Start Date: 2017-05-21
RFC PR: (leave this empty)
Yarn Issue: (leave this empty)

Summary

Allow to select a nested dependency version via the resolutions field of the package.json file.

Motivation

The motivation was initially discussed in yarnpkg/yarn#2763.

Basically, the problem with the current behaviour of yarn is that it is not possible to force the use of a particular version for a nested dependency.

Example

For example, given the following content in the package.json:

  "devDependencies": {
    "@angular/cli": "1.0.3",
    "typescript": "2.3.2"
  }

The yarn.lock file will contain:

"typescript@>=2.0.0 <2.3.0":
  version "2.2.2"
  resolved "https://registry.yarnpkg.com/typescript/-/typescript-2.2.2.tgz#606022508479b55ffa368b58fee963a03dfd7b0c"

typescript@2.3.2:
  version "2.3.2"
  resolved "https://registry.yarnpkg.com/typescript/-/typescript-2.3.2.tgz#f0f045e196f69a72f06b25fd3bd39d01c3ce9984"

Also, there will be:

typescript@2.3.2 in node_modules/typescript
typescript@2.2.2 in node_modules/@angular/cli/node_modules.

Problem

In this context, it is impossible to force the use of typescript@2.3.2 for the whole project (except by flattening the whole project, which we don't want).

It makes sense for typescript as the user intent is clearly to use typescript 2.3.2 for compiling all its project, and with the current behaviour, the angular CLI (responsible of compiling .ts files) will simply use the 2.2.2 version from its node_modules.

Similarly, even using such a content for package.json:

  "devDependencies": {
    "@angular/cli": "1.0.3"
  }

The need could arise for forcing the use of typescript@2.3.2 (or typescript@2.1.0 for that matter).

Why?

In these example, the need does not seem very important (the user could maybe use typescript@2.2.2 or ask the @angular/cli dev team to relax its constraints on typescript), but there could be cases where a nested dependency introduces a bug and the project developer would want to set a specific version for it (see for example this comment).

Related scenario (out of scope of this document)

An extension of this motivation is also the potential need for mapping nested dependencies to others. For example a project developer could want to map typescript@>=2.0.0 <2.3.0 to my-typescript-fork@2.0.0.

See alternatives solutions below also.

Detailed design

The proposed solution is to make the resolutions field of the package.json file to be considered all the time and on a per-package basis (instead of only when the --flat parameter is used).

When a nested dependency is being resolved by yarn, if the resolutions field contains a specification for this package, then it will be used instead.

Special attention to the specific nature of package management in the npm ecosystem is given in this RFC: indeed, it is not unusual to have the same package being present as a nested dependency of multiple packages with different versions. It is thus possible within the resolutions field to express versions either for the whole dependency tree or only for a subset of it, using a syntax relying on glob patterns.

Most of the examples are given with exact dependencies, but note that using a non-exact specification in the resolutions field should be accepted and resolved by yarn like it usually does. This subject is discussed below also.

Any potentially counter-intuitive situation will result in a warning being issued. This subject is discussed at the end of this section.

Examples

We have the following packages and their dependencies:

package-a@1.0.0
 |_ package-d1@1.0.0
     |_ package-d2@1.0.0

package-a@2.0.0
 |_ package-d1@2.0.0
     |_ package-d2@1.0.0

package-b@1.0.0
 |_ package-d1@2.0.0
     |_ package-d2@1.0.0

package-c@1.0.0
 |_ package-a@2.0.0
     |_ package-d1@2.0.0
         |_ package-d2@1.0.0

With:

  "dependencies": {
    "package-a": "1.0.0",
    "package-b": "1.0.0"
  },
  "resolutions": {
    "**/package-d1": "2.0.0"
  }

yarn will use package-d1@2.0.0 for every nested dependency to package-d1 and will behave as expected with respect to the node_modules folder by not duplicating the package-d1 installation.

With:

  "dependencies": {
    "package-a": "1.0.0",
    "package-b": "1.0.0"
  },
  "resolutions": {
    "package-a/package-d1": "3.0.0"
  }

yarn will use package-d1@3.0.0 only for package-a and package-b will still have package-d1@2.0.0 in its own node_modules.

With:

  "dependencies": {
    "package-a": "1.0.0",
    "package-c": "1.0.0"
  },
  "resolutions": {
    "**/package-a": "3.0.0"
  }

package-a will still be resolved to 1.0.0, but package-c will have package-a@3.0.0 in its own node_modules.

With:

  "dependencies": {
    "package-a": "1.0.0",
    "package-c": "1.0.0"
  },
  "resolutions": {
    "package-a": "3.0.0"
  }

yarn will do nothing (see below why).

With:

  "dependencies": {
    "package-a": "1.0.0",
    "package-c": "1.0.0"
  },
  "resolutions": {
    "**/package-a/package-d1": "3.0.0"
  }

yarn will use package-d1@3.0.0 both for package-a and the nested dependency package-a of package-c.

Resolutions

Each sub-field of the resolutions field is called a resolution. It is a JSON field expressed by two strings: the package designation on the left and a version specification on the right.

Package designation

A resolution contains on the left-hand side a glob pattern applied to the dependency tree (and not to the node_modules directory tree, since the latter is the result of yarn resolution being influenced by the resolution).

a/b denotes the directly nested dependency b of the project's dependency a.
**/a/b denotes the directly nested dependency b of all the dependencies and nested dependencies a of the project.
a/**/b denotes all the nested dependencies b of the project's dependency a.
**/a denotes all the nested dependencies a of the project.
a is an alias for **/a (for retro-compatibility, see below, and because if it wasn't such an alias, it wouldn't mean anything as it would represent one of the non-nested project dependencies, which can't be overridden as explained below).
** denotes all the nested dependencies of the project (a bad idea mostly, as well as all other designations ending with **).

Note on single star: * is not authorized in a package resolution because it would introduce too much non-determinism. For example, there is the risk of a referring to package-* at one point to match package-a and package-b, and later on, this would match a new nested dependency package-c that wasn't intended to be matched.

Version specification

A resolution contains on the right-hand side a version specification interpreted via the semver package as usually done in yarn.

Relation to non-nested dependencies

The devDependencies, optionalDependencies and dependencies fields always take precedence over the resolutions field: if the user defines explicitly a dependency there, it means that he wants that version, even if it's specified with a non-exact specification. So the resolutions field only applies to nested-dependencies. Nevertheless, in case of incompatibility between the specification of a non-nested dependency version and a resolution, a warning is issued.

This is coherent with the fact that the package designation package-a can be used safely as an alias of **/package-a: if it wasn't the case, package-a would designate one of the non-nested dependencies and would be ignored.

Retro compatibility for the `resolutions` field

Until now, theresolutions field can contain resolutions of the following form (filled by add --flat or install --flat):

  "resolutions": {
    "package-a": "1.0.0"
  }

With the current proposal, the package designation package-a is an alias for **/package-a: this means the behaviour of yarn with a project whose resolutions field contains resolutions filed by a pre-RFC yarn will be as expected: the nested dependencies will have the fixed version specified.

Relation to the `--flat` option

Before this RFC, --flat is both about populating resolutions field AND taking resolutions field into account when executing the install command (including installation as part of the add command).

This RFC is about taking the resolutions field into account when executing the install command (including installation as part of the add command).

So with this RFC, --flat is now only about populating resolutions field. I does it in the same way as before (using a package designation in the form of package-name).

The only breaking change is that the resolutions field is always considered by yarn, even when --flat is not specified!

Incidently, this resolves this strange situation when two developers would be working on the same project, and one is using --flat while the other is not, and they would get different node_modules contents because of that.

Note that --flat being related to the installation mode (it is used via the install command, but also via the add command but pertains to the installation itself, not the adding), it will continue to behave as before by asking for resolutions of all the nested dependencies of the project even with add.

In the future, --flat will need to be rethought but for now we will keep its behaviour.

`yarn.lock`

This design implies that it is possible to have for a given version specification (e.g., >=2.0.0 <2.3.0) a resolved version that is incompatible with it (e.g., 2.3.2). It is acceptable as long as it is explicitly asked by the user via a resolution.

It is currently the case that such situation would make yarn unhappy and provoke the modification of the yarn.lock (see yarnpkg/yarn#3420).

This feature would remove the need for this behaviour of yarn.

Relation to the `check` command

The default check (without specific options) reads yarn.lock and makes sure that all versions in it match to what is inside node_modules.

We should thus get this for free without extra changes.

`--verify-tree`

--verify-tree was built to make sure that all packages inside node_modules are consistent between each other independently of yarn's resolution logic.

If you force a version that does not match semver requirements of a package, --verify-tree would throw an error.

For now we don't need to make changes to it, but later, we can expand --verify-tree to support the overrides of the resolutions field.

Non-exact version specifications

If there is a non-exact specifications in the resolutions field, the rule is the same: the resolutions field takes precedence over the specification in a nested dependency.

In case the resolutions field is broader than the nested dependency specification, then a warning can be issued. This happens if the the exact version resolved by yarn based on the resolutions specification is incompatible with the nested dependency specification.

For example, if @angular/cli depends on typescript@>=2.0.0 <2.3.0 and the resolutions field contains typescript@>=2.0.0 <2.4.0, then if the latest available version for typescript is 2.2.2, no warning is issued, and if the latest available version for typescript is 2.3.2 then a warning is issued.

The rational behind that is that since the yarn.lock file is only modified by the user (via yarn commands), then a warning will always be issued before such a situation happens and is written to the yarn.lock file.

Warnings in logs

yarn should warn about the following situations:

Unused resolutions.
Incompatible resolutions (see also above the sections about yarn.lock and about broadening non-exact specifications). Basically, an incompatible resolution is used because a package does not correctly express its dependencies. In an ideal world, the package should be fixed at one point or another and the resolution should be removed. In that sense, incompatible resolutions should always be warned about. Furthermore, an incompatible resolution is a potential for unwanted behaviour and should thus never be ignored by the user.

Locality of the resolutions

The resolutions field only apply to the local project and not to the projects that depends on it. It is the same as with lock files in a way.

How We Teach This

This won't have much impact as it extends the current behaviour by adding functionality.

The only breaking change is that resolutions is being considered all the time, but that won't surprise people, this will make yarn behaviour simply more consistent than before (see the comment on --flat above).

The term "resolution" has the same meaning as before, but it is not under the sole control of yarn itself anymore, but also under the control of the user now.

This is an advanced use of yarn, so new users don't really have to know about it in the beginning. Still, it is meant to be used on a potential regular basis, in particular when some packages a project depends on have problems in their how dependencies.

Thus it would make sense to have a bit of the documentation talking about this use case and underlying the fact that resolutions are mostly here on a temporary basis.

Drawbacks

Teaching

It makes yarn behaviour a bit more complex, although more useful. So it can be difficult for users to wrap their head around it. The RFC submitter has seen it happen many times with maven, which is quite complex but complete in its dependency management. Users would get confused and it can take time to understand the implications of manipulation the resolutions field .

Alternatives

Global nested dependencies resolution

Starting from an example, this solution would take the following form in the package.json file:

  "devDependencies": {
    "@angular/cli": "1.0.3",
    "typescript": "2.3.2"
  },
  "resolutions": {
    "typescript": "2.0.2"
  }

yarn would use typescript@2.0.2 for the whole project and that's all. The same kind of consideration (outside of the glob pattern thing) should be followed as with the selected solution of this RFC.

This is basically too simple according to discussions with yarn maintainers.

Mapping version specifications

This is a kind of simplified solution to the "out-of-scope scenario" presented in the Motivations section above (it maps versions but not dependency names).

It was proposed in this comment.

It is similar to the previous alternative but with a version specification in the package designation. This would take this form in the package.json:

  "devDependencies": {
    "@angular/cli": "1.0.3",
    "typescript": "2.2.2",
    "more dependencies..."
  },
  "resolutions": {
    "typescript@>=2.0.0 <2.3.0": "typescript@2.3.2"
  }

yarn would then replace matching version specifications with the user's one. For example a dependency normally resolved to typescript@2.2.2 would be resolved in practice to typescript@2.3.2.

This is too advanced and can be considered a possible extension of this RFC.

Mapping version specifications as well as packages name

Same as the two above but with a different name on the right-hand side of the resolution:

  "devDependencies": {
    "@angular/cli": "1.0.3",
    "typescript": "2.2.2"
  },
  "resolutions": {
    "typescript@>=2.0.0 <2.3.0": "my-typescript-fork@2.3.2"
  }

or even:

  "devDependencies": {
    "@angular/cli": "1.0.3",
    "typescript": "2.2.2"
  },
  "resolutions": {
    "typescript": "my-typescript-fork",
  }

and the version specification would be conserved.

This is too advanced and can be considered a possible extension of this RFC.

Future extensions

The two alternatives discussed in the section just above, "Mapping version specifications" and "Mapping version specifications as well as packages name", can be adapted to the current proposition to support these uses cases as well.

`flatten`

Some notes on --flat and its future with respect to this RFC.

The --flat option of install could be transformed to a flatten command that would:

Fill in the resolutions for all nested dependencies.
Set the flat field in the package.json.

It makes no real sense to have a flattening mode for install:

install already follows the resolutions field with this RFC.
install should be only about building the node_modules directory, not modifying the the package.json IMHO.

Then the flat option in the package.json (and the --flat option of add) would apply not to the installation but to the adding, upgrading, etc (everything that modify the package.json's dependencies). It will ensure that the project stays flattened via the populating of the resolutions field.

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