Component Section

Often, multiple API operations have some common parameters or return the same response structure. To avoid code duplication, you can place the common definitions in the global components section and reference them using $ref. In the example below, duplicate definitions of a User object are replaced with a single component and references to that component. Before:

paths:
  /users/{userId}:
    get:
      summary: Get a user by ID
      parameters:
        ...
      responses:
        '200':
          description: A single user.
          content:
            application/json:
              schema:
                type: object
                properties:
                  id:
                    type: integer
                  name:
                    type: string
  /users:
    get:
      summary: Get all users
      responses:
        '200':
          description: A list of users.
          content:
            application/json:
              schema:
                type: array
                items:
                  type: object
                  properties:
                    id:
                      type: integer
                    name:
                      type: string

After:

paths:
  /users/{userId}:
    get:
      summary: Get a user by ID
      parameters:
        ...
      responses:
        '200':
          description: A single user.
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'
  /users:
    get:
      summary: Get all users
      responses:
        '200':
          description: A list of users.
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/User'
components:
  schemas:
    User:
      type: object
      properties:
        id:
          type: integer
        name:
          type: string

Components Structure

components serve as a container for various reusable definitions – schemas (data models), parameters, responses, examples, and others. The definitions in components have no direct effect on the API unless you explicitly reference them from somewhere outside the components. That is, components are not parameters and responses that apply to all operations; they are just pieces of information to be referenced elsewhere. Under components, the definitions are grouped by type – schemas, parameters and so on. The following example lists the available subsections. All subsections are optional.

components:
  # Reusable schemas (data models)
  schemas:
    ...
  # Reusable path, query, header and cookie parameters
  parameters:
    ...
  # Security scheme definitions (see Authentication)
  securitySchemes:
    ...
  # Reusable request bodies
  requestBodies:
    ...
  # Reusable responses, such as 401 Unauthorized or 400 Bad Request
  responses:
    ...
  # Reusable response headers
  headers:
    ...
  # Reusable examples
  examples:
    ...
  # Reusable links
  links:
    ...
  # Reusable callbacks
  callbacks:
    ...

Each subsection contains one or more named components (definitions):

components:
  schemas:
    User:
      type: object
      ...
    Pet:
      type: object
      ...

The component names can consist of the following characters only:

A..Z a..z 0..9 . _ -

Examples of valid names:

User
New_User
org.example.User
401-Unauthorized

The component names are used to reference the components via $ref from other parts of the API specification:

$ref: '#/components/<type>/<name>'

For example:

$ref: '#/components/schemas/User'

An exception are definitions in securitySchemes which are referenced directly by name (see Authentication).

Components Example

Below is an example of components that contains reusable data schemas, parameters and responses. Other component types (links, examples, and others) are defined similarly.

components:
  #-------------------------------
  # Reusable schemas (data models)
  #-------------------------------
  schemas:
    User:             # Can be referenced as '#/components/schemas/User'
      type: object
      properties:
        id:
          type: integer
          format: int64
        name:
          type: string
    Error:            # Can be referenced as '#/components/schemas/Error'
      type: object
      properties:
        code:
          type: integer
        message:
          type: string
  #-------------------------------
  # Reusable operation parameters
  #-------------------------------
  parameters:
    offsetParam:      # Can be referenced via '#/components/parameters/offsetParam'
      name: offset
      in: query
      description: Number of items to skip before returning the results.
      required: false
      schema:
        type: integer
        format: int32
        minimum: 0
        default: 0
    limitParam:       # Can be referenced as '#/components/parameters/limitParam'
      name: limit
      in: query
      description: Maximum number of items to return.
      required: false
      schema:
        type: integer
        format: int32
        minimum: 1
        maximum: 100
        default: 20
  #-------------------------------
  # Reusable responses
  #-------------------------------
  responses:
    404NotFound:       # Can be referenced as '#/components/responses/404NotFound'
      description: The specified resource was not found.
    ImageResponse:     # Can be referenced as '#/components/responses/ImageResponse'
      description: An image.
      content:
        image/*:
          schema:
            type: string
            format: binary
    GenericError:      # Can be referenced as '#/components/responses/GenericError'
      description: An error occurred.
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/Error'

Externally Defined Components

Individual definitions in components can be specified either inline (as in the previous example) or using a $ref reference to an external definition:

components:
  schemas:
    Pet:
      $ref: '../models/pet.yaml'
      # Can now use use '#/components/schemas/Pet' instead
    User:
      $ref: 'https://api.example.com/v2/openapi.yaml#/components/schemas/User'
      # Can now use '#/components/schemas/User' instead
  responses:
    GenericError:
      $ref: '../template-api.yaml#/components/responses/GenericError'
      # Can now use '#/components/responses/GenericError' instead

Differences From OpenAPI 2.0

OpenAPI 2.0 had separate sections for reusable components – definitions, parameters, responses and securityDefinitions. In OpenAPI 3.0, they all were moved inside components. Also, definitions were renamed to schemas and securityDefinitions were renamed to securitySchemes (note the different spelling: schemAs vs securitySchemEs). The references are changed accordingly to reflect the new structure:

OpenAPI 2.0                    OpenAPI 3.0
'#/definitions/User'         → '#/components/schemas/User'
'#/parameters/offsetParam'   → '#/components/parameters/offsetParam'
'#/responses/ErrorResponse'  → '#/components/responses/ErrorResponse'

References

Components Object