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+# Exclude `node_modules` from macOS Time Machine backups
+
+## Summary
+
+On macOS, npm should provide an opt-in flag that marks a project's `node_modules`
+directory with the extended attribute that tells Time Machine to skip it. This
+prevents `node_modules` — which can be fully reconstructed from `package-lock.json`
+— from filling up backup disks and slowing down backup runs.
+
+## Motivation
+
+`node_modules` directories are, in effect, a local cache derived entirely from
+`package-lock.json`. They can be recreated at any time with a single `npm install`
+and therefore carry no unique, irreplaceable data.
+
+On macOS, Apple's Time Machine backup tool backs up every file that it can see,
+including the often tens-of-thousands of small files inside `node_modules`. This
+has several regrettable side-effects:
+
+- **Slower backups.** Each `npm install` or `npm update` touches many
+  files, forcing Time Machine to scan and snapshot all of them on the next backup
+  run.
+- **Higher backup-disk consumption.** Accumulated snapshots of large
+  `node_modules` trees eat disk space quickly.
+
+The xattr-based "sticky exclusion" mechanism provided by macOS lets any tool
+flag a directory so that it is permanently skipped by Time Machine — even when
+the directory is moved or renamed — without requiring user interaction in System
+Preferences.
+
+## Detailed Explanation
+
+### The macOS sticky-exclusion mechanism
+
+macOS exposes a per-item backup exclusion flag through the extended attribute
+key `com.apple.metadata:com_apple_backup_excludeItem`. When this attribute is
+present on a file or directory with the value `com.apple.backupd`, Time Machine
+(and the underlying `backupd` daemon) skips that item permanently. This is the
+same mechanism used by the Xcode build toolchain for its `DerivedData`
+directories, and is equivalent to running:
+
+```sh
+tmutil addexclusion /path/to/node_modules
+```
+
+### When the attribute is applied
+
+When the opt-in flag is set, npm should apply this attribute on macOS
+(i.e. `process.platform === 'darwin'`) as soon as a `node_modules` directory is
+created on disk — that is, at directory-creation time inside
+`@npmcli/arborist`, before any package files are written into it.
+
+According to the [2021-10-13 meeting notes](https://github.com/npm/rfcs/blob/816944d6fe9c85130a5a7a92883c29c69e5777e2/meetings/2021-10-13.md?plain=1#L94), it was agreed the setting should be first introduced as opt-in.
+
+### Setting the attribute
+
+Because Node.js does not provide a built-in API for extended attributes, npm
+should shell out to the macOS `xattr` command-line utility, which ships with
+every macOS installation:
+
+```sh
+xattr -w com.apple.metadata:com_apple_backup_excludeItem \
+  com.apple.backupd \
+  /path/to/node_modules
+```
+
+Writing the value as a plain UTF-8 string is sufficient — Time Machine respects
+the attribute regardless of whether the value is a raw string or a binary plist.
+The call is made with `child_process.execFile` (non-blocking, errors suppressed)
+so that it does not affect npm's exit code or performance on non-macOS systems
+or when the `xattr` binary is unavailable.
+
+### New configuration option
+
+A new boolean flag `time-machine-exclude` is added to npm's configuration:
+
+| Key                    | Default | Description                                                                                                 |
+| ---------------------- | ------- | ----------------------------------------------------------------------------------------------------------- |
+| `time-machine-exclude` | `false` | When `true`, marks `node_modules` with the macOS Time Machine exclusion xattr after every install on macOS. |
+
+Users who want `node_modules` excluded from their backups set
+`time-machine-exclude=true` in their `.npmrc` (or via `npm config set`).
+
+## Rationale and Alternatives
+
+### Alternative 1 – Opt-out (exclude by default)
+
+npm could apply the xattr on every macOS install and provide a flag to opt back
+in to backups. This would protect the majority of users automatically.
+
+**Drawback:** Silently preventing something from being backed up without the
+user asking for it is likely to surprise some users and has some maintainers have
+[raised concerns](https://github.com/npm/rfcs/issues/471#issuecomment-943215770).
+A user who relies on their backup for disaster recovery may not realise `node_modules`
+is excluded until they attempt a restore. The opt-in approach ensures the user
+has made a deliberate choice.
+
+### Alternative 2 – Write the value as a binary plist
+
+`tmutil addexclusion` writes the value as a binary plist encoding of the string
+`com.apple.backupd`. This is technically the more correct form of the attribute.
+
+**Drawback:** Generating a binary plist in Node.js without a native library
+requires manual `Buffer` construction (or shelling out to an additional tool).
+In practice, Time Machine respects the attribute whether the value is a raw
+string or a binary plist — [many projects](https://github.com/search?q=%22com.apple.metadata%3Acom_apple_backup_excludeItem%22&type=code)
+write it as a plain string with the same effect. Plain strings are simpler
+and carry no additional risk.
+
+### Alternative 3 – Use `tmutil addexclusion`
+
+`tmutil addexclusion /path/to/node_modules` is the official command-line
+interface for adding a "sticky" backup exclusion.
+
+**Drawback:** `tmutil` is significantly slower than writing the xattr directly,
+which would noticeably increase install time. I'm not sure what extra work tmutil
+does which explain the longer execution time.
+
+### Alternative 4 – Use `NSURLIsExcludedFromBackupKey` / `kCFURLIsExcludedFromBackupKey`
+
+Apple's official URL resource property (`NSURLIsExcludedFromBackupKey` in
+Objective-C/Swift, `kCFURLIsExcludedFromBackupKey` in Core Foundation) is the
+high-level API for the same underlying mechanism. This is the approach taken by
+Cargo ([cargo#4386](https://github.com/rust-lang/cargo/pull/4386)).
+
+**Drawback:** Node.js and libuv do not expose this API. Using it would require
+a native add-on, which significantly increases implementation and maintenance
+complexity.
+
+### Alternative 5 – Advising users to use `tmutil` manually
+
+Users can already run `tmutil addexclusion ~/project/node_modules` themselves.
+Some tools (e.g. `Finder`'s "Exclude from backups" checkbox) surface this, but
+the exclusion must be re-applied after `node_modules` is deleted and reinstalled.
+The "sticky exclusion" set by npm persists even across reinstalls because it
+travels with the directory while it exists and is re-applied by npm on the next
+install.
+
+**Drawback:** Relies on the user taking manual action every time they start a
+new project; does not scale.
+
+### Chosen approach
+
+The opt-in default is the conservative choice: npm does not silently alter backup
+behaviour, but provides a first-class, either project-level or global knob for developers who
+actively want their `node_modules` excluded. Writing the xattr value as a plain
+string is the simplest correct implementation — no native add-on, no binary plist
+generation, no dependency on slow system utilities.
+
+## Implementation
+
+### Affected repositories / packages
+
+- **`@npmcli/arborist`** – The code path that creates a new `node_modules`
+  directory on disk is updated to immediately call a helper
+  (`lib/utils/time-machine-exclude.js` or similar) when
+  `process.platform === 'darwin'` and the `time-machine-exclude` config
+  option is `true`.
+
+- **`npm/cli`** – Adds the `time-machine-exclude` configuration key (type:
+  `Boolean`, default: `false`) to `lib/utils/config/definitions.js` and exposes
+  it in the docs.
+
+### Helper implementation sketch
+
+```js
+// lib/utils/time-machine-exclude.js  (inside @npmcli/arborist or npm/cli)
+const { execFile } = require("child_process");
+
+const ATTR_NAME = "com.apple.metadata:com_apple_backup_excludeItem";
+const ATTR_VALUE = "com.apple.backupd";
+
+function excludeFromTimeMachine(nodeModulesPath) {
+  if (process.platform !== "darwin") {
+    return;
+  }
+  execFile("xattr", ["-w", ATTR_NAME, ATTR_VALUE, nodeModulesPath], () => {
+    // errors intentionally ignored — absence of xattr binary or unsupported FS
+    // must not break npm
+  });
+}
+
+module.exports = { excludeFromTimeMachine };
+```
+
+### No native add-on required
+
+The implementation shells out to `/usr/bin/xattr`, which is present on every
+supported macOS version. No native Node.js add-on or additional npm dependency
+is needed.
+
+## Prior Art
+
+- **Rust / Cargo** – Added the same xattr exclusion for `target/` directories in
+  [cargo#4386](https://github.com/rust-lang/cargo/pull/4386). This is the
+  primary inspiration for this RFC.
+- **Python / Poetry** – Implemented the same exclusion for `.venv` directories
+  in [poetry#4599](https://github.com/python-poetry/poetry/pull/4599).
+- **Xcode** – Uses the same mechanism for `DerivedData`; Apple documents it in
+  their backup-exclusion guides.
+
+## Unresolved Questions and Bikeshedding
+
+- **Support broader range of OS and backup software**: The Windows/Linux backup
+  landscape is much more fragmented, with no shared approach for programmatically
+  excluding files/directories. But maybe there are some widely deployed tools which
+  should still be supported?
+- **Config flag naming**: `backup-node-modules` vs `time-machine-exclude` vs
+  `no-backup`? The name should make the purpose clear and follow npm config
+  naming conventions.
+- **Other backup tools**: The xattr is specific to Time Machine / `backupd`. It
+  does not exclude `node_modules` from iCloud Drive sync, Dropbox, or other
+  tools. Those cases may be addressed in a separate RFC.