What are OpenAPI "links"?

What are OpenAPI “links”? #

In today’s interconnected web of APIs, ensuring smooth, seamless integration of various services is crucial. One widely adopted specification for designing APIs is OpenAPI, formerly known as Swagger. OpenAPI not only outlines the structure and functioning of an API but also facilitates better understanding, testing, and automation of API interactions. A noteworthy feature in OpenAPI that helps in creating these well-integrated systems is links. But what exactly are these links, and how do they enhance the API experience? Let’s delve into the concept of OpenAPI links.

Understanding OpenAPI #

Before delving into links, it’s essential to have a foundational understanding of OpenAPI. OpenAPI is a specification for defining APIs that allows both humans and machines to discover and understand the capabilities of a service without access to source code or additional documentation. The power of OpenAPI lies in its ability to describe:

  • The available endpoints (/users, /pets, etc.).
  • Operations on each endpoint (GET, POST, PUT, DELETE, etc.).
  • Input/output parameters, including format and types.
  • Authentication methods.
  • Contact information, license, terms of service, and other metadata.

The OpenAPI documentation is often expressed in YAML or JSON and can be visualized or interacted with using tools like Swagger UI, Postman, or Redoc.

OpenAPI 3.0 introduced a new feature known as links. Inspired by the links concept in RESTful Hypermedia APIs, OpenAPI links provide a way to define relationships and navigations between various API operations. Essentially, links allow you to dynamically determine what subsequent operations can be performed based on the responses of previous operations.

According to the OpenAPI Specification, a link is an annotation within an API response that defines how you can obtain more information or perform further actions from that response. These links can be thought of as actionable metadata.

For instance, when an API call to /users returns a list of users, each user in that list might have a link to retrieve additional details about that specific user with a call to /users/{userId}.

Links in OpenAPI are similar to the relationships seen in relational databases, helping clients navigate a web of API endpoints.

An OpenAPI link typically has the following properties:

  • operationRef/operationId: A reference to the operation that the client should follow.
  • parameters: A mapping of parameters required to invoke the corresponding operation. These can be hardcoded values, expressions, or values extracted from the response.
  • requestBody: Represents the body of the request if the linked operation requires one.
  • description: A brief explanation of the link.
  • server: An optional list of servers to be used for the linked operation.

Here is an example of an OpenAPI link in YAML format:

paths:
  /users:
    get:
      summary: List all users
      responses:
        '200':
          description: A paged array of users
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/User'
          links:
            GetUserById:
              operationId: getUser
              parameters:
                userId: '$response.body#/id'
              
  /users/{userId}:
    get:
      summary: Get a user by ID
      operationId: getUser
      parameters:
      - name: userId
        in: path
        required: true
        schema:
          type: string
      responses:
        '200':
          description: The user details
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'

In this example, when the client receives a list of users from /users, they can follow the link GetUserById to fetch details about a specific user using the userId.

Enhanced Navigation #

Links offer a programmatically understandable way to navigate between different endpoints and operations. They help in defining relationships between operations, making it easier for developers to understand the API’s structure and flow.

Better Documentation #

Relationships between API calls are explicitly documented and not just implied. This leads to cleaner and more comprehensive documentation, which can be critical for onboarding new developers or third-party integrators.

Improved Automation and Tooling #

Automated tools can leverage links to provide a more seamless interaction flow. For example, a tool like Postman can use links to guide users through a series of related API calls, reducing the need for manual lookup and reducing potential errors.

Encourages Design Consistency #

Including links in API design encourages thoughtful consideration of how different parts of the API are related. This can lead to more consistent and predictable API behavior, which is beneficial for both the developers building and those consuming the API.

Easier Integration with Hypermedia #

For APIs that aim to follow RESTful principles, particularly HATEOAS (Hypermedia As The Engine Of Application State), links serve as a stepping stone towards creating truly hypermedia-driven APIs. Although not a full replacement for hypermedia controls found in formats like HAL or JSON-LD, OpenAPI links provide a valuable start.

Implementing links in OpenAPI involves the following steps:

  1. Identify Related Operations: Determine which operations will be related through links. For example, a POST operation that creates a resource and a GET operation that fetches details of that resource.

  2. Define Link Objects: In your OpenAPI specification, define link objects within the responses section of an operation. These links will describe how to transition from the current operation to a related one.

  3. Map Parameters: Ensure proper mapping of parameters between operations. Use expressions to extract necessary data from the responses.

  4. Test Your Implementations: Utilize tools like Swagger UI, Postman, or custom scripts to verify that the links work as intended and that the API navigation flows smoothly.

Real-World Example: GitHub API #

One real-world API that benefits from a link-like system is the GitHub API. For example, when fetching issues for a repository, each issue contains URLs to its comments, labels, etc. While not explicitly using OpenAPI links, the concept of providing navigable pointers is similar.

Here’s a conceptual example aligned with OpenAPI links:

paths:
  /repos/{owner}/{repo}/issues:
    get:
      summary: List repository issues
      parameters:
      - name: owner
        in: path
        required: true
        schema:
          type: string
      - name: repo
        in: path
        required: true
        schema:
          type: string
      responses:
        '200':
          description: A list of issues
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Issue'
          links:
            GetIssue:
              operationId: getIssue
              parameters:
                owner: '$request.path.owner'
                repo: '$request.path.repo'
                issue_number: '$response.body#/number'
              
/components/schemas/Issue:
  type: object
  properties:
    number:
      type: integer
    title:
      type: string
    body:
      type: string
    ...

  /repos/{owner}/{repo}/issues/{issue_number}:
    get:
      summary: Get a single issue
      operationId: getIssue
      parameters:
      - name: owner
        in: path
        required: true
        schema:
          type: string
      - name: repo
        in: path
        required: true
        schema:
          type: string
      - name: issue_number
        in: path
        required: true
        schema:
          type: integer
      responses:
        '200':
          description: The issue details
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Issue'

Conclusion #

OpenAPI links are a vital component for creating navigable and understandable API designs. By defining relationships between different API operations, links offer enhanced navigation, better documentation, improved automation, design consistency, and easier integration with hypermedia-driven approaches.

For those aiming to design modern, scalable, and developer-friendly APIs, leveraging OpenAPI links will undoubtedly be a step in the right direction. As you design your next API, consider how links could provide meaningful guidance and context for your clients, and explore the various tools and documentation available at sites like OpenAPI Initiative to help you get started.

Remember, the end goal is to make APIs more intuitive, expressive, and easier to use – and OpenAPI links are a powerful feature to help achieve that objective.

This website is not affiliated with the OpenAPI Initiative.