Limit dependencies shared between workspaces

accepted/0000-workspace-dependency-sharing.md

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+# Limit dependencies shared between workspaces
+
+## Summary
+
+For all workspaces within a project, the default behavior of dependency
+resolution will be as follows:
+
+- Dependencies on other workspaces within the project will _always_ be
+  resolved to the corresponding sibling workspace.
+- Peer dependencies of multiple workspaces within the project will always
+  be shared with one another, raising an `ERESOLVE` if this is impossible.
+- All other dependencies will not be hoisted above the workspace location.
+  However, if a satisfying version of a dependency is listed as a root
+  project dependency, then that will be used.
+
+Thus, all dependencies of workspace projects which are not
+`peerDependencies` or dependencies on sibling workspaces within the
+project, will never be hoisted above the workspace level _unless_ they are
+explicitly listed as a root project dependency.
+
+The ability to load transitive dependencies without an explicit dependency
+on them is out of scope for this proposal, and covered by
+[`isolated-mode`](https://github.com/npm/rfcs/blob/main/accepted/0042-isolated-mode.md).
+
+### Example
+
+A project contains the following workspace definition:
+
+```json
+{
+  "workspace": ["packages/*"]
+}
+```
+
+Workspaces exist at `packages/foo`, `packages/bar`, and `packages/baz`.
+
+`packages/foo/package.json` contains:
+
+```json
+{
+  "name": "foo",
+  "version": "1.0.0",
+  "peerDependencies": {
+    "react": "16"
+  },
+  "dependencies": {
+    "bar": "1.x"
+  },
+  "devDependencies": {
+    "webpack": "4",
+    "webpack-cli": "4.9"
+  }
+}
+```
+
+`packages/bar/package.json` contains:
+
+```json
+{
+  "name": "bar",
+  "version": "1.0.0",
+  "peerDependencies": {
+    "react": "16"
+  },
+  "dependencies": {
+    "baz": "1.x"
+  },
+  "devDependencies": {
+    "webpack": "5",
+    "webpack-cli": "4.9"
+  }
+}
+```
+
+`packages/baz/package.json` contains:
+
+```json
+{
+  "name": "baz",
+  "version": "2.0.0",
+  "dependencies": {
+    "foo": "1.x"
+  }
+}
+```
+
+In this example:
+
+* The `bar` loaded by `foo` will be the one in `packages/bar`.
+* `webpack-cli` is duplicated in both `foo` and `bar` packages, instead of
+  only `webpack` being duplicated.
+* The `baz` loaded by `bar` will be the one in `packages/bar`, _even though
+  this is an invalid version_, because they are sibling workspaces within a
+  project.  (This will be flagged as an error in `npm ls`.)
+* The `react` loaded by both `foo` and `bar` will be the same instance.
+* If `baz` adds a peerDep on `react@17`, then this will cause an `ERESOLVE`
+  error, because the peer dependency cannot be resolved.  (This can be
+  worked around by `baz` also listing `react` in `devDependencies`.)
+
+## Motivation
+
+A common use case for workspaces is to develop multiple related packages as
+a single overarching project, so that they can be more easily kept in sync
+and tested with one another.
+
+The current implementation is somewhat naive.  Workspaces are treated
+roughly the same as `file:` dependencies from the root project, with some
+additional semantics applied.  Any shared dependencies are deduplicated up
+to the top-most `node_modules` folder if possible, as they would be if
+they were simple `file:` dependencies.
+
+This RFC proposes more specific guidelines for which types of dependencies
+are shared in various scenarios.
+
+There are use cases where the naive workspaces implementation, while
+simple, does not provide the ideal user experience.
+
+Implementation details are out of scope for this RFC.  This proposal
+focuses exclusively on the user experience and expectations for which
+packages are shared between workspaces in a project.
+
+### Shadow Dependencies
+
+A naive implementation of workspaces makes it easy to come to rely on
+undeclared dependencies.  For example, consider a project with this
+`package.json` file:
+
+```json
+{
+  "workspaces": [
+    "packages/*"
+  ]
+}
+```
+
+and this folder structure:
+
+```
+root
++-- packages
+    +-- a
+    |   +-- package.json
+    |   +-- index.js
+    +-- b
+        +-- package.json
+        +-- index.js
+```
+
+If `a` declares a dependency on `foo`, and `b` does _not_ declare that
+dependency, then the code in `b` can call `require('foo')` and it will work
+without any errors in development.  But when `b` is published, and
+subsequently installed as a standalone project, it fails to find the `foo`
+dependency.
+
+### Unexpectedly divergent Intra-Project Dependencies
+
+A common goal of a workspaces project is to ensure that all workspaces in
+the project will work together when they depend on one another.
+
+Consider a project containing many workspaces, including a shared library
+`foo` which many of the other workspaces depend on.
+
+At some point, the developer bumps the version of `foo` from 1 to 2, with
+the intent of testing the new changes within all the other workspaces.
+However, as this is no longer a satisfying dependency for those other
+workspaces, the naive workspaces implementation will result in a package
+tree like this:
+
+```
+root
++-- node_modules
+|   +-- foo (link: ../packages/foo)
+|   +-- bar (link: ../packages/bar) (forgot to update dependency!)
+|   +-- ... all other workspaces
++-- packages
+    +-- foo (v2)
+    +-- bar
+    |   +-- node_modules
+    |       +-- foo (v1)
+    +-- other workspaces
+        +-- node_modules (empty, getting foo@2 from top level)
+```
+
+If the user does not note the error, and publishes all of their packages
+together, they may end up releasing software that is incompatible with
+itself, and could have been detected during development.
+
+### Unable to support divergent peer dependencies
+
+It may be valuable in some cases to define multiple packages that each have
+a peer dependency on a _different_ version of some common module.
+
+For example, imagine a plugin that is designed to work alongside React, and
+has adapters to implement its functionality in different versions of React.
+
+```json
+{
+  "name": "my-react-plugin",
+  "version": "1.2.3",
+  "description": "Works with React v15 through v17!",
+  "optionalDependencies": {
+    "my-react-plugin-15": "*",
+    "my-react-plugin-16": "*",
+    "my-react-plugin-17": "*"
+  }
+}
+```
+
+Then, each adapter has a `package.json` like this:
+
+```json
+{
+  "name": "my-react-plugin-15",
+  "version": "1.2.3",
+  "peerDependencies": {
+    "react": "15"
+  }
+}
+```
+
+When a user runs `npm install my-react-plugin`, it'll try to install all
+three optional dependencies, but 2 of them will always fail by virtue of
+the conflicting `react` peer dependency, and they will get one that works
+with their system.
+
+Unfortunately, doing this in a naive workspace implementation, where all
+`peerDependencies` are deduplicated to the top level means that the project
+will necessarily always fail to install with an `ERESOLVE` error, because
+the workspace projects cannot all be installed at the same time in the same
+place.
+
+Making this more complicated, if the intent of using workspaces is to
+ensure that all the workspaces within the project _may_ be used together,
+then isolating all conflicting `peerDependencies` would silently
+cause problems!
+
+The workaround for this case is for the packages with divergent
+peerDependencies to list their peerDependencies in `devDependencies` as
+well, so that they may be nested.
+
+## Detailed Explanation
+
+The following expectations should not be silently violated without explicit
+user intent.
+
+1. A workspace package that has a dependency on another workspace package
+   within the same project, should have its dependency met by the
+   package within the workspace, and not by fetching the dependency from
+   the registry.  If the workspace package does not satisfy the dependency
+   specification from another workspace package, then a warning will be
+   displayed.
+2. A workspace that has a peer dependency should share the same instance of
+   that peer dependency with all other workspace projects, unless
+   marked as a non-peer (ie, dev) dependency as well.
+3. A workspace should not be able to load any package that it does not have
+   an explicit direct dependency or implicit transitive dependency on, with
+   the exception of dependencies declared at a level higher in the
+   directory tree.
+
+## Rationale and Alternatives
+
+In the initial naive workspaces implementation in npm, all dependencies are
+shared if possible, because workspaces are treated more or less the same as
+local `file:` link dependencies.
+
+Thus:
+
+1. Workspaces that depend on another workspace package will resolve to the
+   other workspace within the project if and only if the dependency is
+   satisfied, without warning that their dependency is being met by a
+   version of a workspace package outside the project.
+2. Workspaces with conflicting peer dependencies cannot coexist within a
+   project.  (But they do share peer dependencies by default.)
+3. Workspaces share many (or all) of their dependencies, since any
+   dependency that can be deduplicated to the root project level will be,
+   and can load any dependency installed in the root project.
+
+The `nohoist` option has been proposed, as implemented in
+[yarn](https://classic.yarnpkg.com/blog/2018/02/15/nohoist/) and
+[lerna](https://github.com/lerna/lerna/blob/main/doc/hoist.md) workspaces.
+Downsides of this approach are discussed in **Prior Art** below, but
+if possible, it would be good for any implementation to _also_ support
+`nohoist`, even if it is not the preferred/canonical way to express the
+user's intent.
+
+## Implementation
+
+Implementation of dependency tree layout is out of scope for this RFC.
+There are numerous ways to approach the problem technically, which may be
+the subject of future RFCs or iterated on separately from the definition of
+the intent.
+
+In its simplest form,
+
+* Any edge from a workspace, with a name matching any other workspace
+  within the project, will be satisfied by a link to that workspace (either
+  within the workspace itself, or by walking up to the top level).
+* For non-peer dependencies, the `PlaceDep` class in `@npmcli/arborist`
+  will stop at any target location where `target.isWorkspace` is true,
+  rather than continuing its search to a higher level.
+
+## Prior Art
+
+The `nohoist` option has been proposed, and is implemented in
+[yarn](https://classic.yarnpkg.com/blog/2018/02/15/nohoist/) and
+[lerna](https://github.com/lerna/lerna/blob/main/doc/hoist.md) workspaces.
+
+For example:
+
+```json
+{
+  "name": "workspace-example",
+  "version": "1.0.0",
+  "workspaces": {
+    "packages": [
+      "packages/*"
+    ],
+    "nohoist": [
+      "**/react-native/**"
+    ]
+  }
+}
+```
+
+However:
+
+1. This declaration uses glob patterns to refer to packages within the
+   dependency graph, which is a UX challenge.  (See discussion regarding
+   the [`overrides` feature](https://github.com/npm/rfcs/pull/129).)
+2. The wording of "nohoist" is overly specific to a particular
+   implementation.  That is, the declaration describes the "what" of
+   package folder tree implementation, instead of the "why" of dependency
+   graph resolution semantics.
+3. Specifying a list of things that are _not_ to be hoisted introduces a
+   "double-negative" UX problem, which is best avoided.
+
+## Unresolved Questions and Bikeshedding
+
+* Is it reasonable that a workspace with a dependency on a sibling
+  workspace should _always_ resolve to its sibling?  Do we need an escape
+  hatch for this, like we have for duplicating peerDeps in devDeps?
+* This will lead to more duplication in large workspace projects.  Are we
+  comfortable saying "use isolated-mode if you care about that"?