title: “Go Modules: v2 and Beyond” date: 2019-11-07 by:
This post is part 4 in a series.
Note: For documentation on developing modules, see Developing and publishing modules.
As a successful project matures and new requirements are added, past features and design decisions might stop making sense. Developers may want to integrate lessons they've learned by removing deprecated functions, renaming types, or splitting complicated packages into manageable pieces. These kinds of changes require effort by downstream users to migrate their code to the new API, so they should not be made without careful consideration that the benefits outweigh the costs.
For projects that are still experimental — at major version v0 — occasional breaking changes are expected by users. For projects which are declared stable — at major version v1 or higher — breaking changes must be done in a new major version. This post explores major version semantics, how to create and publish a new major version, and how to maintain multiple major versions of a module.
Modules formalized an important principle in Go, the import compatibility rule:
If an old package and a new package have the same import path, the new package must be backwards compatible with the old package.
By definition, a new major version of a package is not backwards compatible with the previous version. This means a new major version of a module must have a different module path than the previous version. Starting with v2, the major version must appear at the end of the module path (declared in the module statement in the go.mod file). For example, when the authors of the module github.com/googleapis/gax-go developed v2, they used the new module path github.com/googleapis/gax-go/v2. Users who wanted to use v2 had to change their package imports and module requirements to github.com/googleapis/gax-go/v2.
The need for major version suffixes is one of the ways Go modules differs from most other dependency management systems. Suffixes are needed to solve the diamond dependency problem. Before Go modules, gopkg.in allowed package maintainers to follow what we now refer to as the import compatibility rule. With gopkg.in, if you depend on a package that imports gopkg.in/yaml.v1 and another package that imports gopkg.in/yaml.v2, there is no conflict because the two yaml packages have different import paths — they use a version suffix, as with Go modules. Since gopkg.in shares the same version suffix methodology as Go modules, the Go command accepts the .v2 in gopkg.in/yaml.v2 as a valid major version suffix. This is a special case for compatibility with gopkg.in: modules hosted at other domains need a slash suffix like /v2.
The recommended strategy is to develop v2+ modules in a directory named after the major version suffix.
github.com/googleapis/gax-go @ master branch /go.mod → module github.com/googleapis/gax-go /v2/go.mod → module github.com/googleapis/gax-go/v2
This approach is compatible with tools that aren't aware of modules: file paths within the repository match the paths expected by go get in GOPATH mode. This strategy also allows all major versions to be developed together in different directories.
Other strategies may keep major versions on separate branches. However, if v2+ source code is on the repository's default branch (usually master), tools that are not version-aware — including the go command in GOPATH mode — may not distinguish between major versions.
The examples in this post will follow the major version subdirectory strategy, since it provides the most compatibility. We recommend that module authors follow this strategy as long as they have users developing in GOPATH mode.
This post uses github.com/googleapis/gax-go as an example:
$ pwd /tmp/gax-go $ ls CODE_OF_CONDUCT.md call_option.go internal CONTRIBUTING.md gax.go invoke.go LICENSE go.mod tools.go README.md go.sum RELEASING.md header.go $ cat go.mod module github.com/googleapis/gax-go go 1.9 require ( github.com/golang/protobuf v1.3.1 golang.org/x/exp v0.0.0-20190221220918-438050ddec5e golang.org/x/lint v0.0.0-20181026193005-c67002cb31c3 golang.org/x/tools v0.0.0-20190114222345-bf090417da8b google.golang.org/grpc v1.19.0 honnef.co/go/tools v0.0.0-20190102054323-c2f93a96b099 ) $
To start development on v2 of github.com/googleapis/gax-go, we'll create a new v2/ directory and copy our package into it.
$ mkdir v2 $ cp -v *.go v2 'call_option.go' -> 'v2/call_option.go' 'gax.go' -> 'v2/gax.go' 'header.go' -> 'v2/header.go' 'invoke.go' -> 'v2/invoke.go' $
Now, let's create a v2 go.mod file by copying the current go.mod file and adding a /v2 suffix to the module path:
$ cp go.mod v2/go.mod $ go mod edit -module github.com/googleapis/gax-go/v2 v2/go.mod $
Note that the v2 version is treated as a separate module from the v0 / v1 versions: both may coexist in the same build. So, if your v2+ module has multiple packages, you should update them to use the new /v2 import path: otherwise, your v2+ module will depend on your v0 / v1 module. For example, to update all github.com/my/project references to github.com/my/project/v2, you can use find and sed:
$ find . -type f \
-name '*.go' \
-exec sed -i -e 's,github.com/my/project,github.com/my/project/v2,g' {} \;
$
Now we have a v2 module, but we want to experiment and make changes before publishing a release. Until we release v2.0.0 (or any version without a pre-release suffix), we can develop and make breaking changes as we decide on the new API. If we want users to be able to experiment with the new API before we officially make it stable, we can publish a v2 pre-release version:
$ git tag v2.0.0-alpha.1 $ git push origin v2.0.0-alpha.1 $
Once we are happy with our v2 API and are sure we don't need any other breaking changes, we can tag v2.0.0:
$ git tag v2.0.0 $ git push origin v2.0.0 $
At that point, there are now two major versions to maintain. Backwards compatible changes and bug fixes will lead to new minor and patch releases (for example, v1.1.0, v2.0.1, etc.).
Major version changes result in development and maintenance overhead and require investment from downstream users to migrate. The larger the project, the larger these overheads tend to be. A major version change should only come after identifying a compelling reason. Once a compelling reason has been identified for a breaking change, we recommend developing multiple major versions in the master branch because it is compatible with a wider variety of existing tools.
Breaking changes to a v1+ module should always happen in a new, vN+1 module. When a new module is released, it means additional work for the maintainers and for the users who need to migrate to the new package. Maintainers should therefore validate their APIs before making a stable release, and consider carefully whether breaking changes are really necessary beyond v1.