Depending on other APIs

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)

Registering the dependency API

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

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

Depending on the registered API

To add a dependency on another API, you must add a dependencies.yml to declare which APIs you wish to depend on.

$
fern/
>
├─ fern.config.json
>
├─ generators.yml
>
├─ dependencies.yml
>
└─ definition/
>
  ├─ api.yml
>
  ├─ imdb.yml

Your dependencies.yml has a list of all the APIs you wish to depend:

1
dependencies:
2
   "@fern/some-dependency": "0.0.1"

Next, you need 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
>
├─ generators.yml
>
├─ dependencies.yml
>
└─ definition/
>
  ├─ api.yml
>
  ├─ imdb.yml
>
  └─ my-folder
>
    └─ __package__.yml  

1
export:
2
  dependency: "@fern/some-dependency"

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