How does OpenAPI handle polymorphism? #

Polymorphism is a ubiquitous concept in software engineering and design, allowing for interfaces and classes to take on multiple forms. This is crucial for designing flexible and maintainable systems. OpenAPI, a powerful specification for building APIs, includes robust support for polymorphism to enable these qualities in API design. This article delves into how OpenAPI handles polymorphism, discussing key concepts, examples, and best practices.

Understanding Polymorphism #

Before exploring how OpenAPI handles polymorphism, it’s essential to understand the concept itself. Polymorphism allows one interface to be used for a general class of actions, and the specific action is determined by the exact nature of the situation.

In the context of APIs, polymorphism can enable an API endpoint to return different types of objects based on the input or other conditions. For example, an animal endpoint might return data for a dog, cat, or bird object, each with different attributes.

Polymorphism in OpenAPI #

OpenAPI supports polymorphism primarily through the use of the oneOf, anyOf, allOf, and discriminator keywords. These keywords allow API designers to specify schemas that can represent multiple types. Let’s break down each of these keywords:

oneOf #

The oneOf keyword allows you to specify that the payload must be valid against exactly one of the specified schemas. This is useful when you expect just one type out of several possible types.

Example #

components:
  schemas:
    Pet:
      type: object
      properties:
        id:
          type: integer
        name:
          type: string
        petType:
          type: string
      required:
        - id
        - name
        - petType

    Dog:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          properties:
            bark:
              type: boolean
          required:
            - bark

    Cat:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          properties:
            hunts:
              type: boolean
          required:
            - hunts

    Bird:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          properties:
            wingspan:
              type: integer
          required:
            - wingspan

    NewPet:
      oneOf:
        - $ref: '#/components/schemas/Dog'
        - $ref: '#/components/schemas/Cat'
        - $ref: '#/components/schemas/Bird'

In this example, when dealing with a NewPet, it must conform to exactly one of Dog, Cat, or Bird.

anyOf #

The anyOf keyword allows you to specify that the payload must be valid against any (one or more) of the specified schemas. This is a bit less restrictive than oneOf.

Example #

components:
  schemas:
    MixedPet:
      anyOf:
        - $ref: '#/components/schemas/Dog'
        - $ref: '#/components/schemas/Cat'
        - $ref: '#/components/schemas/Bird'

In this example, a MixedPet can conform to any (potentially more than one) of the Dog, Cat, or Bird schemas.

allOf #

The allOf keyword is used to indicate that the payload must be valid against all of the specified schemas. This is useful for composing schemas from multiple parts.

Example #

components:
  schemas:
    BasePet:
      type: object
      properties:
        id:
          type: integer
        name:
          type: string
      required:
        - id
        - name

    Dog:
      allOf:
        - $ref: '#/components/schemas/BasePet'
        - type: object
          properties:
            bark:
              type: boolean
          required:
            - bark

In this example, a Dog must conform to both BasePet and the added properties specific to Dog.

discriminator #

The discriminator keyword helps in determining which schema should be used when multiple schemas are possible. This is particularly useful when implementing polymorphism with oneOf.

Example #

components:
  schemas:
    Pet:
      type: object
      required:
        - petType
      properties:
        petType:
          type: string
      discriminator:
        propertyName: petType

    Dog:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          properties:
            bark:
              type: boolean
          required:
            - bark

    Cat:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          properties:
            hunts:
              type: boolean
          required:
            - hunts

    Bird:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - type: object
          properties:
            wingspan:
              type: integer
          required:
            - wingspan

    NewPet:
      oneOf:
        - $ref: '#/components/schemas/Dog'
        - $ref: '#/components/schemas/Cat'
        - $ref: '#/components/schemas/Bird'
      discriminator:
        propertyName: petType

In this example, the discriminator field petType helps to determine which schema to use when validating the NewPet.

Best Practices for Polymorphism in OpenAPI #

Use discriminator with oneOf for Clarity #

When using oneOf, specifying a discriminator can greatly enhance the readability and usability of your API. It makes it clear which subtype is expected and improves client generation.

Be Cautious with anyOf #

Using anyOf can make your API less predictable, as it allows multiple types to be valid. Ensure this is the desired behavior before opting for anyOf.

Validate Extensively #

Extensive validation and testing are crucial when implementing polymorphism in your API. Automated tests can ensure that your API handles all possible types correctly.

Documentation #

Document the behavior of polymorphic endpoints clearly. Tools like Swagger UI can automatically generate useful documentation for your OpenAPI specification.

Conclusion #

Polymorphism is a robust feature of OpenAPI, providing flexibility and robustness to API design. By leveraging oneOf, anyOf, allOf, and discriminator, API designers can build flexible systems that accommodate various data types while maintaining clear and concise documentation.

For more detailed information, the official OpenAPI Documentation is an excellent resource, as well as their tools like Swagger Editor for testing and validating your OpenAPI specifications.

By adhering to best practices and understanding these key concepts, you can effectively utilize polymorphism in your API design, creating more versatile and maintainable systems.