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Why is GO111MODULE everywhere, and everything about Go Modules

maelvls profile image Maël Valais Originally published at Updated on ・8 min read

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

GO111MODULE=on go get
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In this short post, I will explain why GO111MODULE exists, 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 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
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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).

So, why is GO111MODULE everywhere?!

Now that we know that GO111MODULE can be very useful 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.

I use GO111MODULE=on very often when I want to switch between the latest version and the HEAD version of gopls (the Go Language Server):

GO111MODULE=on go get
GO111MODULE=on go get
GO111MODULE=on go get
GO111MODULE=on go get
GO111MODULE="on" go get
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The pitfall of go.mod being silently updated

And to make things even worse, some projects have an even more complicated one-liners:

(cd && GO111MODULE=on go get
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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.

That's yet another Go ideocracy: 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 is 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.

I hope that (sooner or later) we will have 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.

  • First caveat: we all use go get to install dev dependencies, so moving to go install would kind of not work (habits...)
  • Second caveat: go install doesn’t allow you to give a version (e.g., @latest or @v1.4.5), and go run either by the way. So go install isn't that useful after all... 😞
$ export GO111MODULE=on

$ go get              # ✅

$ go install          # ❌
can t load package: package cannot use path@version syntax in GOPATH mode

$ go run              # ❌
package can only use path@version syntax with 'go get'
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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
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then you will immediately realize that it is wrong: 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

That’s one of the weird things with go get: sometimes, it serves the purpose of installing binaries or downloading packages. But with Go modules, if you are in a repo with a go.mod, it will silently add the package you go get to your go.mod.

That’s one of the catches of Go modules! 😁

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
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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

During the migration 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.

This is where direnv comes 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
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The GOPRIVATE disables the Go Proxy (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

At my company, we use a lot of private repositories. As explained above, we can use GOPRIVATE in order to tell 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 ""
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Illustration by Bailey Beougher, from The Illustrated Children's Guide to Kubernetes.

Update 22 June 2020: it said use replace instead of just replace

Discussion (23)

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alexshtin profile image
Alex Shtin

Great post, thanks for sharing. One question. We use GO111MODULE=off to install tools like goimport and not to pollute our go.mod file:

GO111MODULE=off go get -u

How is it worse than:

(cd && GO111MODULE=on go get -u


daodennis profile image

Thank you for this article Maël!
You might find this as a breath of fresh air, regarding "Remember that go get also updates your go.mod"
go build, go test, and others - stop updating go.mod by default

Seems like there is a flag that certain Go tools have, the "-mod=readonly" flag. Might be worth adding to your article.

suntong profile image

That's such a helpful post, so many of my questions got answered here. One more:

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.

I haven't fully got this working yet. Could you elaborate more on this a bit more please?

  • what go mod vendor is actually doing, how would it be different without doing it? Would -mod=vendor work without such step? and, -mod=vendor is for go build right?
  • how to put things in vendor/? Can you give a detailed example please -- i.e., Should I put mine in the GOPATH way, or the go mod way (with twisted name and added versions), Can I not put everything, but only a selectful of them that go get is failing to get?


maelvls profile image
Maël Valais Author

Hi! Thanks for the kind words!!! 🙂

  • go mod vendor will vendor all the .go files that are imported by your project (it is smart and only vendors the files actually used, not the whole repo for each import path). It keeps track of what is being vendored in vendor/modules.txt (not sure but I think this module list is only "for the humans" so that we can see in a diff what import paths have been added to vendor/)
  • To put things into vendor/, you first need to have modules turned on (go.mod at the root of your project) and then you can run go mod vendor once and here you go. Whenever you add import paths in your project, you also want to re-run go mod vendor to add/delete any missing/unused vendored files.
  • After having run go mod vendor you can fall back to using the good old GOPATH way (i.e. GO111MODULE=off) and not care about modules anymore.
  • Before Go 1.14, go build would by default use the modules in $GOPATH/pkg/mod instead of vendor/ and you had to run go build -mod=vendor
  • With Go 1.14 and above, the vendor/ folder is used by default and if you want to force using the $GOPATH/pkg/mod you can use go build -mod=mod.

Should I put my vendor/ in the GOPATH way or the go mod way

I think go mod vendor puts things in the GOPATH way (except it only puts the files that are "actually used"). Not sure I answered the question properly though 😞

Can I not put everything, but only a selectful of them that go get is failing to get?

Ah, right, I see the use case: you want to be able to manually add anything that go get doesn't know how to download (e.g. private networks, corporate firewalls).

I guess you can definitely do that: go mod vendor for anything that works and manually copy-paste the import paths that are failing. I guess you would have to update vendor/modules.txt too but not sure. The important part is that the hash of the files you manually add must match the hashes in the go.sum (I guess). 👍

suntong profile image

Thanks a lot for such detailed explanation.

domq profile image

Thanks Maël, you single-handedly saved me about 8 hours of RTFM. Now I can get started on a project that enjoys sane, 21st-century style version control of its dependencies. And thanks for the direnv link, which appears to work with Emacs (although I can't vouch for that just yet)

sandeepmeenuga profile image

Would you share something about govendor( vs Go111module=on . I've a doubt about Go111module=on which saves one version of packages only in GOPATH but Govendor works as individual project's versions which exactly as node_modules. Any views appreciated.

maelvls profile image
Maël Valais Author

Hi sameen,

Unfortunately, I do not know about govendor. But from what I understand, when building a binary with GO111MODULE=on, a single version of each dependency will be used for the whole build. Russ Cox calls it minimal version selection (see his blog post on Dec 3rd, 2019).

I've a doubt about Go111module=on which saves one version of packages only in GOPATH

What do you mean? GO111MODULE=on will remember (in go.sum) the version of dependencies and save each dependency copy in GOPATH/pkg/mod (i.e. one git clone at a fixed version if you will).

Note that GO111MODULE=on works very differently from node_modules since a single version is used per package, even when the package is used in multiple dependencies across the dependency tree.

martinflower profile image
Martin Flower

Thanks Maël

Maybe you can answer the following, related, question:

If I'm using Go 1.14, with go.mod, no vendor folder, should I have a GOPATH environment variable? Does this affect GO111MODULE in any way?



kelepirci profile image
Ferhat Ozkasgarli

What a software language. A simple start require great effort and time. It must be great to become GO developer ( ͡° ͜ʖ ͡°).

Wanderful article to sooth my pain while I was trying to figure out the GOPATH and its minions. :D

I thank and salute your sir ...

murphyke profile image
Kevin Murphy

Hi, your example able says use replace => ../beers ... but I think the word use is not supposed to be there. Thanks for the post, btw.

maelvls profile image
Maël Valais Author

Good point, thank you!!
I fixed that in the article 👍

rafalskolasinski profile image

It could be good to add note on behaviour in Go 1.14. Does it differ from one in Go 1.13?

maelvls profile image
Maël Valais Author

Definitely, I’ll try to find time to update to 1.14 🙂

rafalskolasinski profile image

BTW, this is a go to post about understanding GO111MODULES. IMO should be linked / part of official docs.

maelvls profile image
Maël Valais Author

Done! I updated with 1.14 and two "new" pitfalls 🙂

1oglop1 profile image
Jan Gazda

Reading the discussion I'm glad I'm not a lone who find Go things counter intuitive.
This article has helped me to solve the problem when go get did not create binary and exited with success.

gcdcoder profile image
Gustavo Castillo

Great article thanks for sharing.

jy226 profile image
Jaafar Yahya

Great Article !

themrkumar profile image

Great Article. Please also make a tutorial about Go modules

folkengine profile image
Chris Baker

This was really useful. Thanks!

benkame profile image
Ben Ka

Very helpful! That spares me lot of pain.

xmpace profile image

Helps me a lot, thank you!