OpenAPI overview and best practices

OpenAPI is a standard for describing RESTful APIs. OpenAPI defines an API’s core elements, like endpoints, request and response data formats, authentication methods, and more.

Several versions of the OpenAPI Specification are in circulation: 2.0 (known as Swagger), 3.0, and 3.1.

Speakeasy supports OpenAPI versions 3.0 and 3.1 and recommends OpenAPI version 3.1 for all projects. The advantage of OpenAPI version 3.1 is its full compatibility with JSON Schema (opens in a new tab), providing access to a large ecosystem of tools and libraries.

OpenAPI best practices

OpenAPI offers extensive flexibility and can describe any HTTP API, including REST APIs and even RPC-based calls. The OpenAPI Specification provides several valid approaches to achieving the same result.

With this flexibility, constructing an OpenAPI document suitable for code generation requires careful consideration. Speakeasy recommends following specific best practices when writing OpenAPI documents. The sections below outline key points to consider when creating an OpenAPI description.

servers

Add multiple servers to define different environments or versions. This is especially useful for separating production and testing environments.

operationId

The operationId value is a unique identifier for the operation. It must be unique in the document and is case sensitive. Strongly recommended to always define an operationId, though not required.

In code generation

The operationId value creates the name of the method generated for the operation. Follow a consistent pattern for naming operations, for example, listDrinks, createDrink, updateDrink, and deleteDrink.

When generating a spec from an API framework, ensure operationId values are human-readable. Some frameworks, like FastAPI, create long operationId identifiers that result in non-idiomatic method names.

Examples

Adding Examples improves OpenAPI document usability by providing examples that illustrate the expected request and response structures.

$ref

In OpenAPI, the $ref keyword references components defined in the Components Object. These components are commonly used for reusable elements like schemas, parameters, responses, and examples.

In code generation

Component schemas describe the request and response bodies of operations, serving as the basis for generating SDK types. Using components prevents issues where multiple types are defined for the same data structure.

Dedicated Error Classes

Define dedicated error classes by creating response objects with specific HTTP status codes, such as 400 Bad Request or 404 Not Found, accompanied by clear descriptions and structured schemas to convey detailed error information. If the error class name doesn’t clearly indicate the error type, use the x-speakeasy-name-override extension to rename it.

title

The title property provides a human-readable title for each schema, improving the readability of the OpenAPI document.

Description

Use the description field to provide clear and concise information about the purpose, behavior, and expected usage of API elements.

openapi.yaml

openapi: 3.1.0
info:
  title: The Speakeasy Bar
  version: 1.0.0
servers:
  - url: https://speakeasy.bar
    description: The production server
  - url: https://speakeasy.bar/testing
    description: The testing server
security:
  - apiKey: []
paths:
  /drinks:
    get:
      tags:
        - drinks
      operationId: listDrinks
      summary: Get a list of drinks
      responses:
        "200":
          description: A list of drinks
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: "#/components/schemas/Drink"
              examples:
                exampleResponse:
                  summary: Example response for a list of drinks
                  value:
                    drinks:
                      - name: "Coffee"
                        price: 2.5
                      - name: "Tea"
                        price: 1.8
        "400":
          description: Bad request
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/BadRequest"
components:
  schemas:
    Drink:
      type: object
      title: Drink
      properties:
        name:
          type: string
        price:
          type: number
      description: A response containing a list of drinks.
    BadRequest:
      type: object
      title: BadRequest
      properties:
        error:
          type: string
        message:
          type: string
  securitySchemes:
    apiKey:
      type: apiKey
      name: Authorization
      in: header

Extending OpenAPI

The OpenAPI Specification lacks an exhaustive vocabulary for describing API functionality. To overcome specification gaps, add extension fields to an OpenAPI document that describe additional metadata and functionality.

Extensions typically follow a naming format of x-<vendor>-<function>, where <vendor> is the name of the vendor or tool that created the extension and <function> is the goal accomplished by the extension.

A range of Speakeasy extensions help prepare OpenAPI documents for code generation. Some of the most commonly used extensions are described below.

x-speakeasy-name-override

This extension overrides the name of a class, operation, or parameter. The most common use case is overriding operationId values in an OpenAPI document to simplify the generated SDK method names.

When operationId identifiers follow a consistent pattern, define the name override globally using a regular expression to match the operationId and replace it with a new name.

In this instance, the SDK will contain a method menu.drinks.list() rather than the longer menu.drinks.list_drinks_v2_get().

x-speakeasy-group

Sometimes, the tags in an OpenAPI document may already serve an unrelated purpose, such as autogenerating labels in documentation. In this scenario, using something other than tags to organize and group methods may be necessary.

Add the x-speakeasy-group field to any operation in the OpenAPI document to define custom namespaces and override any tags associated with the method.

In this case, the listDrinks operation is added to a menu.drinks namespace rather than a menu namespace.

x-speakeasy-usage-example

Documentation forms an important part of any SDK. This extension allows selecting which operation is featured at the top of the README.md.

Select the API operation that users frequently use. At a Speakeasy, that would likely be getting the list of drinks on offer.

x-speakeasy-example

Another useful documentation extension is x-speakeasy-example, which provides an example value to be used in the Authentication section of the SDK README.md. This example signals to users that the SDK should be instantiated with their security token.

openapi.yaml

openapi: 3.1.0
info:
  title: The Speakeasy Bar
  version: 1.0.0
servers:
  - url: https://speakeasy.bar
    description: The production server
security:
  - apiKey: []
x-speakeasy-name-override:
  - operationId: ^list_.*
    methodNameOverride: list
paths:
  /drinks:
    get:
      tags:
        - menu
      x-speakeasy-group: menu.drinks
      operationId: list_drinks_v2_get
      x-speakeasy-usage-example: true
      summary: Get a list of drinks
      responses:
        "200":
          description: A list of drinks
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: "#/components/schemas/Drink"
components:
  schemas:
    Drink:
      type: object
      properties:
        name:
          type: string
        price:
          type: number
  securitySchemes:
    apiKey:
      type: apiKey
      name: Authorization
      in: header
      x-speakeasy-example: "<YOUR_API_KEY>"