What is the Go module system? What do go.mod and go.sum contain?

Modules are Go's dependency management system (stable since Go 1.13). A module is a collection of related packages identified by a module path (typically a repository URL). The module's root contains a go.mod file that records the module path, the minimum Go version, and all required dependencies.

// Initialise a module
// $ go mod init github.com/alice/myapp

// go.mod — human-editable; commit to version control
// ─────────────────────────────────────────────────
// module github.com/alice/myapp
//
// go 1.22
//
// require (
//     github.com/gin-gonic/gin v1.9.1
//     golang.org/x/sync        v0.6.0
// )

// go.sum — machine-managed; commit to version control
// Contains SHA-256 hashes of each downloaded module zip
// Guarantees tamper-proof, reproducible builds

// Key commands:
// go get github.com/pkg@v1.2.3  — add / upgrade dependency
// go mod tidy                    — remove unused, add missing deps
// go mod download                — pre-download all deps
// go list -m all                 — list entire dependency graph

// Minimum Version Selection (MVS):
// Go ALWAYS picks the minimum version that satisfies all requirements.
// Nothing upgrades silently — builds are reproducible.

// Major version imports (breaking changes need new import path):
// import "github.com/alice/pkg/v2"  — v2 has breaking API changes

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