Documents from Julia code

The authoring API is intentionally smaller than the ingestion and client pipeline. It maps common Julia endpoint declarations to an OpenAPI 3.2.0 document: describe each endpoint as an OpenAPI.Operation and pass the collection to OpenAPI.document.

using OpenAPI, JSON

struct Widget
    id::Int
    tags::Vector{String}
end

operations = [
    OpenAPI.Operation(
        id = "get_widget",
        method = :GET,
        path = "/v1/widgets/{id}",
        params = [
            OpenAPI.Param("id", :path, Int),
            OpenAPI.Param("verbose", :query, Bool; required = false),
        ],
        responsetype = Widget,
    ),
]

document = OpenAPI.document(
    operations;
    title = "Widgets",
    version = "1.0.0",
)

println(JSON.json(document; pretty = 2))

{
  "openapi": "3.2.0",
  "info": {
    "title": "Widgets",
    "version": "1.0.0"
  },
  "paths": {
    "/v1/widgets/{id}": {
      "get": {
        "operationId": "get_widget",
        "parameters": [
          {
            "name": "id",
            "in": "path",
            "required": true,
            "schema": {
              "type": "integer",
              "format": "int64"
            }
          },
          {
            "name": "verbose",
            "in": "query",
            "required": false,
            "schema": {
              "type": "boolean"
            }
          }
        ],
        "responses": {
          "200": {
            "description": "success",
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/Widget"
                }
              }
            }
          },
          "default": {
            "description": "unexpected error",
            "content": {
              "application/json": {
                "schema": {
                  "type": "object",
                  "properties": {
                    "error": {
                      "type": "object",
                      "properties": {
                        "message": {
                          "type": "string"
                        },
                        "code": {
                          "type": "integer"
                        }
                      }
                    }
                  }
                }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Widget": {
        "type": "object",
        "properties": {
          "id": {
            "type": "integer",
            "format": "int64"
          },
          "tags": {
            "type": "array",
            "items": {
              "type": "string"
            }
          }
        },
        "additionalProperties": false,
        "required": [
          "id",
          "tags"
        ]
      }
    }
  }
}

Named struct types encountered in parameter, body, and response types are collected under components/schemas and referenced by $ref; OpenAPI.schemaof documents the exact Julia-type-to-schema mapping.

The result is a plain JSON object, so the same document can be served by an application, written to a file, or fed straight back into the generation pipeline (OpenAPI.client accepts in-memory documents).

OpenAPI.jl does not depend on a server framework. Framework packages can add optional OpenAPI.operations and OpenAPI.register! methods to expose their routes as Operations and serve the generated document. Servo.jl provides its OpenAPI adapter from a downstream package extension.