Plug'n'Play

PnP API

Are you a library author trying to make your library compatible with the Plug'n'Play installation strategy? Do you want to use the PnP API for something awesome? If the answer to any of these questions is yes, make sure to visit the PnP API page after reading the introduction!

Unveiled in September 2018, Plug'n'Play is a new innovative installation strategy for Node. Based on prior works from other languages (for example autoload from PHP), it presents interesting characteristics that build upon the regular commonjs require workflow in an almost completely backward-compatible way.

The node_modules problem

The way installs used to work was simple: when running yarn install Yarn would generate a node_modules directory that Node was then able to consume thanks to its builtin Node Resolution Algorithm. In this context, Node didn't have to know the first thing about what a "package" was: it only reasoned in terms of files. "Does this file exist here? No? Let's look in the parent node_modules then. Does it exist here? Still no? Too bad...", and it kept going until it found the right one. This process was vastly inefficient, and for a lot of reasons:

• The node_modules directories typically contained gargantuan amounts of files. Generating them could make up for more than 70% of the time needed to run yarn install. Even having preexisting installations wouldn't save you, as package managers still had to diff the existing node_modules with what it should have been.

• Because the node_modules generation was an I/O-heavy operation, package managers didn't have a lot of leeways to optimize it much further than just doing a simple file copy - and even though we could have used hardlinks or copy-on-write when possible, we would still have needed to diff the current state of the filesystem before making a bunch of syscalls to manipulate the disk.

• Because Node had no concept of packages, it also didn't know whether a file was meant to be accessed (versus being available by the sheer virtue of hoisting). It was entirely possible that the code you wrote worked one day in development but broke later in production because you forgot to list one of your dependencies in your package.json.

• Even at runtime, the Node resolution had to make a bunch of stat and readdir calls to figure out where to load every single required file from. It was extremely wasteful, and was part of why booting Node applications took so much time.

• Finally, the very design of the node_modules folder was impractical in that it didn't allow package managers to properly dedupe packages. Even though some algorithms could be employed to optimize the tree layout (hoisting), we still ended up unable to optimize some particular patterns - causing not only the disk usage to be higher than needed, but also some packages to be instantiated multiple times in memory.

Fixing node_modules

When you think about it, Yarn already knows everything there is to know about your dependency tree - it even installs it on the disk for you. So the question becomes: why do we leave it to Node to locate the packages? Why don't we simply tell Node where to find them, and inform it that any require call to X by Y was meant to access the files from a specific set of dependencies? It's from this postulate that Plug'n'Play was created.

In this install mode (now the default starting from Yarn v2), Yarn generates a single .pnp.js file instead of the usual node_modules. Instead of containing the source code of the installed packages, the .pnp.js file contains a map linking a package name and version to a location on the disk, and another map linking a package name and version to its set of dependencies. Thanks to this efficient system, Yarn can tell Node exactly where to look for files being required - regardless of who asks for them!

This approach has various benefits:

• Since we only need to generate a single text file instead of tens of thousands, installs are now pretty much instantaneous - the main bottleneck becomes the number of dependencies in your project rather than your disk performance.

• Installs are more stable and reliable due to reduced I/O operations, which are prone to fail (especially on Windows, where writing and removing files in batch may trigger various unintended interactions with Windows Defender and similar tools).

• Perfect optimization of the dependency tree (aka perfect hoisting) and predictable package instantiations.

• The generated .pnp.js file can be committed to your repository as part of the Zero-Installs effort, removing the need to run yarn install in the first place.

• Faster application startup, because the Node resolution doesn't have to iterate over the filesystem hierarchy nearly as much as before (and soon won't have to do it at all!).

PnP loose mode

Because the hoisting heuristics aren't standardized and predictable, PnP operating under strict mode will prevent packages to require dependencies that they don't explicitly list (even if one of their other dependencies happens to depend on it). This may cause issues with some packages.

To address this problem, Yarn ships with a "loose" mode which will cause the PnP linker to work in tandem with the node-modules hoister - we will first generate the list of packages that would have been hoisted to the top-level in a typical node_modules install, then remember this list as what we call the "fallback pool".

Note that because the loose mode directly calls the node-modules hoister, it follows the exact same implementation as the true algorithm used by the node-modules linker!

At runtime, packages that require unlisted dependencies will still be allowed to access them if any version of the dependency ended up in the fallback pool (which packages exactly are allowed to rely on the fallback pool can be tweaked with pnpFallbackMode).

Note that the content of the fallback pool is undetermined - should a dependency tree contains multiple versions of the same package, there's no telling which one will be hoisted to the top-level! For this reason, a package accessing the fallback pool will still generate a warning (via the process.emitWarning API).

This mode is an in-between between the strict PnP linker and the node_modules linker.

In order to enable loose mode, make sure that the nodeLinker option is set to pnp (the default) and add the following into your local .yarnrc.yml file:

pnpMode: loose

More information about the pnpMode option.

Caveat

Because we emit warnings (instead of throwing errors) on resolution errors, applications can't catch them. This means that the common pattern of trying to require an optional peer dependency inside a try/catch block will print a warning at runtime if the dependency is missing, even though it shouldn't. This doesn't have any other runtime implications other than the fact that an incorrect warning that sometimes causes confusion is emitted, so it can be safely ignored.

This is the reason why, unlike we originally planned, PnP loose mode won't be the default starting from 2.1. It will continue being supported as an alternative, helping in the transition to the default and recommended workflow - PnP strict mode.

Caveats and work-in-progress

Over the years that led to Plug'n'Play being designed and adopted as the main install strategy, various projects came up with their own implementation of the Node Resolution Algorithm - usually to circumvent shortcomings of the require.resolve API. Such projects can be Webpack (enhanced-resolve), Babel (resolve), Jest (jest-resolve), Metro (metro-resolver), ...

Compatibility Table

The following compatibility table gives you an idea of the integration status with various tools from the community. Note that only CLI tools are listed there, as frontend libraries (such as react, vue, lodash, ...) don't reimplement the Node resolution and as such don't need any special logic to take advantage of Plug'n'Play:

Suggest an addition to this table

Native support

A lot of very common frontend tools now support Plug'n'Play natively!

Support via plugins

Incompatible

The following tools unfortunately cannot be used with pure Plug'n'Play install (even under loose mode).

Important: Even if a tool is incompatible with Plug'n'Play, you can still enable the node-modules plugin. Just follow the instructions and you'll be ready to go in a minute 🙂

This list is kept up-to-date based on the latest release we've published starting from the v2. In case you notice something off in your own project please try to upgrade Yarn and the problematic package first, then feel free to file an issue. And maybe a PR? 😊

Frequently Asked Questions

Packages are stored inside Zip archives: How can I access their files?

When using PnP, packages are stored and accessed directly inside the Zip archives from the cache. The PnP runtime (.pnp.js) automatically patches Node's fs module to add support for accessing files inside Zip archives. This way, you don't have to do anything special:

const {readFileSync} = require(`fs`);

// Looks similar to `/path/to/.yarn/cache/lodash-npm-4.17.11-1c592398b2-8b49646c65.zip/node_modules/lodash/ceil.js`
const lodashCeilPath = require.resolve(`lodash/ceil`);

console.log(readFileSync(lodashCeilPath));

Different behaviours based on workspace / not-a-workspace

Back when PnP was implemented, the compatibility wasn't as good as it is now. To help with the transition, we designed a fallback mechanism: if a package tries to access an unlisted dependency, we still allow to resolve it if the top-level package lists it as a dependency. We allow this because there's no resolution ambiguity, as there's a single top-level package in any project. Unfortunately, it may cause confusing behaviors depending on how your project is setup - when that happens, remember that PnP is always right, and that the only reason it works when not in a workspace is due to some extra laxism.

Regardless, this behaviour was just a patch, and will eventually be removed to clear up the confusion. You can prepare for that by setting pnpFallbackMode to none, which will disable the fallback mechanism altogether.