setup-node/docs/adrs/0000-caching-dependencies.md

# 0. Caching dependencies
Date: 2021-05-21

Status: Proposed

# Context
`actions/setup-node` is the 2nd most popular action in GitHub Actions. A lot of customers use it in conjunction with [actions/cache](https://github.com/actions/cache) to speed up dependencies installation.  
See more examples on proper usage in [actions/cache documentation](https://github.com/actions/cache/blob/main/examples.md#node---npm).

# Goals & Anti-Goals
Integration of caching functionality into `actions/setup-node` action will bring the following benefits for action users:
- Decrease the entry threshold for using the cache for Node.js dependencies and simplify initial configuration
- Simplify YAML pipelines because no need additional steps to enable caching
- More users will use cache for Node.js so more customers will have fast builds!

As the first stage, we will add support for NPM dependencies caching. We can consider adding the same functionality for Yarn later.

We don't persue the goal to provide wide customization of caching in scope of `actions/setup-node` action. The purpose of this integration is covering ~90% of basic use-cases. If user needs flexible customization, we should advice them to use `actions/cache` directly.

# Decision
- Add `cache` input parameter to `actions/setup-node`. For now, input will accept the following values: 
  - `npm` - enable caching for npm dependencies
  - `''` - disable caching (default value)
  - Potentially, we will be able to extend this input to support Yarn
- Cache feature will be disabled by default to make sure that we don't break existing customers. We will consider enabling cache by default in next major release (`v3`)
- Add optional input `package-lock-path` that will allow to specify path to `package-lock.json` file path:
  - If input is not defined, action will try to search `package-lock.json` or `yarn.lock` (npm 7.x supports `yarn.lock` files) files in the repository root and throw error if no one is found
  - If input contains file path, action will use the specified file
  - if input contains wildcards (like `**/package-lock.json`), hash of multiple files will be used
- The hash of file provided in `package-lock-path` input will be used as cache key (the same approach like [actions/cache](https://github.com/actions/cache/blob/main/examples.md#node---npm) recommends)
- The following key cache will be used `${{ runner.os }}-npm-${{ hashFiles('<package-lock-path>') }}`
- Action will cache global npm cache directory (retrieved via `npm config get cache`)

# Example of real use-cases
Default use case when `package-lock.json` or `yarn.lock` are located in repository root:
```yml
steps:
- uses: actions/checkout@v2
- uses: actions/setup-node@v2
  with:
    node-version: '14'
    cache: npm
```

More flexible solution for monorepos:
```yml
steps:
- uses: actions/checkout@v2
- uses: actions/setup-node@v2
  with:
    node-version: '14'
    cache: npm
    package-lock-path: service1/yarn.lock
```

# Release process

As soon as functionality is implemented, we will release minor update of action. No need to bump major version since there are no breaking changes for existing users.
After that, we will update [starter-workflows](https://github.com/actions/starter-workflows/blob/main/ci/node.js.yml) and [GitHub Action documentation](https://docs.github.com/en/actions/guides/building-and-testing-nodejs#example-caching-dependencies).