Overview

Module is go support for dependency management. A module by definition is a collection of related packages with go.mod at its root.  The go.mod file defines the

• Module import path

• Dependency requirements of the module for a successful build. It defines both module’s dependencies requirement and also locks them to their correct version.

A dependency of a module can be of two kinds

• Direct -A direct dependency is a dependency which the module directly imports.

• Indirect – It is the dependency that is imported by the module’s direct dependencies. Also, any dependency that is mentioned in the go.mod file but not imported in any of the source files of the module is also treated as an indirect dependency.

Let’s explore some ways of adding dependency to your project

• Directly adding it to the go.mod file
• Do a go get
• Add the dependency to your source code and do a go mod tidy

Before looking at each of the ways, again let’s create a module first

go mod init learn

This command will create a go.mod file in the same directory.  Since it is an empty module it doesn’t have any direct dependency specified yet. Now let’s explore different ways of adding a dependency

Directly adding it to the go.mod file

We can  add direct dependency to the go.mod file too. Let’s do that. Add below dependency to the go.mod file

require github.com/pborman/uuid v1.2.1

With this dependency go.mod file will look like below

module learn

go 1.14

require github.com/pborman/uuid v1.2.1

Now we need to download the newly added dependency as well. Fo that we can use the below command

go mod download

This command will download the github.com/pborman/uuid module as well all its dependencies. Also it will update the go.sum file with the checksum and version of all direct and indirect dependencies. go build as well as go install also will download the dependencies and also build the binary. go run will also download and and run the binary as well. go mod download command is used when you want to pre download the dependencies without build or running it.

Do a go get

Simply doing a go get will also the add the dependency in the go.mod file. Remove the uuid dependency we added above from go.mod file and clean up go.sum file. Now run below command

export GO111MODULE=on
go get github.com/pborman/uuid

Now check the contents of go.mod file. Do a cat go.mod

module learn

go 1.14

require github.com/pborman/uuid v1.2.1 //indirect

The dependency will be marked as //indirect as it is not being used in any of the source files. Once you do a go build after using this in the source files, the //indirect will be removed automatically by go. Also it will update the go.sum file with the checksum and version of all direct and indirect dependencies.

Add the dependency to your source code and do a go mod tidy

This method we already saw in the above example.  Basically, go mod tidy command makes sure that your go.mod files reflect the dependencies that you have actually used in your project. When we run go mod tidy command then it will do two things

• Add any dependency which is imported in the source files

• Remove any dependency which is mentioned in the go.mod file but not imported in any of the source files.

Let’s see an example. Remove the dependency we added earlier in the go.mod file. Your go.mod file should like below

module learn

go 1.14

Also let’s create a file named uuid.go in the same directory with below contents

uuid.go

package main

import (
	"fmt"
	"strings"

	"github.com/pborman/uuid"
)

func main() {
	uuidWithHyphen := uuid.NewRandom()
	uuid := strings.Replace(uuidWithHyphen.String(), "-", "", -1)
	fmt.Println(uuid)
}

Notice that we have imported the dependency in the uuid.go as well

"github.com/pborman/uuid"

Let’s run the below command

go mod tidy

This command will download all the dependencies that are required in your source files and update go.mod file with that dependency. After running this command let’s now let’s again examine the contents of go.mod file

Do a cat go.mod

module learn

go 1.14

require github.com/pborman/uuid v1.2.1

It lists direct dependency which was specified in the uuid file along with exact version of the dependency as well. Now let’s check the go.sum file as well. Do a cat go.sum

github.com/google/uuid v1.0.0 h1:b4Gk+7WdP/d3HZH8EJsZpvV7EtDOgaZLtnaNGIu1adA=
github.com/google/uuid v1.0.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
github.com/pborman/uuid v1.2.1 h1:+ZZIw58t/ozdjRaXh/3awHfmWRbzYxJoAdNJxe/3pvw=
github.com/pborman/uuid v1.2.1/go.mod h1:X/NO0urCmaxf9VXbdlT7C2Yzkj2IKimNn4k+gtPdI/k=

Now let’s see one more example where go mod tidy will remove the dependency from go.mod file if it is not required in the source files. To illustrate this let’s remove the uuid.go file that we created above. Now run the command

go mod tidy -v

It will give below output

unused github.com/pborman/uuid

Now examine the contents of go.mod file.  It will be as below

module learn

go 1.14

The

require github.com/pborman/uuid v1.2.1

line will be removed as it is not required in any of the source files. Also all entries of github.com/pborman/uuid and its depedencies will be removed from the go.sum file as well.