(This article has been graciously translated to Russian here. Huge thanks to Akhmad Karimov.)

I wrote a brief introduction to Go modules and in it I talked briefly about Go modules proxies and now that Go 1.11 is out, I thought I’d play a bit these proxies to figure our how they’re supposed to work.

Why

One of the goals of Go modules is to provide reproducible builds and it does a very good job by fetching the correct and expected files from a repository.

But what if the servers are offline? What if the repository simply vanishes?

One way teams deal with these risks is by vendoring the dependencies, which is fine. But Go modules offers another way: the use of a module proxy.

The Download Protocol

When Go modules support is enabled and the go command determines that it needs a module, it first looks at the local cache (under $GOPATH/pkg/mods). If it can’t find the right files there, it then goes ahead and fetches the files from the network (i.e. from a remote repo hosted on Github, Gitlab, etc.)

If we want to control what files go can download, we need to tell it to go through our proxy by setting the GOPROXY environment variable to point to our proxy’s URL. For instance:

export GOPROXY=http://gproxy.mycompany.local:8080

The proxy is nothing but a web server that responds to the module download protocol, which is a very simple API to query and fetch modules. The web server may even serve static files.

A typical scenario would be the go command trying to fetch github.com/pkg/errors:

The first thing go will do is ask the proxy for a list of available versions. It does this by making a GET request to /{module name}/@v/list. The server then responds with a simple list of versions it has available:

v0.8.0
v0.7.1

The go will determine which version it wants to download — the latest unless explicitly told otherwise¹. It will then request information about that given version by issuing a GET request to /{module name}/@v/{module revision} to which the server will reply with a JSON representation of the struct:

type RevInfo struct {
    Version string    // version string
    Name    string    // complete ID in underlying repository
    Short   string    // shortened ID, for use in pseudo-version
    Time    time.Time // commit time
}

So for instance, we might get something like this:

{
    "Version": "v0.8.0",
    "Name": "v0.8.0",
    "Short": "v0.8.0",
    "Time": "2018-08-27T08:54:46.436183-04:00"
}

The go command will then request the module’s go.mod file by making a GET request to /{module name}/@v/{module revision}.mod. The server will simply respond with the contents of the go.mod file (e.g. module github.com/pkg/errors.) This file may list additional dependencies and the cycle restarts for each one.

Finally, the go command will request the actual module by getting /{module name}/@v/{module revision}.zip. The server should respond with a byte blob (application/zip) containing a zip archive with the module files where each file must be prefixed by the full module path and version (e.g. github.com/pkg/errors@v0.8.0/), i.e. the archive should contain:

github.com/pkg/errors@v0.8.0/example_test.go
github.com/pkg/errors@v0.8.0/errors_test.go
github.com/pkg/errors@v0.8.0/LICENSE
...

And not:

errors/example_test.go
errors/errors_test.go
errors/LICENSE
...

This seems like a lot when written like this, but it’s in fact a very simple protocol that simply fetches 3 or 4 files:

1. The list of versions (only if go does not already know which version it wants)
2. The module metadata
3. The go.mod file
4. The module zip itself

Creating a simple local proxy

To try out the proxy support, let’s create a very basic proxy that will serve static files from a directory. First we create a directory where we will store our in-site copies of our dependencies. Here’s what I have in mine:

$ find . -type f
./github.com/robteix/testmod/@v/v1.0.0.mod
./github.com/robteix/testmod/@v/v1.0.1.mod
./github.com/robteix/testmod/@v/v1.0.1.zip
./github.com/robteix/testmod/@v/v1.0.0.zip
./github.com/robteix/testmod/@v/v1.0.0.info
./github.com/robteix/testmod/@v/v1.0.1.info
./github.com/robteix/testmod/@v/list

These are the files our proxy will serve. You can find these files on Github if you’d like to play along. For the examples below, let’s assume we have a devel directory under our home directory; adapt accordingly.

$ cd $HOME/devel
$ git clone https://github.com/robteix/go-proxy-blog.git

Our proxy server is simple (it could be even simpler, but I wanted to log the requests):

package main

import (
    "flag"
    "log"
    "net/http"
)

func main() {
    addr := flag.String("http", ":8080", "address to bind to")
    flag.Parse()

    dir := "."
    if flag.NArg() > 0 {
        dir = flag.Arg(0)
    }

    log.Printf("Serving files from %s on %s\n", dir, *addr)

    h := handler{http.FileServer(http.Dir(dir))}

    panic(http.ListenAndServe(*addr, h))
}

type handler struct {
    h http.Handler
}

func (h handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    log.Println("New request:", r.URL.Path)
    h.h.ServeHTTP(w, r)
}

Now run the code above:

$ go run proxy.go -http :8080 $HOME/devel/go-proxy-blog
2018/08/29 14:14:31 Serving files from /home/robteix/devel/go-proxy-blog on :8080

$ curl http://localhost:8080/github.com/robteix/testmod/@v/list
v1.0.0
v1.0.1

Leave the proxy running and move to a new terminal. Now let’s create a new test program. we create a new directory $HOME/devel/test and create a file named test.go inside it with the following code:

package main

import (
    "github.com/robteix/testmod"
)

func main() {
    testmod.Hi("world")
}

And now, inside this directory, let’s enable Go modules:

$ go mod init test

And we set the GOPROXY variable:

export GOPROXY=http://localhost:8080

Now let’s try building our new program:

$ go build
go: finding github.com/robteix/testmod v1.0.1
go: downloading github.com/robteix/testmod v1.0.1

And if you check the output from our proxy:

2018/08/29 14:56:14 New request: /github.com/robteix/testmod/@v/list
2018/08/29 14:56:14 New request: /github.com/robteix/testmod/@v/v1.0.1.info
2018/08/29 14:56:14 New request: /github.com/robteix/testmod/@v/v1.0.1.mod
2018/08/29 14:56:14 New request: /github.com/robteix/testmod/@v/v1.0.1.zip

So as long as GOPROXY is set, go will only download files from out proxy. If I go ahead and delete the repository from Github, things will continue to work.

Using a local directory

It is interesting to note that we don’t even need our proxy.go at all. We can set GOPROXY to point to a directory in the filesystem and things will still work as expected:

export GOPROXY=file://home/robteix/devel/go-proxy-blog

If we do a go build now², we’ll see exactly the same thing as with the proxy:

$ go build
go: finding github.com/robteix/testmod v1.0.1
go: downloading github.com/robteix/testmod v1.0.1

Of course, in real life, we probably will prefer to have a company/team proxy server where our dependencies are stored, because a local directory is not really much different from the local cache that go already maintains under $GOPATH/pkg/mod, but still, nice to know that it works.

There is a project called Athens that is building a proxy and that aims — if I don’t misunderstand it — to create a central repository of packages à la npm.

This post is also available in other languages:

• Russian: Введение в модули Go
• Uzbek: Go modullariga kirish

The upcoming version 1.11 of the Go programming language will bring experimental support for modules, a new dependency management system for Go. A few days ago, I wrote a quick post about it. Since that post went live, things changed a bit and as we’re now very close to the new release, I thought it would be a good time for another post with a more hands-on approach. So here’s what we’ll do: we’ll create a new package and then we’ll make a few releases to see how that would work.

Creating a Module

So first things first. Let’s create our package. We’ll call it “testmod”. An important detail here: this directory should be outside your $GOPATH because by default, the modules support is disabled inside it. Go modules is a first step in potentially eliminating $GOPATH entirely at some point.

$ mkdir testmod
$ cd testmod

Our package is very simple:

package testmod

import "fmt" 

// Hi returns a friendly greeting
func Hi(name string) string {
   return fmt.Sprintf("Hi, %s", name)
}

The package is done but it is still not a module. Let’s change that.

$ go mod init github.com/robteix/testmod
go: creating new go.mod: module github.com/robteix/testmod

This creates a new file named go.mod in the package directory with the following contents:

module github.com/robteix/testmod

Not a lot here, but this effectively turns our package into a module. We can now push this code to a repository:

$ git init 
$ git add * 
$ git commit -am "First commit" 
$ git push -u origin master

Until now, anyone willing to use this package would go get it:

$ go get github.com/robteix/testmod

And this would fetch the latest code in master. This still works, but we should probably stop doing that now that we have a Better Way™. Fetching master is inherently dangerous as we can never know for sure that the package authors didn’t make change that will break our usage. That’s what modules aims at fixing.

Quick Intro to Module Versioning

Go modules are versioned, and there are some particularities with regards to certain versions. You will need to familiarize yourself with the concepts behind semantic versioning.

More importantly, Go will use repository tags when looking for versions, and some versions are different of others: e.g. versions 2 and greater should have a different import path than versions 0 and 1 (we’ll get to that.) As well, by default Go will fetch the latest tagged version available in a repository. This is an important gotcha as you may be used to working with the master branch. What you need to keep in mind for now is that to make a release of our package, we need to tag our repository with the version. So let’s do that.

Making our first release

Now that our package is ready, we can release it to the world. We do this by using version tags. Let’s release our version 1.0.0:

$ git tag v1.0.0
$ git push --tags

This creates a tag on my Github repository marking the current commit as being the release 1.0.0.

Go doesn’t enforce that in any way, but a good idea is to also create a new branch (“v1”) so that we can push bug fixes to.

$ git checkout -b v1
$ git push -u origin v1

Now we can work on master without having to worry about breaking our release.

Using our module

Now we’re ready to use the module. We’ll create a simple program that will use our new package:

package main

import (
 	"fmt"

 	"github.com/robteix/testmod"
)

func main() {
 	fmt.Println(testmod.Hi("roberto"))
}

Until now, you would do a go get github.com/robteix/testmod to download the package, but with modules, this gets more interesting. First we need to enable modules in our new program.

$ go mod init mod

As you’d expect from what we’ve seen above, this will have created a new go.mod file with the module name in it:

module mod

Things get much more interesting when we try to build our new program:

$ go build
go: finding github.com/robteix/testmod v1.0.0
go: downloading github.com/robteix/testmod v1.0.0

As we can see, the go command automatically goes and fetches the packages imported by the program. If we check our go.mod file, we see that things have changed:

module mod
require github.com/robteix/testmod v1.0.0

And we now have a new file too, named go.sum, which contains hashes of the packages, to ensure that we have the correct version and files.

github.com/robteix/testmod v1.0.0 h1:9EdH0EArQ/rkpss9Tj8gUnwx3w5p0jkzJrd5tRAhxnA=
github.com/robteix/testmod v1.0.0/go.mod h1:UVhi5McON9ZLc5kl5iN2bTXlL6ylcxE9VInV71RrlO8=

Making a bugfix release

Now let’s say we realized a problem with our package: the greeting is missing ponctuation! People are mad because our friendly greeting is not friendly enough. So we’ll fix it and release a new version:

// Hi returns a friendly greeting
func Hi(name string) string {
-       return fmt.Sprintf("Hi, %s", name)
+       return fmt.Sprintf("Hi, %s!", name)
}

We made this change in the v1 branch because it’s not relevant for what we’ll do for v2 later, but in real life, maybe you’d do it in master and then back-port it. Either way, we need to have the fix in our v1 branch and mark it as a new release.

$ git commit -m "Emphasize our friendliness" testmod.go
$ git tag v1.0.1
$ git push --tags origin v1

Updating modules

By default, Go will not update modules without being asked. This is a Good Thing™ as we want predictability in our builds. If Go modules were automatically updated every time a new version came out, we’d be back in the uncivilized age pre-Go1.11. No, we need to tell Go to update a modules for us.

We do this by using our good old friend go get:

• run go get -u to use the latest minor or patch releases (i.e. it would update from 1.0.0 to, say, 1.0.1 or, if available, 1.1.0)
• run go get -u=patch to use the latest patch releases (i.e., would update to 1.0.1 but not to 1.1.0)
• run go get package@version to update to a specific version (say, github.com/robteix/testmod@v1.0.1)

In the list above, there doesn’t seem to be a way to update to the latest major version. There’s a good reason for that, as we’ll see in a bit.

Since our program was using version 1.0.0 of our package and we just created version 1.0.1, any of the following commands will update us to 1.0.1:

$ go get -u
$ go get -u=patch
$ go get github.com/robteix/testmod@v1.0.1

After running, say, go get -u our go.mod is changed to:

module mod
require github.com/robteix/testmod v1.0.1

Major versions

According to semantic version semantics, a major version is different than minors. Major versions can break backwards compatibility. From the point of view of Go modules, a major version is a different package completely. This may sound bizarre at first, but it makes sense: two versions of a library that are not compatible with each other are two different libraries.

Let’s make a major change in our package, shall we? Over time, we realized our API was too simple, too limited for the use cases of our users, so we need to change the Hi() function to take a new parameter for the greeting language:

package testmod

import (
 	"errors"
 	"fmt" 
) 

// Hi returns a friendly greeting in language lang
func Hi(name, lang string) (string, error) {
 	switch lang {
 	case "en":
 		return fmt.Sprintf("Hi, %s!", name), nil
 	case "pt":
 		return fmt.Sprintf("Oi, %s!", name), nil
 	case "es":
 		return fmt.Sprintf("¡Hola, %s!", name), nil
 	case "fr":
 		return fmt.Sprintf("Bonjour, %s!", name), nil
 	default:
 		return "", errors.New("unknown language")
 	}
}

Existing software using our API will break because they (a) don’t pass a language parameter and (b) don’t expect an error return. Our new API is no longer compatible with version 1.x so it’s time to bump the version to 2.0.0.

I mentioned before that some versions have some peculiarities, and this is the case now. Versions 2 and over should change the import path. They are different libraries now.

We do this by appending a new version path to the end of our module name.

module github.com/robteix/testmod/v2

The rest is the same as before, we push it, tag it as v2.0.0 (and optionally create a v2 branch.)

$ git commit testmod.go -m "Change Hi to allow multilang"
$ git checkout -b v2 # optional but recommended
$ echo "module github.com/robteix/testmod/v2" > go.mod
$ git commit go.mod -m "Bump version to v2"
$ git tag v2.0.0
$ git push --tags origin v2 # or master if we don't have a branch

Updating to a major version

Even though we have released a new incompatible version of our library, existing software will not break, because it will continue to use the existing version 1.0.1. go get -u will not get version 2.0.0.

At some point, however, I, as the library user, may want to upgrade to version 2.0.0 because maybe I was one of those users who needed multi-language support.

I do it but modifying my program accordingly:

package main

import (
 	"fmt"
 	"github.com/robteix/testmod/v2" 
)

func main() {
 	g, err := testmod.Hi("Roberto", "pt")
 	if err != nil {
 		panic(err)
 	}
 	fmt.Println(g)
}

And then when I run go build, it will go and fetch version 2.0.0 for me. Notice how even though the import path ends with “v2”, Go will still refer to the module by its proper name (“testmod”).

As I mentioned before, the major version is for all intents and purposes a completely different package. Go modules does not link the two at all. That means we can use two incompatible versions in the same binary:

package main
import (
 	"fmt"
 	"github.com/robteix/testmod"
 	testmodML "github.com/robteix/testmod/v2"
)

func main() {
 	fmt.Println(testmod.Hi("Roberto"))
 	g, err := testmodML.Hi("Roberto", "pt")
 	if err != nil {
 		panic(err)
 	}
 	fmt.Println(g)
}

This eliminates a common problem with dependency management: when dependencies depend on different versions of the same library.

Tidying it up

Going back to the previous version that uses only testmod 2.0.0, if we check the contents of go.mod now, we’ll notice something:

module mod
require github.com/robteix/testmod v1.0.1
require github.com/robteix/testmod/v2 v2.0.0

By default, Go does not remove a dependency from go.mod unless you ask it to. If you have dependencies that you no longer use and want to clean up, you can use the new tidy command:

$ go mod tidy

Now we’re left with only the dependencies that are really being used.

Vendoring

Go modules ignores the vendor/ directory by default. The idea is to eventually do away with vendoring[^0]. But if we still want to add vendored dependencies to our version control, we can still do it:

$ go mod vendor

This will create a vendor/ directory under the root of your project containing the source code for all of your dependencies.

Still, go build will ignore the contents of this directory by default. If you want to build dependencies from the vendor/ directory, you’ll need to ask for it.

$ go build -mod vendor

I expect many developers willing to use vendoring will run go build normally on their development machines and use -mod vendor in their CI.

Again, Go modules is moving away from the idea of vendoring and towards using a Go module proxy for those who don’t want to depend on the upstream version control services directly.

There are ways to guarantee that go will not reach the network at all (e.g. GOPROXY=off) but these are the subject for a future blog post.

Conclusion

This post may seem a bit daunting, but I tried to explain a lot of things together. The reality is that now Go modules is basically transparent. We import package like always in our code and the go command will take care of the rest.

When we build something, the dependencies will be fetched automatically. It also eliminates the need to use $GOPATH which was a roadblock for new Go developers who had trouble understanding why things had to go into a specific directory.

Vendoring is (unofficially) being deprecated in favour of using proxies.[^0] I may do a separate post about the Go module proxy. (Update: it’s live.) [^0]: I think this came out a bit too strong and people left with the impression that vendoring is being removed right now. It isn’t. Vendoring still works, albeit slightly different than before. There seems to be a desire to replace vendoring with something better, which may or may not be a proxy. But for now this is just it: a desire for a better solution. Vendoring is not going away until a good replacement is found (if ever.)

Update: much of this article has been rendered obsolete by changes made to Go modules since. Check this more recent post that’s up to date.

Had some free time in my hands, so I decided to check out the new Go modules. For those unaware, the next Go release (1.11) will include a new functionality that aims at addressing package management called “modules”. It will still be marked as experimental, so things may very well change a lot.

Here’s how it works. Let’s say we create a new package:

rselbach@wile ~/code $ mkdir foobar
rselbach@wile ~/code $ cd foobar/

Our foobar.go will be very simple:

package foobar

import (
"fmt"
"io"

"github.com/pkg/errors"
)

func WriteGreet(w io.Writer, name string) error {
if _, err := fmt.Fprintln(w, "Hello", name); err != nil {
return errors.Wrapf(err, "Could not greet %s", name)
}

return nil
}

Notice we’re using Dave Cheney’s excellent errors package. Until now, go get would fetch whatever it found on that package’s repository. If Dave ever decided to change something drastic in the package, our code would probably break without us even knowing about it until too late.

That’s where Go modules come into play, as it will help us (1) formalize the dependency and (2) lock a particular version of it to our code. First we need to turn on the support for modules in our package. We do this by using the new go mod command and giving out module a name:

$ go mod -init -module github.com/rselbach/foobar
go: creating new go.mod: module github.com/rselbach/foobar

After that, you will notice that a new file called go.mod will be created in our package root. For now, it only contains the module name we gave it above:

module github.com/rselbach/foobar

But it does more than that, for now the go command is aware that we are in a module-aware package and will behave accordingly. See what happens when we first try to build this:

$ go build
go: finding github.com/pkg/errors v0.8.0
go: downloading github.com/pkg/errors v0.8.0

It sees that we are using an external package and then it goes out, finds the latest version of it, downloads it, and adds it to our go.mod file:

module github.com/rselbach/foobar

require github.com/pkg/errors v0.8.0

From now one, whenever someone uses our package, Go will also download version 0.8.0 of Dave’s errors package. If version 0.9.0 completely breaks compatibility, it will not break our code for it will continue to be compiled with correct version.

If we change go.mod to require version 0.7.0, our next go build will fetch it as well:

$ go build
go: finding github.com/pkg/errors v0.7.0
go: downloading github.com/pkg/errors v0.7.0

If you’re wondering where the packages are, they’re stored under $GOPATH/src/mod:

$ ls -l ~/go/src/mod/github.com/pkg/
total 0
dr-xr-xr-x 13 rselbach staff 416 20 Jul 07:44 errors@v0.7.0
dr-xr-xr-x 14 rselbach staff 448 20 Jul 07:40 errors@v0.8.0

What about vendoring?

Good thing you asked. This is where I don’t like Go modules’ approach. It specifically aims at doing away with vendoring. Versioning is obviously taken care of by the modules functionality itself while disponibility is supposed to be taken care of by something like caching proxies.

I don’t see it. First of all, not everybody is Google. Maintaining cache proxies is extra work that many organizations don’t have the resources for. Also, what about when we’re working from home? Or on the commute? Sure, a VPN solves this, but then it’s one more thing we need to maintain.

That doesn’t mean you can’t vendor with Go modules. It’s simply not the default way it works. Let’s see how it works though. Let’s say we want to vendor our dependencies:

$ go mod -vendor
rselbach@wile ~/code/foobar $ ls -la
total 24
drwxr-xr-x 6 rselbach staff 192 20 Jul 08:01 .
drwxr-xr-x 33 rselbach staff 1056 20 Jul 07:14 ..
-rw-r--r-- 1 rselbach staff 252 20 Jul 07:22 foobar.go
-rw-r--r-- 1 rselbach staff 72 20 Jul 07:43 go.mod
-rw-r--r-- 1 rselbach staff 322 20 Jul 07:44 go.sum
drwxr-xr-x 4 rselbach staff 128 20 Jul 08:01 vendor

Notice how go mod now has created a vendor directory in our module root. Inside it you’ll find the source code for the modules we are using. It also contains a file with a list of packages and versions:

$ cat vendor/modules.txt
# github.com/pkg/errors v0.7.0
github.com/pkg/errors

We can now add this to our repository. But it’s not all done. Surprisingly, when modules support is enabled, the go command will ignore our vendor directory completely. In order to actually use our vendor directory, we need to explicitely tell go build to use it

go build -v -getmode=vendor

This essentially emulates the behaviour of previous version of Go.