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OpenAPI for API Testing

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OpenAPI is a standard specification format for describing HTTP APIs.

An OpenAPI file describes the API in a structured way so that tools can understand the endpoints, methods, request payloads, response payloads, headers, status codes, and authentication rules.

OpenAPI Is The Specification

OpenAPI is the standard. It is not a single tool or product.

An OpenAPI specification can be written as JSON or YAML. The file can then be used by many different tools, including REST clients, documentation generators, API validators, mock servers, SDK generators, and browser-based interfaces.

Swagger is one family of tools that works with OpenAPI. Swagger UI is a free-to-use browser interface that can render an OpenAPI file and let people issue requests from a web page.

What Tools Can Do With OpenAPI

Tools can read an OpenAPI file to:

  • create request templates in REST API clients
  • generate interactive API documentation
  • compare API responses with the documented response structure
  • create mock API servers
  • generate SDK or client code
  • support exploratory testing by showing available endpoints and data shapes

The more complete the OpenAPI file is, the more value those tools can provide.

Viewing An OpenAPI File

You can view an OpenAPI file in any text editor because it is plain JSON or YAML.

If you are new to OpenAPI then a dedicated tool is easier because it can render the file as navigable documentation.

Examples on this site:

These APIs also expose versioned OpenAPI JSON files:

  • OpenAPI v 3.0 JSON for tools that support OpenAPI 3.0
  • OpenAPI v 3.1 JSON, which is the default openapi.json version
  • OpenAPI v 3.2 JSON, which can describe QUERY as a native HTTP method

OpenAPI For Testing

When testing, the OpenAPI file is a useful source of truth, but it is not the same as the running API.

The specification can help you identify:

  • documented endpoints
  • expected request payloads
  • expected status codes
  • required fields
  • content types
  • unsupported methods

It can also help you spot gaps. If the running API behaves differently from the OpenAPI description then either the API or the documentation may need to change.

Standard And Permissive Files

The API Challenges practice APIs provide two styles of OpenAPI output.

The standard validation files describe intended usage. These are useful when you want tools to guide you through normal valid requests.

The less-validating, permissive files are useful for testing because they relax some of the constraints that browser tools and REST clients might otherwise enforce before a request is sent.

Examples:

OpenAPI In REST Clients

Most REST clients can import an OpenAPI file and create a starter collection of requests.

This is often the most flexible testing workflow because REST clients are not limited by browser security rules in the same way as browser-hosted API interfaces.

OpenAPI UIs

OpenAPI UIs render an OpenAPI file as human-friendly API reference documentation.

They can help you explore endpoints, methods, parameters, request bodies, response schemas, authentication requirements, and examples without reading the raw JSON or YAML directly. Some OpenAPI UIs also include a browser request console so you can try documented API calls from the rendered page.

  • Swagger UI and Tools - Swagger is a family of tools that work with OpenAPI files. Swagger UI renders an OpenAPI file as interactive API documentation and can create request forms for browser API calls.
  • OpenAPI Explorer - OpenAPI Explorer is a web component for rendering OpenAPI documentation, resources, models, and browser API calls inside a site or app.
  • Scalar - Scalar provides open source OpenAPI and Swagger support for API references, a REST API client, hosted docs, and related API tooling.
  • Stoplight Elements - Stoplight Elements provides React and web components for building interactive OpenAPI and Markdown powered API documentation.
  • Zudoku - Zudoku is an open source documentation framework for building customizable developer documentation around OpenAPI documents.
  • Redoc - Redoc is an open source OpenAPI documentation viewer that renders API reference pages. The open source version is primarily a viewer, not a request-sending client.

OpenAPI Tools

The OpenAPI ecosystem has many tools for validation, mocking, documentation, code generation, and test support.

Related OpenAPI UI Tool Pages

Read the Online Clients summary and the Swagger UI about page when you want to compare browser interfaces for exploring an OpenAPI file.

 

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