Package Management in Go
Brief overview of package management in Go — pre and post Go modules.
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Package management is one of the things Go has always missed. One of the major drawbacks of the previous (pre 1.11)
go get was lack of support for managing dependency versions and enabling reproducible builds. The community has developed package managers and tools like Glide, dep, and many others serving as de-facto solutions for versioning dependencies.
go getfor production builds.” — said no one ever.
Go’s implementation of package management traces its origins back to Google (which has a giant monolithic repository for all their source code). Let’s break down what’s wrong with ‘pre-go module’ package management tooling.
- Versioning dependencies.
- Vendoring dependencies.
- The necessity of
go get by default didn’t support module versioning. The idea behind the first version of go’s package management was — no need for module versioning, no need for 3rd-party module repositories, you build everything from your current branch.
Pre Go 1.11, adding a dependency meant cloning that dependency’s source code repo in your
GOPATH. That was about it. There was no concept of versions. Rather, it always pointed to the current master branch at the time of cloning. Another major issue cropped up when different projects needed different versions of a dependency — which wasn’t possible either.
Package vendoring is commonly referred to as the case where dependent packages are stored in the same place as your project. That usually means your dependencies are checked into your source management system, such as Git.
Consider this case — A uses dependency B, which uses a feature of dependency C introduced in version 1.5 of C, B must be able to ensure that A’s build uses C 1.5 or later. Pre Go 1.5, there was no mechanism for carrying dependency code alongside commands without rewriting import paths.
GOPATH exists for two main reasons:
- In Go, the
importdeclaration references a package via its fully qualified import path.
GOPATHexist so that from any directory inside
GOPATH/srcthe go tool can compute the absolute import path of the package in question.
- A location to store dependencies fetched by
What’s wrong with this?
GOPATHdoesn’t allow checking out the source of a project in a directory of choice like they are used to with other languages.
GOPATHdoes not let the developer have more than one copy of a project (or its dependencies) checked out at the same time.
Introducing Go Modules
Go 1.11 introduces preliminary support for Go modules. From Go Wiki,
A module is a collection of related Go packages that are versioned together as a single unit. Modules record precise dependency requirements and create reproducible builds.
Go modules brings three important features built-in:
go.modfile similar to
A machine-generated transitive dependency description -
GOPATHlimitation. Modules can be in any path.
$ go help mod Go mod provides access to operations on modules. Note that support for modules is built into all the go commands, not just 'go mod'. For example, day-to-day adding, removing, upgrading, and downgrading of dependencies should be done using 'go get'. See 'go help modules' for an overview of module functionality. Usage: go mod <command> [arguments] The commands are: download download modules to local cache edit edit go.mod from tools or scripts graph print module requirement graph init initialize new module in current directory tidy add missing and remove unused modules vendor make vendored copy of dependencies verify verify dependencies have expected content why explain why packages or modules are needed Use "go help mod <command>" for more information about a command.
Relevant discussion thread.
Migrating to Go Modules
To use Go modules, update Go to version
>= 1.11. Since
GOPATH is going away, one can activate module support in one of these two ways:
- Invoke the
gocommand in a directory outside of the
GOPATH/srctree, with a valid
go.modfile in the current directory.
- Go modules don’t work if the source is under
GOPATH. To override this behavior, invoke the
GO111MODULE=onenvironment variable set.
Let’s start porting by following these simple steps:
GOPATHisn’t necessary anymore, move the module out of
- From the project root, create the initial module definition -
go mod init github.com/username/repository. The best part is,
go modautomatically converts dependencies from existing package managers like
glideand six others. This will create a file called
go.modwith the module name and dependencies with its versions.
$ cat go.mod module github.com/deepsourcelabs/cli go 1.12 require ( github.com/certifi/gocertifi v0.0.0-20190410005359-59a85de7f35e github.com/getsentry/raven-go v0.2.0 github.com/pkg/errors v0.0.0-20190227000051-27936f6d90f9
go buildto create a
go.sumfile which contains the expected cryptographic checksums of the content of specific module versions. This is to ensure that future downloads of these modules retrieve the same bits as the first download. Note that go.sum is not a lock file.
$ cat go.sum github.com/certifi/gocertifi v0.0.0-20190410005359-59a85de7f35e h1:9574pc8MX6rF/QyO14SPHhM5KKIOo9fkb/1ifuYMTKU= github.com/certifi/gocertifi v0.0.0-20190410005359-59a85de7f35e/go.mod h1:GJKEexRPVJrBSOjoqN5VNOIKJ5Q3RViH6eu3puDRwx4= github.com/getsentry/raven-go v0.2.0 h1:no+xWJRb5ZI7eE8TWgIq1jLulQiIoLG0IfYxv5JYMGs= github.com/getsentry/raven-go v0.2.0/go.mod h1:KungGk8q33+aIAZUIVWZDr2OfAEBsO49PX4NzFV5kcQ= github.com/pkg/errors v0.0.0-20190227000051-27936f6d90f9 h1:dIsTcVF0w9viTLHXUEkDI7cXITMe+M/MRRM2MwisVow= github.com/pkg/errors v0.0.0-20190227000051-27936f6d90f9/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
Note on versioning: To maintain backward compatibility, if the module is version v2 or higher, the major version of the module must be included as a
/vNat the end of the module paths used in
go list -m all lists the current module and all its dependencies.
$ go list -m all github.com/deepsourcelabs/cli github.com/certifi/gocertifi v0.0.0-20190410005359-59a85de7f35e github.com/getsentry/raven-go v0.2.0 github.com/pkg/errors v0.0.0-20190227000051-27936f6d90f9
go listoutput, the current module, also known as the main module, is always the first line, followed by dependencies sorted by module path.
List Available Versions of A Package
go list -m -versions github.com/username/repository lists available versions of a package.
$ go list -m -versions github.com/getsentry/raven-go github.com/getsentry/raven-go v0.1.0 v0.1.1 v0.1.2 v0.2.0
Add a Dependency
Adding a dependency is implicit. After importing a dependency in code, running
go build or
go test command gets the latest version of the module and adds it to
go.mod file. If you would like to add a dependency explicitly, run
go get github.com/username/repository.
Upgrade/Downgrade a Dependency
go get email@example.com downloads and sets the specific version of the dependency and updates
$ go get firstname.lastname@example.org go: finding github.com/getsentry/raven-go v0.1.2 go: downloading github.com/getsentry/raven-go v0.1.2 go: extracting github.com/getsentry/raven-go v0.1.2
$ cat go.mod module github.com/deepsourcelabs/marvin-go go 1.12 require ( github.com/certifi/gocertifi v0.0.0-20190410005359-59a85de7f35e github.com/getsentry/raven-go v0.1.2 github.com/pkg/errors v0.0.0-20190227000051-27936f6d90f9 )
$ cat go.sum github.com/certifi/gocertifi v0.0.0-20190410005359-59a85de7f35e h1:9574pc8MX6rF/QyO14SPHhM5KKIOo9fkb/1ifuYMTKU= github.com/certifi/gocertifi v0.0.0-20190410005359-59a85de7f35e/go.mod h1:GJKEexRPVJrBSOjoqN5VNOIKJ5Q3RViH6eu3puDRwx4= github.com/getsentry/raven-go v0.1.2 h1:4V0z512S5mZXiBvmW2RbuZBSIY1sEdMNsPjpx2zwtSE= github.com/getsentry/raven-go v0.1.2/go.mod h1:KungGk8q33+aIAZUIVWZDr2OfAEBsO49PX4NzFV5kcQ= github.com/getsentry/raven-go v0.2.0 h1:no+xWJRb5ZI7eE8TWgIq1jLulQiIoLG0IfYxv5JYMGs= github.com/getsentry/raven-go v0.2.0/go.mod h1:KungGk8q33+aIAZUIVWZDr2OfAEBsO49PX4NzFV5kcQ= github.com/pkg/errors v0.0.0-20190227000051-27936f6d90f9 h1:dIsTcVF0w9viTLHXUEkDI7cXITMe+M/MRRM2MwisVow= github.com/pkg/errors v0.0.0-20190227000051-27936f6d90f9/go.mod h1:bwawxfHBFNV+L2hUp1rHADufV3IMtnDRdf1r5NINEl0=
When using modules, the go command completely ignores vendor directories. For backward compatibility with older versions of Go, or to ensure that all files used for a build are stored together in a single file tree, run
go mod vendor.
This creates a directory named
vendor in the root directory of the main module and stores all the packages from dependency modules there.
Note: To build using the main module’s top-level vendor directory, run '
go build -mod=vendor'.
Remove Unused Dependencies
go mod tidy trims unused dependencies and updates
GOPATH not needed anymore?
Which version is pulled by default?
The go.mod file and the go command more generally use semantic versions as the standard form for describing module versions, so that versions can be compared to determine which should be considered earlier or later than another. A module version like
v1.2.3 is introduced by tagging a revision in the underlying source repository. Untagged revisions can be referred to using a “pseudo-version” like
v0.0.0-yyyymmddhhmmss-abcdefabcdef, where the time is the commit time in UTC and the final suffix is the prefix of the commit hash.
go.sum be checked into version control?
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