagger, called swagger-ui (https://swagger.io/swagger-ui/), which actually renders this data from swagger.json nicely into an interactive web page that only provides documentation, but also allows interaction with the service:

We will have a look at and utilize the swagger-ui Node.js module to provide the API that we will develop later in the book, with up-to-date documentation.