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13 Nov2019

Why is GO111MODULE everywhere, and everything about Go Modules (updated with Go 1.20)


You might have noticed that GO111MODULE=on is flourishing everywhere. Many readmes have that:

GO111MODULE=on go get

Note that go get has been deprecated for installing binaries since Go 1.17 and will be become impossible in Go 1.18. If you are using Go 1.16 or above, you should use instead:

go install

In this short post, I will explain why GO111MODULE was introduced in Go 1.11, its caveats and interesting bits that you need to know when dealing with Go Modules.

Table of content:


First off, let’s talk about GOPATH. When Go was first introduced in 2009, it was not shipped with a package manager. Instead, go get would fetch all the sources by using their import paths and store them in $GOPATH/src. There was no versioning and the ‘master’ branch would represent a stable version of the package.

Go Modules (previously called vgo – versioned Go) were introduced with Go 1.11. Instead of using the GOPATH for storing a single git checkout of every package, Go Modules stores tagged versions with go.mod keeping track of each package’s version.

Since then, the interaction between the ‘GOPATH behavior’ and the ‘Go Modules behavior’ has become one of the biggest gotchas of Go. One environment variable is responsible for 95% of this pain: GO111MODULE.

The GO111MODULE environment variable

GO111MODULE is an environment variable that can be set when using go for changing how Go imports packages. One of the first pain-points is that depending on the Go version, its semantics change.

GO111MODULE can be set in two different ways:

  • The environment variable in your shell, e.g.: export GO111MODULE=on.
  • The “hidden” global configuration only known by go env using go env -w GO111MODULE=on (only available since Go 1.12).

If Go seems to behave the way you think it should, it is recommended to check whether, in the past, you have set the global configuration using go env -w GO111MODULE=on (or off):

go env GO111MODULE

Note that the environment variable takes precedence over the value that was stored using go env -w GO111MODULE.

To unset the global configuration, you can do:

go env -u GO111MODULE

GO111MODULE with Go 1.11 and 1.12

  • GO111MODULE=on will force using Go modules even if the project is in your GOPATH. Requires go.mod to work.

  • GO111MODULE=off forces Go to behave the GOPATH way, even outside of GOPATH.

  • GO111MODULE=auto is the default mode. In this mode, Go will behave

    • similarly to GO111MODULE=on when you are outside of GOPATH,
    • similarly to GO111MODULE=off when you are inside the GOPATH even if a go.mod is present.

Whenever you are in your GOPATH and you want to do an operation that requires Go modules (e.g., go get a specific version of a binary), you need to do:

GO111MODULE=on go get

GO111MODULE with Go 1.13

Using Go 1.13, GO111MODULE’s default (auto) changes:

  • behaves like GO111MODULE=on anywhere there is a go.mod OR anywhere outside the GOPATH even if there is no go.mod. So you can keep all your repositories in your GOPATH with Go 1.13.
  • behaves like GO111MODULE=off in the GOPATH with no go.mod.

GO111MODULE with Go 1.14

The GO111MODULE variable has the same behavior as with Go 1.13.

Note that some slight changes in behaviors unrelated to GO111MODULE happened:

  • The vendor/ is picked up automatically. That has the tendency of breaking Gomock (issue) which were unknowingly not using vendor/ before 1.14.
  • You still need to use cd && GO111MODULE=on go get when you don’t want to mess up your current project’s go.mod (that’s so annoying).

GO111MODULE with Go 1.15

Nothing with regards to GO111MODULE has changed with Go 1.15.

GO111MODULE with Go 1.16

As of Go 1.16, the default behavior is GO111MODULE=on, meaning that if you want to keep using the old GOPATH way, you will have to force Go not to use the Go Modules feature:

export GO111MODULE=off

The best news in Go 1.16 is that we finally get a dedicated command for installing Go tools instead of relying on the does-it-all go get that keeps updating your go.mod. Instead of:

# Old way
(cd && go get

you can now run:

go install

One caveat is that the semantics of go install is slightly different from go get. As detailed on the Go blog:

In order to eliminate ambiguity about which versions are used, there are several restrictions on what directives may be present in the program’s go.mod file when using this install syntax. In particular, replace and exclude directives are not allowed, at least for now. In the long term, once the new go install program@version is working well for enough use cases, we plan to make go get stop installing command binaries. See issue 43684 for details.

As an example, you cannot (as of April 2021) install the master version of gopls:

% go install
go: downloading v0.1.1-0.20210316190639-9e9211a98eaa
go: downloading v0.0.0-20210316190639-9e9211a98eaa
go install
        The go.mod file for the module providing named packages contains one or
        more replace directives. It must not contain directives that would cause
        it to be interpreted differently than if it were the main module.

The replace directive in the go.mod file looks like this:

replace => ../

Fortunately, the gopls project makes sure to remove the replace directive before each release, which means you can use the latest tag:

go install
#                                   ^^^^^^

GO111MODULE with Go 1.17

Go 1.17 was released on August 16, 2021. As for 1.16, GO111MODULE=on is the default behavior, and GO111MODULE=auto is equivalent to GO111MODULE=on. If you still want to use the GOPATH way, you will have to force Go not to use the Go Modules feature using GO111MODULE=off (see the section about direnv).

The three important changes that may affect how you use GO111MODULE are the following:

Faster downloading of dependencies if you are using Git to fetch modules

If you decide to update the go line in your go.mod to take advantage of the new module graph pruning, your go.mod will want to get updated. The first time you will want to use go build, you will see the following error:

go build ./...
go: updates to go.mod needed; to update it:
        go mod tidy

After running go mod tidy, you will see that your go.mod changed quite a lot with a new require block:

diff --git a/go.mod b/go.mod
index 3af719e..26ed354 100644
--- a/go.mod
+++ b/go.mod
@@ -1,12 +1,12 @@

-go 1.12
+go 1.17

   require ( v2.0.1 v1.4.9 // indirect v1.4.4
- v1.4.3
+ v1.5.2 v1.0.0 v1.2.0 v1.3.0
@@ -37,8 +37,25 @@ require ( v0.0.0-20200804184101-5ec99f83aff1 // indirect v0.0.0-20201116205149-79184cff4dfe // indirect v1.33.2
- v1.25.0
+ v1.27.1 v1.0.0-20190902080502-41f04d3bba15 // indirect v1.62.0 // indirect v3.0.0-20200615113413-eeeca48fe776 // indirect
+require (
+ v1.0.0 // indirect
+ v1.1.1 // indirect
+ v1.3.0 // indirect
+ v0.5.4 // indirect
+ v1.0.0 // indirect
+ v1.0.0 // indirect
+ v1.0.1 // indirect
+ v1.0.0 // indirect
+ v0.0.0-20190812154241-14fe0d1b01d4 // indirect
+ v0.4.0 // indirect
+ v0.0.0-20190507164030-5867b95ac084 // indirect
+ v1.0.5 // indirect
+ v1.2.0 // indirect
+ v2.3.0 // indirect

The new require block adds even more // indirect lines. Thanks to these new lines, Go commands such as go get have less downloading to do (the mechanism is called lazy modules loading). Previously, go get had to download every single project in order to access their go.mod files, even if some of these projects were not actually used by your code. That was a big problem for git-based projects, and less of a problem for people using GOPROXY (i.e., everyone else) since the go.mod can be fetched without fetching the whole repository when using the GOPROXY mechanism.

For example, using the GOPROXY mechanism, the number of HTTP GET calls is reduced from 252 to 169 (49% less HTTP requests) with the cert-manager project for the tag v1.4.0. The number of HTTP calls was calculated using mitmproxy using GOCACHE=off. Although the number of HTTP calls has been reduced, I did not notice any significant time reduction.

The biggest difference will happen for people who are relying on Git for downloading repositories. Each time Go needs to read a go.mod, it needs to download the whole Git repository. We can force Go to clone the Git repositories by using GOPRIVATE=*:

# With Go 1.16:
$ time GOPATH=$(mktemp -d) GOPRIVATE='*' go get ./...
161.65s user 34.07s system 38% cpu 8:23.58 total

# With Go 1.17:
$ time GOPATH=$(mktemp -d) GOPRIVATE='*' go get ./...
158.09s user 33.39s system 39% cpu 7:59.63 total

That’s 5% faster (24 seconds less with a 30 Mbit/s DSL connection). I would have expected a better result, but that number will depend on the go.mod you have!

Installing binaries with GO111MODULE=on go get is deprecated

In Go 1.17, the use of go get to install binaries is now showing the following warning:

$ GO111MODULE=on go get
go get: installing executables with 'go get' in module mode is deprecated.
     Use 'go install pkg@version' instead.
     For more information, see
     or run 'go help get' or 'go help install'.

To work around this warning, you can use go install that was taught how to deal with @version in Go 1.16:

go install

Notice that @version implies the Go module mode, meaning that you don’t have to add GO111MODULE=on at the beginning of the command when you are not in a Go project that has a go.mod.

go run knows about @version (finally!)

The go run command was taught how to deal with @version! Like go install, it implies GO111MODULE=on. Up to now, if you wanted to run a binary once, you had two ways:

  • If you were running the binary from a go.mod enabled project, you could specify the version of the binary you want to run in your go.mod and then run go run -mod=mod.
# Before Go 1.17:
go get
go run -mod=mod

# With Go 1.17:
go run

This is great if you use //go:generate to generate mocks. For example, in the project [users-grpc]( master/schema/placeholder.go), I was able to replace my //go:generate directive:

// Old way, before 1.17:
//go:generate go run -mod=mod -build_flags=-mod=mod -package mocks -destination ./mock_service.go -source=../  user.go
// New way, Go 1.17:
//go:generate go run -package mocks -destination ./mock_service.go -source=../user.go

GO111MODULE with Go 1.18

If you still need to use go get to install a binary, you will need to set GO111MODULE=off. The recommended way is to switch to using go install instead. Without GO111MODULE=off, go get will only update your go.mod. Otherwise, it won’t do anything. All the READMEs on the Internet have to be updated; if they don’t, people will get confused by not seeing the binary being installed.

GO111MODULE with Go 1.19

Nothing with regards to GO111MODULE has changed with Go 1.19.

GO111MODULE with Go 1.20

Nothing with regards to GO111MODULE has changed with Go 1.20.

Why was GO111MODULE everywhere? (Go 1.15 and below)

Now that we know that GO111MODULE was very useful with Go 1.15 and below for enabling the Go Modules behavior, here is the answer: that’s because GO111MODULE=on allows you to select a version. Without Go Modules, go get fetches the latest commit from master. With Go Modules, you can select a specific version based on git tags.

Before Go 1.16 got released, I used to use GO111MODULE=on very often when I wanted to switch between the latest version and the HEAD version of gopls (the Go Language Server):

# With Go 1.15.
GO111MODULE=on go get
GO111MODULE=on go get
GO111MODULE=on go get
GO111MODULE=on go get
GO111MODULE="on" go get

This became much easier with Go 1.16 though. I could do the same thing with:

# With Go 1.16.
go install

The pitfall of go.mod being silently updated (Go 1.15 and below)

With Go 1.15 and below, our only option to install binaries was to use go get. You may have encountered this weird one-liner in READMEs:

(cd && GO111MODULE=on go get

Note: the @latest suffix will use the latest git tag of gopls. Note that -u (which means ‘update’) is not needed for @v0.1.8 since this is a ‘fixed’ version, and updating a fixed version does not really make sense. It is also interesting to note that with @v0.1, go get will fetch the latest patch version for that tag.

So, why did we need to use this contrived command that calls a subshell and moves to your HOME? That’s yet another Go ideocracy that was fixed with Go 1.16: in Go 1.15 and below, by default (and you can’t turn that off), if you are in a folder that has a go.mod, go get will update that go.mod with what you just installed. And in the case of development binaries like gopls or kind, you definitely don’t want to have these appearing in the go.mod file!

So the workaround for anyone using Go 1.15 and below was to give a one-liner that makes sure that you won’t be in a go.mod-enabled folder: (cd && go get) does exactly that.

Fortunately, with Go 1.16, there is now a clear separation of concerns between go get that is adding a dependency to your go.mod (like npm install) and go install that is meant to install a binary without messing up your go.mod. With Go 1.16, the above go get becomes:

go install

The -u and @version pitfall

I have been bitten multiple times by this: when using go get @latest (for a binary, at least), you should avoid using -u so that it uses the dependencies as defined in go.sum. Otherwise, it will update all the dependencies to their latest minor revision… And since a ton of projects choose to have breaking changes between minor versions (e.g. v0.2.0 to v0.3.0), using -u has a large chance of breaking things.

So if you see this:

# Both -u and @latest!
GO111MODULE=on go get -u

then you will immediately realize that (1) it uses the old preGo .1.16 way of installing a Go binary, and (2) you want to be using the recorded versions given in go.sum when go-getting a binary!

Rebecca Stambler reminds us that we should not use -u in conjunction with a version:

-u should not be used in conjunction with the @latest tag, as it will give you incorrect versions of the dependencies.

But it’s kind of hidden in this issue… I guess it is written somewhere in the Go help (btw, what a hideous help compared to git help) but that kind of caveat should be more visible: maybe print a warning when installing a binary with both @version and -u?

Caveats when using Go Modules

Now, let’s go through some caveats I encountered when working with Go Modules.

Remember that go get also updates your go.mod

Before Go 1.16 came out, that was one of the weird things with go get: sometimes, it served the purpose of installing binaries or downloading packages. And with Go modules, if you were in a repo with a go.mod, it would silently add the binary that you were go get-ing to your go.mod!

Fortunately, with Go 1.16, go install has learnt about the @version suffix. With go install foo@version, your local go.mod won’t be affected! And in Go 1.18, go get won’t install binaries anymore, and will only ever be used for adding dependencies to your go.mod.

And if you would like to run go test or go build without having go.mod updated, you can use the flag -mod=readonly. For example:

go test -mod=readonly ./...
go build -mod=readonly ./...

Where are the sources of the dependencies with Go Modules

When using Go Modules, the packages that are used during go build are stored in $GOPATH/pkg/mod. When trying to inspect an ‘import’ in vim or VSCode, you might end up in the GOPATH version of the package instead of the pkg/mod one used during compilation.

A second issue that arises is when you want to hack one of your dependencies, for example for testing purposes.

Solution 1: use go mod vendor + go build -mod=vendor. That will force go to use the vendor/ files instead of using the $GOPATH/pkg/mod ones. This option also solves the problem of vim and VSCode not opening the right version of a package’s file.

Solution 2: add a ‘replace’ line at the end of your go.mod:

replace => ../beers

where ../beers is a local copy I made of the dependency I want to inspect and hack.

Set GO111MODULE on a per-folder basis with direnv

On older versions of Go (1.15 and below), while migrating from GOPATH-based projects (mainly using Dep) to Go Modules, I found myself struggling with two different places: inside and outside GOPATH. All Go Modules had to be kept outside of GOPATH, which meant my projects were in different folders. To remediate that, I used GO111MODULE extensively. I would keep all my projects in the GOPATH, and for the Go Modules-enabled projects, I would set export GO111MODULE=on.

Note: since the default behavior in Go 1.16 is now GO111MODULE=on, this trick isn’t necessary anymore.

This is where direnv came in handy. Direnv is a lightweight command written in Go that will load a file, .envrc, whenever you enter a directory and .envrc is present. For every Go Module-enabled project, I would have this .envrc:

# .envrc
export GO111MODULE=on
export GOFLAGS=-mod=vendor

The GOPRIVATE environment variable disables the Go Proxy (introduced in Go 1.13) for certain import paths. I also found useful to set -mod=vendor so that every command uses the vendor folder (go mod vendor).

Private Go Modules and Dockerfile

Many companies choose to use private repositories as import paths. As explained above, we can use GOPRIVATE in order to tell Go (as of Go 1.13) to skip the package proxy and fetch our private packages directly from Github.

But what about building Docker images? How can go get fetch our private repositories from a docker build?

Solution 1: vendoring

With go mod vendor, no need to pass Github credentials to the docker build context. We can just put everything in vendor/ and the problem is solved. In the Dockerfile, -mod=vendor will be required, but developers don’t even have to bother with -mod=vendor since they have access to the private Github repositories anyway using their local Git config

  • Pros: faster build on CI (~10 to 30 seconds less)
  • Cons: PRs are bloated with vendor/ changes and the repo’s size might be big

Solution 2: no vendoring

If vendor/ is just too big (e.g., for Kubernetes controllers, vendor/ is about 30MB), we can very well do it without vendoring. That would require to pass some form of GITHUB_TOKEN as argument of docker build, and in the Dockerfile, set something like:

git config --global url."https://foo:${GITHUB_TOKEN}".insteadOf ""

Illustration by Bailey Beougher, from The Illustrated Children’s Guide to Kubernetes.

  • Update 22 June 2020: it said use replace instead of just replace.
  • Update 8 April 2021: update with Go 1.16.
  • Update 20 Sept 2021: update with Go 1.17.
  • Update 18 Feb 2023: mention the go env GO111MODULE gotcha reported by Josh Soref. I also mentioned Go 1.18, Go 1.19, and Go 1.20. Finally, I reformulated “Why is GO111MODULE everywhere” to “Why was GO111MODULE everywhere in Go 1.15 and below” thanks to Ed Randall’s remark. I also added a note about -mod=readonly after a remark from daodennis, and fixed an issue reported by DongHo Jung.
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