Project structure

Configuring fern starts with the fern folder, which contains your API definitions, SDK generators, and your CLI version.

Fern recommends a multi-repository structure with your fern folder in a source repository (containing your API definitions and generation configuration) and each generated SDK in its own separate repository.

Directory structure

When you run fern init (for the Fern Definition) or fern init --spec-type path/to/spec (for other specs), your fern folder is initialized with the following files:

$
fern/
>
  ├─ fern.config.json # Root-level config for entire Fern project
>
  ├─ generators.yml # Defines SDKs and docs to generate
>
  └─ spec-folder/ # definition, openapi, asyncapi, etc.
>
    └─ spec-file.yml # API definition file

fern.config.json file

Every fern folder has a single fern.config.json file. This file stores the organization and the version of the Fern CLI that you are using.

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{
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    "organization": "your-organization",
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    "version": "0.31.2"
4
}

Every time you run a fern CLI command, the CLI downloads itself at the correct version to ensure determinism.

generators.yml

The generators.yml file includes information about where your API definition file is located. It also configures the SDKs you’re generating for your API.

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api:
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  specs:
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    - openapi: spec-file.yml 

API definition file

For Fern Definition, your API configuration is split across two files: api.yml for API-wide configuration and separate .yml files for your actual endpoint and type definitions. See What is a Fern Definition? for more information.

For the other specification formats (OpenAPI, AsyncAPI, OpenRPC, and gRPC), you’ll have a single self-contained specification file.

Multiple API definitions

The fern folder can house multiple API definitions. When you have multiple APIs, nest your definition files within a top-level apis folder.

Each API must have a separate generators.yml file that specifies the location of the API definition and configures SDK generation.

Below is an example of what a fern folder containing multiple APIs might look like. Your exact structure might vary depending on your organization’s conventions and needs – the important part is that each API has its own generators.yml file.

$
fern/
>
  ├─ fern.config.json
>
  ├─ generators.yml # Optional: top-level configuration for all APIs
>
  └─ apis/
>
    └─ first-api/ # First API
>
        └─ spec-folder/ # definition, openapi, asyncapi, etc.
>
          ├─ generators.yml # Required: points to API spec and configures SDKs
>
          └─ spec-file.yml # API Definition file
>
    └─ second-api/ # Second API
>
        └─ spec-folder/ # definition, openapi, asyncapi, etc.
>
          ├─ generators.yml # Required: points to API spec and configures SDKs
>
          └─ spec-file.yml # API Definition file

Docs configuration

APIs in docs are specified by setting the api-name property.