What is the “externalDocs” field in OpenAPI? #

Introduction #

The OpenAPI Specification (OAS) is a standard for defining APIs, enabling both humans and computers to discover and understand the capabilities of a web service without access to source code or other documentation. One of the key features of OpenAPI is its comprehensive documentation capabilities. In this context, the externalDocs field plays a significant role. This article explores the externalDocs field in OpenAPI, elaborating on its purpose, structure, and usage.

Understanding OpenAPI #

Before diving into the specifics of the externalDocs field, it is essential to have a basic understanding of what OpenAPI is. OpenAPI, originally known as Swagger, is an open-source framework for designing, building, documenting, and consuming RESTful web services. By adhering to a standard format, OpenAPI provides a detailed specification of API endpoints, data models, and authentication mechanisms, among other aspects.

The primary objective of OpenAPI is to make APIs easier to understand and interact with by providing a machine-readable specification format. This specification can then be used to generate API documentation, client libraries in various programming languages, and even API tests.

For more in-depth information on OpenAPI, you can visit the OpenAPI Initiative website.

The Role of `externalDocs` in OpenAPI #

In any API documentation, there might be scenarios where referencing external resources becomes necessary. For example, one might want to cite detailed guides, additional documentation, tutorials, terms of service, or examples from external sources. The externalDocs field in OpenAPI is designed to fulfill this exact purpose.

What is `externalDocs`? #

The externalDocs field is a means to specify external documentation for an API or any individual component within the API. It enables API providers to link to external resources, offering users additional context or details that are not contained within the OpenAPI document itself.

Structure of `externalDocs` #

The externalDocs field is an object with two optional properties:

description: A short description of the target documentation. It’s a human-readable string providing some context about what the external resource represents.
url: A string containing the URL to the external documentation. This must be a valid URL format.

Usage Examples #

The externalDocs field can be applied at various levels within an OpenAPI document, such as the top-level specification, operations, schemas, and more. Below are some typical usage scenarios:

1. Top-Level Documentation #

Here is an example of externalDocs used at the top level of an API specification:

openapi: 3.0.3
info:
  title: Sample API
  version: 1.0.0
externalDocs:
  description: Find more info here
  url: https://example.com/documentation

In this example, externalDocs gives a link to a more detailed documentation page for the entire API.

2. Operation-Level Documentation #

You can also use the externalDocs field within individual operations to link to specific external resources relevant to that endpoint:

paths:
  /users:
    get:
      summary: Get a list of users
      externalDocs:
        description: More info on user retrieval
        url: https://example.com/user-retrieval
      responses:
        '200':
          description: A list of users

3. Schema-Level Documentation #

Similarly, externalDocs can be applied to schemas to link to external definitions or more detailed descriptions of the data models used:

components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: integer
        name:
          type: string
      externalDocs:
        description: Detailed schema for the User entity
        url: https://example.com/user-schema

Benefits of Using `externalDocs` #

Modularity: By using externalDocs, you can keep your OpenAPI document concise and maintainable, placing detailed guides and extensive documentation externally.
Rich Resource Linking: externalDocs allows you to dynamically link to various resources like online tutorials, API usage examples, terms of service, and privacy agreements, making your documentation richer and more user-friendly.
Consistent Updates: External documentation can be updated independently of the OpenAPI file, ensuring that users always have access to the most current information.

Considerations #

While externalDocs offers numerous benefits, there are some considerations to keep in mind:

URL Management: Make sure that the URLs provided in externalDocs are maintained properly. Broken links can frustrate users and adversely affect the credibility of your documentation.
Context Appropriateness: The external documentation linked should be directly relevant to the context it is linked from. Irrelevant or excessive linking can overwhelm or confuse the users.
Security: Ensure that external links are secure (using HTTPS) and point to trustworthy resources to prevent security risks.

Real-World Examples #

To provide more context, let’s look at some real-world uses of the externalDocs field in open-source API documentation.

GitHub API #

In the GitHub REST API documentation, externalDocs is used to link to detailed guides and additional resources. For instance, under some of their endpoints, they provide links to their broader documentation or usage examples.

Google APIs #

Google’s APIs, such as those provided by Google Cloud, often include external documentation links to offer comprehensive examples and tutorials. They make extensive use of externalDocs for helping developers understand the broader context and usage patterns of their APIs.

Swagger Petstore Example #

The Swagger Petstore is a popular minimal example used to demonstrate OpenAPI features. It uses externalDocs to link to additional documentation for the Petstore API, helping users explore advanced features and updates.

Conclusion #

The externalDocs field in OpenAPI is a powerful tool for creating rich, modular, and maintainable API documentation. By judiciously linking to external resources, API providers can offer comprehensive information that aligns seamlessly with the concise specification of APIs. Understanding and correctly implementing the externalDocs field can significantly enhance the utility and user experience of your API documentation.

For more information, you can refer to the OpenAPI Specification documentation.

By leveraging externalDocs, you are not just limited to describing your API within the confines of the OpenAPI document itself but can extend to a plethora of additional resources, thereby providing a much more enriched developer experience.