Fern Definition

api.yml reference

A Fern directory has a special file called api.yml, which includes all the API-wide configuration.

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fern/
├─ fern.config.json
├─ generators.yml
└─ definition/
  ├─ api.yml # <---
  └─ imdb.yml

API name

This name is used to uniquely identify your API in your organization. If you just have one API, then api is a sufficient name.

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name: api

API description

You can define a top level API description. This description will come through in the OpenAPI spec and Postman collection.

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name: api
docs: |  # <---
  ## Header
  This API provides access to...

Authentication

You can specify the authentication scheme used by your API.

Out of the box, Fern supports Bearer and Basic authentication schemes.

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name: api
auth: bearer
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name: api
auth: basic

You can also create your own authentication schemes.

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name: api
auth: MyAuthScheme
auth-schemes:
  MyAuthScheme:
    header: X-API-Key
    type: string

Global headers

You can specify headers that are meant to be included on every request:

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name: api
headers:
  X-App-Id: string

Global path parameters

You can specify path parameters that are meant to be included on every request:

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name: api
path-parameters:
  userId: string

Global query parameters

You cannot yet specify query parameters that are meant to be included on every request. If you'd like to see this feature, please upvote this issue.

Environments

You can specify the environments where your backend is deployed. These are useful in most generated outputs, like SDKs and in Postman Collections.

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name: api
environments:
  Production: https://www.yoursite.com
  Staging:
    docs: This staging environment is helpful for testing!
    url: https://www.staging.yoursite.com

You can also provide a default environment:

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name: api
environments:
  Production: https://www.yoursite.com
    Staging:
      docs: This staging environment is helpful for testing!
      url: https://www.staging.yoursite.com
default-environment: Production

Multiple URLs per environment

You can specify multiple URLs per environment. This is helpful if you have a microservice architecture, and you want a single SDK to interact with multiple servers.

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environments:
  Production:
    urls:
      Auth: https://auth.yoursite.com
      Plants: https://plants.yoursite.com
  Staging:
    urls:
      Auth: https://auth.staging.yoursite.com
      Plants: https://plants.staging.yoursite.com

If you choose to use this feature, you must specify a url for each service you define:

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service:
  url: Auth
  base-path: /auth
  ...

Error discrimination strategy

In order to generate SDKs idiomatically, Fern needs to know how to differentiate between different errors when parsing an endpoint response.

Discriminate by status code

You can specify Fern to discriminate by status code. This means on each endpoint, every error that's listed must have a different HTTP status code.

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name: api
error-discrimination:
  strategy: status-code

Discriminate by error name

You can specify Fern to discriminate by error name. If you select this strategy, then Fern will assume that every error response has an extra property denoting the error name.

If you use Fern to generate server-side code, then this option provides the most flexibility. Otherwise, you'll probably want to use the status code discrimination strategy.

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name: api
error-discrimination:
  strategy: property
  property-name: errorName

Global errors

You can import and list errors that will be thrown by every endpoint.

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imports:
  commons: commons.yml

errors:
  - commons.NotFoundError
  - commons.BadRequestError