Go Language Modularization

Internal

• Go Language

Overview

Go modularization builds upon the concepts of package and module. Packages provide a namespace for their members, and they are a way to encapsulate code, hide implementation details and only expose features, such as variables, functions or type definitions that are meant to be publicly consumed. Packages can be published as part of modules. Modules have been introduced in Go 1.11.

Packages

Packages

Standard Library

Go comes with a set of over 100 "built-in" packages, which are available as part of the locally installed Go development environment.

Standard library package documentation is available online here:

https://pkg.go.dev/std

The standard library is a good source of code examples, comments and style guidance.

Standard library packages:

archive	atomic	bytes	container	database	encoding	errors
filepath	flag	fmt	io	ioutil	encoding/json	hash
log	math	net	net/http	os	path	rand
reflect	regexp	runtime	slices	sort	strings	strconv
sync	text/template	testing	time	unicode

pkg.go.dev

The place to look for published third-party packages is

https://pkg.go.dev

Modules

Modules

Module-Aware or GOPATH Mode

https://go.dev/ref/mod#mod-commands

The compiler must locate packages on the local file system every time it handles an import statement.

The go tool has two modes of resolving package dependencies: module-aware mode or GOPATH mode.

In module-aware mode, the go commands use go.mod files to find versioned dependencies and typically load packages out of the module cache, downloading modules if they are missing. As of Go 1.16, the module-aware mode is enabled by default, regardless of whether go.mod is present or not. The behavior can be controlled with the GO111MODULE environment variable.

In GOPATH mode, go commands use the value of the GOPATH environment variable and vendor directories to resolve packages.

Also see:

GoLand Module-Aware or GOPATH Mode

GOPATH

Workspace

This section needs refactoring after reading:

• Tutorial: Getting started with multi-module workspaces https://go.dev/doc/tutorial/workspaces
• Get familiar with workspaces https://go.dev/blog/get-familiar-with-workspaces

The workspace is a concept introduced in Go 1.18. A workspace allows organizing the code for a project that has several modules which share a common list of dependencies. The workspace maintains metadata, especially dependency metadata, in a file called go.work. The dependencies declared in this file can span modules and anything declared in go.work will override dependencies in the modules's go.mod. The packages and modules maintained in a workspace are managed with the go tool.

A workspace may contain multiple projects.

The standard workspace layout is:

. ← GOPATH should point to this directory, it contains src, pkg and bin
│
├─ src 
│   ├─ a 
│   │  └─ b
│   │     └─color  # "color" package directory, with the "a/b/color" import path
│   │        ├─ colors.go 
│   │        ├─ aux.go 
│   │        └─ ... 
│   │
│   ├─ weight  # "weight" package directory, with the "weight" import path
│   │   ├─ weights.go 
│   │   ├─ aux.go 
│   │   └─ ... 
│   │
│   ├─ novaordis.com
│   │   └─ tools
│   │       └─ hammer # "hammer" package directory, with the "novaordis.com/tools/hammer" import path
│   │           └─ ... 
│   │
│   └─ github.com
│       └─ blue-org
│           └─ tools
│               └─ wrench # "wrench" package directory, with the "github.com/blue-org/tools/wrench" import path
│                   ├─  .git
│                   └─ ... 
│ 
├─ pkg 
│   └─ darwin_amd64 
│       ├─ weights.go 
│       ├─ a/b/color.a 
│       ├─ novaordis.com/tools/hammer.a
│       └─ github.com/bue-org/tools/wrench.a
└─ bin

Content

src

The src subdirectory holds source code. Each package resides in a directory whose name relative to ${GOPATH}/src represents the package's import path.

The src subdirectory may contain multiple version-control repository workareas.

pkg

The build tool stores compiled packages in the pkg directory, under ${GOOS}_${GOARCH} subdirectories.

bin

The bin directory is where the executables are stored.

vendor

TODO: expand this and link to Go_Packages#Vendoring.

Relationship between Workspace and GOPATH

GOPATH should point to the root of the workspace, the directory that contains src, pkg and bin. Further research is required.

Microservice-based Project Layout

Microservices in Go | Project Layout

Program

Go programs are constructed by linking together packages. There must be a main package, which contains the main(), to trigger the linker.

Project

Project

Repository

A Go repository typically contains only one module, located in the root of the repository. Repository may contain more than one module.

Packages, Modules, Projects and Repositories

Designing your project, to live in its own repository and host a single module in its root directory will help keep maintenance simpler, particularly over time as you publish new versions.

It is possible to maintain more than one module in a project and repository TODO: https://go.dev/doc/modules/managing-source#multiple-module-source.