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A brief Introduction to OpenAPI Specification

Open API specifications are increasingly becoming a popular way to describe and document APIs. The benefits of using an Open API specification include the ability to generate documentation and specification from the client code , and the fact that many API management tools support the Open API specification.
As I promised in a previous article in this article we'll take a look at what an Open API specification is, and how you can use it to document your APIs.

Glance at the OAS

OpenAPI Specification (formerly Swagger Specification) is an API description format for REST APIs. An OpenAPI file allows you to describe your entire API, including:
β€’ The available endpoints (URLs) and methods (e.g. GET, POST, etc.)
β€’ The input and output for each endpoint
β€’ The authentication methods

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In addition to documenting the API, an Open API specification can also be used to generate documentation and client code. Many API management tools support the Open API specification, which makes it easy to create and maintain APIs.
Some of the main component of OAS are path, parameters, responses and security.Each of them are JSON object that holds properties and array.
The info field contains high level information about the API, version of the document, license, contact, and description.
The server field describe the root endpoint for the API, later using for construct the entire URL from the path.

 "info": {
        "description": "This is a sample server Petstore server.  You can find out more about Swagger at []( or on [, #swagger](  For this sample, you can use the api key `special-key` to test the authorization filters.",
        "version": "1.0.6",
        "title": "Swagger Petstore",
        "termsOfService": "",
        "contact": {
            "email": ""
        "license": {
            "name": "Apache 2.0",
            "url": ""
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The Path Object holds the individual endpoints and their operations. Then the path is appended to the URL from the Server Object in order to reconstruct full URL.
For example here is a POST request to /pet path.

"/pet": {
            "post": {
                "tags": ["pet"],
                "summary": "Add a new pet to the store",
                "description": "",
                "operationId": "addPet",
                "consumes": ["application/json", "application/xml"],
                "produces": ["application/json", "application/xml"],
                "parameters": [{
                    "in": "body",
                    "name": "body",
                    "description": "Pet object that needs to be added to the store",
                    "required": true,
                    "schema": {
                        "$ref": "#/definitions/Pet"
                "responses": {
                    "405": {
                        "description": "Invalid input"
                "security": [{
                    "petstore_auth": ["write:pets", "read:pets"]
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The OAS can be written in JSON format or YAML.
Let's explain this JSON code from the official swagger web site .

  • /pets is the endpoint

  • POST is the method

  • parameter list the parameters for the specific endpoint

  • schema $ref is a reference to another part of the specification that add additional information about the POST request

  • 405 is the status code

  • security is a list of permissions for the endpoint

Many more pathways, as well as extra HTTP method operations for the same path, would most likely be defined in an OpenAPI specification.

Next, the OpenAPI file describe components object. The Components Object allows for the definition of metadata about the API. The metadata can then be reused across the API, making the API definition more concise. The Components Object acts as a container for all the schemas, responses, parameters, examples, and security schemes that your API uses.

"components": {
    "schemas": {
      "overviewSegment": {
        "type": "object",
        "properties": {
          "success": {
            "type": "boolean",
            "example": true
          "message": {
            "type": "string",
            "example": "Success View Segment"
          "code": {
            "type": "string",
            "example": "200"
          "data": {
            "$ref": "#/components/schemas/overviewSegment_data"
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OpenAPI vs Swagger.

OpenAPI and Swagger are two popular frameworks for defining and documenting APIs. Both are open source projects and are widely used in the API community.

Swagger started as an open source project in 2011. It was acquired by SmartBear in 2015 and renamed to OpenAPI Specification.

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OpenAPI Specification is now managed by the OpenAPI Initiative, which is hosted by the Linux Foundation.

OpenAPI Specification (formerly Swagger Specification) is an API description format for REST APIs. An OpenAPI file allows you to describe your entire API, including:

  • The available endpoint(s) and methods (e.g. GET, POST, etc.)
  • The inputs and outputs for each endpoint
  • The data types used
  • Authentication methods

Swagger is a set of open-source tools that use OpenAPI Specification to help you develop, document, and consume REST APIs. The Swagger tools include:

  • Swagger Editor: A web-based editor where you can write OpenAPI specs.
  • Swagger Codegen: A library that generates code (e.g. client SDKs, server stubs) from OpenAPI specs.
  • Swagger UI: A web-based user interface that shows the documentation for an OpenAPI-defined API.

If you're just getting started with API development, we recommend using OpenAPI Specification. It's a great way to define and document your APIs. Once you're comfortable with OpenAPI Specification, you can start using Swagger tools to speed up your development process.

OpenAPI Tools

The ability to use the vast tools ecosystem that surrounds OpenAPI is a significant advantage.
Many open-source and commercial tools assist API developers in developing, testing, documenting, and generating support code for their API.
I've selected some of the most popular OpenAPI tools in a few key areas below.

Since the rise of cloud computing about a decade ago, there has been an explosion of accessible technology.
Developers working hard to make things better, cooler, or slicker.
It's difficult to keep track of tools as they change and others arise.
In addition, because adopting new tools takes time, it becomes important to vet new tools for usage.
It is no different in the API design environment.
πŸ§ͺ Testing: Postman, Citrus, APIFortress, Postwoman, Everest.
🐞 Validation: OpenAPILint, Spectral, CherryBomb.
🧰 Editors: SwaggerHub, Visual Studio Code extension, KaiZen OpenAPI Editor, Stoplight, Insomnia Designer, Optic, and Curio.
πŸ“„ Documentation: Swagger UI, Widdershins, and openapi-viewer.
♾️ Code Generation: OpenAPI-generator, swagger-node-codegenGoogle Gnostic, and Gen.

My next article will be on generating an OpenAPI Specification.
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Top comments (4)

jansche profile image
Jan Schenk (he/him)

Hi @nathan20,
person from Postman here. Thanks for this article. What's your take on the API First approach?

nathan20 profile image

Hey Jan! I think that API First is the ideal approach, it coincides with microservices then can reduce costs in addition provide better scalability and availability. Maybe I will talk about it in a next story. What is your opinion?

jansche profile image
Jan Schenk (he/him)

I definitely believe a product's success depends on how interoperable it is designed. Hence API First is the only approach that I would recommend to anyone. But I might also be a bit biased. ^^

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nathan20 profile image

I believe it is depend also on the product itself, but in general I would also recommend API First. If you want we can continue to discuss on discord we have a server dedicated to API and API security :) Nathan20#3270