Go Project Layout

Structure follows size. The biggest layout mistake in Go is copying a microservice skeleton for a 500-line tool — or growing a 50-package service inside a flat directory. Match the layout to the project.

1. Pick the Layout by Project Size

Never start with empty pkg/, api/, docs/, build/ directories "for later". Add structure when the code demands it, not before.

2. Module Naming

# ✅ Good — repository path, lowercase
go mod init github.com/acme/payment-service

# ❌ Bad — not fetchable, uppercase, or vanity without DNS
go mod init PaymentService
go mod init payment_service

The last path element should match what users will see: for a library, it becomes the default import name.

3. Service Layout (the default for APIs and workers)

payment-service/
├── cmd/
│   └── payment-api/
│       └── main.go         # flag/env parsing, wiring, Run() — nothing else
├── internal/
│   ├── domain/             # core types, business rules; zero external deps
│   ├── service/            # use cases orchestrating domain + stores
│   ├── store/              # data access implementations (postgres/, redis/)
│   ├── handler/            # HTTP/gRPC adapters
│   └── config/             # config loading and validation
├── migrations/             # if the service owns a database
├── go.mod
├── Makefile
└── README.md

Rules:

• internal/ by default — the compiler enforces that nobody outside the module imports it. Promote to a public package only on demand.
• pkg/ only when external consumers exist AND the module also has private code. When in doubt, don't create it.
• Dependencies point inward: handler → service → domain ← store. domain imports neither store nor handler.

4. Thin main, Runnable Run

Keep main.go to wiring plus a delegating call, so the app is testable:

func main() {
    if err := run(context.Background(), os.Args[1:], os.Getenv); err != nil {
        fmt.Fprintln(os.Stderr, err)
        os.Exit(1)
    }
}

func run(ctx context.Context, args []string, getenv func(string) string) error {
    cfg, err := config.Load(getenv)
    if err != nil {
        return fmt.Errorf("load config: %w", err)
    }

    db, err := store.Open(ctx, cfg.DatabaseURL)
    if err != nil {
        return fmt.Errorf("open db: %w", err)
    }
    defer db.Close()

    svc := service.New(store.NewUserRepo(db))
    srv := handler.NewServer(cfg.Addr, svc)
    return srv.ListenAndServe(ctx)
}

• os.Exit appears exactly once, in main.
• run takes its dependencies (args, getenv) so tests can call it.
• No init() functions for wiring — explicit construction order only.

5. Library Layout

retry/
├── retry.go            # package retry — the API, in the root
├── retry_test.go
├── backoff.go          # same package, split by topic
├── internal/
│   └── clock/          # implementation details users must not import
├── examples_test.go    # Example* functions shown in godoc
└── go.mod

• The root directory IS the package. No src/, no lib/.
• One package per concept. Resist util, common, helpers — name packages after what they provide (retry, clock, httpsign).

6. Naming Rules for Directories and Packages

• Package name == directory name, short, lowercase, no underscores: store/postgres, not store/postgres_impl.
• Don't stutter: payment.Service, not payment.PaymentService.
• Binary names in cmd/ are user-facing: cmd/payment-api, hyphenated is fine (directory only holds package main).

7. Files That Belong at the Root

• go.mod, go.sum, README.md, LICENSE, Makefile, .golangci.yml, Dockerfile (single-binary projects).
• Do NOT create: src/ (un-idiomatic), vendor/ (unless the team explicitly vendors), one-file packages like types/ or models/ that become dumping grounds.

Scaffolding Procedure

1. Ask/decide: tool, library, or service? How many binaries?
2. go mod init <repo-path>.
3. Create only the directories the first feature needs.
4. Write main.go with the thin-main pattern above.
5. Add Makefile targets: build, test, lint.
6. Verify: go build ./... and go vet ./... pass on the skeleton.

Verification Checklist

1. Layout matches project size — no empty scaffolding directories
2. Module path is the fetchable repository path
3. All non-public packages live under internal/
4. main.go is thin: parse, wire, call run, exit
5. os.Exit only in main; no wiring in init()
6. Dependencies flow inward; domain has zero infrastructure imports
7. No util/common/helpers/models grab-bag packages
8. Package names match directories, lowercase, no stutter
9. go build ./... passes on the fresh skeleton