Cómo se resuelven los peers

Una de las mejores características de pnpm es que en un proyecto, una versión específica de un paquete siempre tendrá un conjunto de dependencias. Hay una excepción a la regla, los paquetes con peer dependencies.

Las Peer dependencies son resueltas desde dependencias instaladas más arriba en el árbol de dependencias, ya que comparten el mismo padre. Esto significa que si [email protected] tiene dos pares (bar@^1 y baz@^1) entonces podria tener difentes juegos de dependencias en el mismo proyecto.

- foo-parent-1
  - [email protected]
  - [email protected]
  - [email protected]
- foo-parent-2
  - [email protected]
  - [email protected]
  - [email protected]

In the example above, [email protected] is installed for foo-parent-1 and foo-parent-2. Ambos paquetes también tienen bar y baz , pero dependen de versiones diferentes de baz. As a result, [email protected] has two different sets of dependencies: one with [email protected] and the other one with [email protected]. To support these use cases, pnpm has to hard link [email protected] as many times as there are different dependency sets.

Normally, if a package does not have peer dependencies, it is hard linked to a node_modules folder next to symlinks of its dependencies, like so:

node_modules
└── .pnpm
    ├── [email protected]
    │   └── node_modules
    │       ├── foo
    │       ├── qux   -> ../../[email protected]/node_modules/qux
    │       └── plugh -> ../../[email protected]/node_modules/plugh
    ├── [email protected]
    ├── [email protected]

However, if foo has peer dependencies, there may be multiple sets of dependencies for it, so we create different sets for different peer dependency resolutions:

node_modules
└── .pnpm
    ├── [email protected][email protected][email protected]
    │   └── node_modules
    │       ├── foo
    │       ├── bar   -> ../../[email protected]/node_modules/bar
    │       ├── baz   -> ../../[email protected]/node_modules/baz
    │       ├── qux   -> ../../[email protected]/node_modules/qux
    │       └── plugh -> ../../[email protected]/node_modules/plugh
    ├── [email protected][email protected][email protected]
    │   └── node_modules
    │       ├── foo
    │       ├── bar   -> ../../[email protected]/node_modules/bar
    │       ├── baz   -> ../../[email protected]/node_modules/baz
    │       ├── qux   -> ../../[email protected]/node_modules/qux
    │       └── plugh -> ../../[email protected]/node_modules/plugh
    ├── [email protected]
    ├── [email protected]
    ├── [email protected]
    ├── [email protected]
    ├── [email protected]

We create symlinks either to the foo that is inside [email protected][email protected][email protected] or to the one in [email protected][email protected][email protected]. As a consequence, the Node.js module resolver will find the correct peers.

If a package has no peer dependencies but has dependencies with peers that are resolved higher in the graph, then that transitive package can appear in the project with different sets of dependencies. For instance, there's package [email protected] with a single dependency [email protected]. [email protected] has a peer dependency c@^1. [email protected] will never resolve the peers of [email protected], so it becomes dependent from the peers of [email protected] as well.

Here's how that structure will look in node_modules. In this example, [email protected] will need to appear twice in the project's node_modules - resolved once with [email protected] and again with [email protected].

node_modules
└── .pnpm
    ├── [email protected][email protected]
    │   └── node_modules
    │       ├── a
    │       └── b -> ../../[email protected][email protected]/node_modules/b
    ├── [email protected][email protected]
    │   └── node_modules
    │       ├── a
    │       └── b -> ../../[email protected][email protected]/node_modules/b
    ├── [email protected][email protected]
    │   └── node_modules
    │       ├── b
    │       └── c -> ../../[email protected]/node_modules/c
    ├── [email protected][email protected]
    │   └── node_modules
    │       ├── b
    │       └── c -> ../../[email protected]/node_modules/c
    ├── [email protected]
    ├── [email protected]