Understanding Go: GO111MODULE

GO111MODULE is a setting in Go programming language that controls the behavior of Go modules, which is a feature added in Go version 1.11 to help manage dependencies in Go projects.

Before the introduction of Go modules, Go relied on the GOPATH environment variable to manage dependencies. In this model, all Go packages and their dependencies were stored in a single directory called GOPATH. This approach was simple, but it led to several problems, including version conflicts, the need to manually manage dependencies, and difficulties in sharing code.

Go modules were introduced to address these issues and provide a better way to manage dependencies. A module is a collection of Go packages that are versioned together, with a specific version defined by a unique module path and a semantic version number.

When a Go project is initialized with go mod init, it creates a go.mod file that specifies the project’s dependencies and their versions. The go.mod file is then used to download and manage the dependencies when building the project.

Modules are identified by a module path: A module path is a unique identifier for a module that consists of a domain name and a module name. For example, github.com/golang/go is the module path for the Go standard library. When you import a package in your project, the module system uses the module path to locate the package and its dependencies.

GO111MODULE is an environment variable that controls the behavior of the Go module system. The Go module system is a feature introduced in Go version 1.11 to manage dependencies and versioning of packages in a Go project.

The GO111MODULE environment variable has three possible values:

1. GO111MODULE=off: This value disables the Go module system and uses the legacy GOPATH mode instead. In this mode, dependencies are downloaded and stored in the GOPATH directory, and the go command looks for packages in the directories specified by the GOPATH environment variable.
2. GO111MODULE=on: This value enables the Go module system and uses modules to manage dependencies. In this mode, the go command looks for the go.mod file in the project directory to determine the required dependencies and their versions. If the file exists, the command downloads the required dependencies and stores them in a local cache, which can be shared between projects.
3. GO111MODULE=auto: This value enables the Go module system if a go.mod file is present in the project directory. If no go.mod file is found, the legacy GOPATH mode is used.

GO111MODULE is usually set as an environment variable before running the Go command. For example, you can set it as follows:

export GO111MODULE=on

Or, you can set it only for a specific command:

GO111MODULE=on go build

Using the Go module system allows developers to manage their project dependencies in a more organized and efficient way. By specifying the required packages and versions in the go.mod file, the module system ensures that the project can be built consistently across different environments.