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How to use allOf in OpenAPI

Use of allOf comes from the desire for reuse. When you have a single source of truth, maintenance is easier.

This makes sense. You might want to reuse a lot of things. But allOf is not appropriate in many cases and can result in illogical schemas.

How do you know when to use allOf and when to avoid it? This article covers:

  • how to use allOf
  • how allOf is evaluated
  • valid use cases
  • common language patterns that warn that allOf use is not appropriate

Usage of allOf

Declare allOf as an array of schemas.

All of these keywords must be set to an array, where each item is a schema.

This works in YAML.

allOf:
  - title: time
    type: object
    properties:
      time:
        type: string
  - title: date
    type: object
    properties:
      date:
        type: string

And it works in JSON. The remainder of this article uses YAML for schema definitions.

{
  "allOf": [
    {
      "title": "time",
      "type": "object",
      "properties": {
        "time": {
          "type": "string"
        }
      }
    },
    {
      "title": "date",
      "type": "object",
      "properties": {
        "date": {
          "type": "string"
        }
      }
    }
  ]
}

Evaluation of allOf

A goal of JSON Schema is to be able to evaluate if JSON is valid or invalid with the defined schema.

From the definition of allOf, it is treated like a logical AND:

Must be valid against all of the subschemas

$time && $date

Based on our prior allOf declaration which requires time and date schemas, the following JSON would match the schemas:

{
  "time": "08:15:00+06:00",
  "date": "2022-01-22"
}

Does the following JSON match the allOf too?

{
  "date": "2022-01-22"
}

The "time" property is missing, and you may think that it only matches the date schema. However, neither schema, including the time schema, has any required properties. Therefore, it matches all of the schemas.

In the same way, the following JSON matches the allOf schemas too.

{
  "temperature": 25,
  "unit": "C"
}

That doesn't seem right. But it is. The schema declares what some properties types must be if they are present. It didn't declare them as required.

The following schema is invalid, because date is not a string.

{
  "temperature": 25,
  "unit": "C",
  "date": 22
}

If you declare a media type examples in your OpenAPI definition, and turn on the no-invalid-media-type-examples rule, Redocly evaluates the examples against the schema to help you evaluate them. You can also do this be evaluating real API responses with API testing. Contact us if you're interested in doing that.

Valid cases

There are times when schemas are a combination of two pre-existing schemas. If you find yourself wanting to add "with minor exceptions", then do not use the allOf keyword, no matter how tempting.

For example, let's say you have a resource for User.

title: User
required:
  - id
  - email
type: object
properties:
  id:
    type: string
  name:
    type: string
  email:
    type: string
  avatar:
    type: string
  phone:
    type: string
  dob:
    type: string
  createdAt:
    type: string
  recentLogInAt:
    type: string

And then you have other resources that use an excerpt of the User schema such as id, name, email, and avatar. (The topic of API design is different from the topic of API description, and this article doesn't cover if you should design an API this way.)

In order to reuse that excerpt of the User schema, you could rework the schema as follows.

title: UserExcerpt
required:
  - id
  - email
type: object
properties:
  id:
    type: string
  name:
    type: string
  email:
    type: string
  avatar:
    type: string

Then, you could use that in the User and any other schemas with allOf.

title: User
allOf:
  - $ref: '#/components/schemas/UserExcerpt'
  - type: object
    properties:
      phone:
        type: string
      dob:
        type: string
      createdAt:
        type: string
      recentLogInAt:
        type: string

And another schema could reuse it similarly.

title: ParkingSpot
allOf:
  - $ref: '#/components/schemas/UserExcerpt'
  - type: object
    properties:
      licenseExpiration:
        type: string
      spotId:
        type: string

Siblings to $refs

OpenAPI 3.0 has a limitation related to reuse. Schemas have some properties that are informational and do not impact the validation of JSON. Two of those properties are summary and description.

A common use case is the desire to reuse the schema by change the description due to the context.

OpenAPI 3.1 allows for siblings next to the $ref.

type: object
properties:
  transactionId:
    $ref: '#/components/schemas/ResourceId'
    description: ID of the transaction.

OpenAPI 3.0 and prior do not allow for siblings next to the $ref, but the allOf keyword could be used above it as a "workaround".

type: object
properties:
  transactionId:
    description: ID of the transaction.
    allOf:
      - $ref: '#/components/schemas/ResourceId'

This behavior confuses people, and some people think of it as a way to override the reference object properties. This is only for informational properties and not for properties that are used for evaluation.

Illogical schemas from allOf misuse

Word to watch out for that could indicate misuse:

  • override
  • extend

Type override is invalid

Overriding a description and summary is allowed. From an evaluation perspective, it works because the description is going to match any type.

The following example references a ResourceId schema and its type is a string. Therefore, the following example is illogical because transactionId cannot be an integer and a string.

type: object
properties:
  transactionId:
    description: ID of the transaction.
    type: integer
    allOf:
      - $ref: '#/components/schemas/ResourceId'

Different types are invalid

The following example demonstrates illogical schemas where types mismatch within a list of schemas provided to the allOf keyword.

allOf:
  - $ref: '#/components/schemas/Foo'
  - $ref: '#/components/schemas/Bar'

If Foo and Bar are not of the same type, then the logical AND cannot be true. For example, something cannot be a string and an object at the same time. Sometimes, this is more difficult to notice when using reference objects. However, it's clear when written the following way.

allOf:
  - type: string
  - type: object

Closed schemas and allOf are invalid

Even when the schemas are of the same type, there can still be illogical conflicts when using the allOf keyword.

The following schema demonstrates and illogical conflict.

allOf:
  - type: object
    properties:
      date:
        type: string
    additionalProperties: false
  - type: object
    properties:
      time:
        type: string
    additionalProperties: false

The additionalProperties: false means that the schema cannot have any additional properties.

The following is invalid, because it matches the first schema, but the second schema does not have date declared as a property and it declares there cannot be any additional properties.

{
  "date": "2022-01-22"
}

Summary

In Swagger 2.0 or OpenAPI 3.0, use allOf to override a description or summary of a schema. In OpenAPI 3.1, use a sibling to the $ref to override a description or summary. Do NOT override any other properties including type.

Use the allOf keyword as a logical AND. Be aware of common illogical combinations:

  • mismatched types
  • schemas where additional properties are not allowed

The allOf keyword may be used with the discriminator. Also, allOf is almost always used with at least one reference object.

Consider cases where the schema is the same with one minor exception as a possible design problem. Refactoring to use allOf may not be a good idea for those scenarios.

 
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