Fern allows you to import other APIs into your API.

This is often useful if:

  • you want to reuse another API’s types in your API
  • you want to combine multiple microservices’ APIs into one SDK (similar to the AWS SDK)

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Registering the dependency API

The first step is to register the API you want to depend on. To do this, use the register compiler command:

$ fern register
[some-dependency]: Uploading definition...
[some-dependency]: Registered @fern/some-dependency:0.0.1

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Depending on the registered API

To add a dependency on another API, you simply create a folder in your Fern Definition to “house” the dependency. Inside the folder, create a special file __package__.yml which specifies the dependency and version you want to add.

fern/
├─ fern.config.json
└─ api/ # your API
  ├─ generators.yml
  └─ definition/
    ├─ api.yml
    ├─ imdb.yml
    └─ my-folder
      └─ __package__.yml
__package__.yml
export:
  name: @fern/some-dependency
  version: 0.0.1

When you compile the API with fern generate, the __package__.yml file will effectively be replaced with the API you’re depending on.