Create 0000-caching-dependencies.md

Author: Maxim Lobanov

docs/adrs/0000-caching-dependencies.md

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+# 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
+
+As a result, more users will use the cache for Node.js builds and will be happy with 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 folder path, action will try to search `package.lock.json` file in the specified folder
+  - 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>') }}`
+
+# 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).