Go Architecture & Domain Modeling

Targets Go 1.26. See STACK.md for pinned dependency versions.

1. Constants & Enums

• Enums: Custom types with iota and an implemented String() method. Avoid bare primitives. Use 1 << iota for bitmasks.
• Errors: Export package-level sentinel errors (var ErrNotFound = ...) for errors.Is(). Use errors.Join to aggregate multiple errors.
• Grouping: When a file declares multiple type, const, or var at package level, consolidate each kind into a single parenthesized block (type (...), const (...), var (...)) rather than repeating the keyword. Keep unrelated groups separated by a blank line inside the block.

2. Structures & Memory

• Design: Explicit struct tags, standard audit fields, composition via embedding.
• Optimization: Order fields largest → smallest to minimize padding.
• Semantics: Pointers for mutation or mutex-protected state; values for small, immutable payloads.
• Validation: Validate() error for structs taking external input; tag-based validation via go-playground/validator.
• Ordering: Use cmp (cmp.Compare[T], cmp.Or, cmp.Less) as the default ordering toolkit.

3. Instantiation

• Constructors: Always provide New... to initialize maps, slices, channels safely.
• Configuration: Functional Options pattern for complex setup; never large config structs passed by value.

4. Interfaces

• Design: Define interfaces where they're used, not where implemented. Keep them small (1–3 methods).
• Signatures: Accept interfaces (for easy mocking); return concrete structs.

5. Stdlib defaults

Prefer stdlib over hand-rolled or third-party when stdlib now covers it.

• slices, maps — collection helpers (Sort, Contains, Index, Clone, Equal, Concat, etc.)
• cmp — Compare, Less, Or, Ordered — the default ordering toolkit.
• iter — iter.Seq[T], iter.Seq2[K,V] for sequence-shaped returns (see §6).
• log/slog — structured logging; default for new code. Reach for zap/zerolog only when slog's allocator perf is provably insufficient.
• errors — errors.Is, errors.As, errors.Join.

6. Iterators (Go 1.23+)

• Range-over-func: for x := range seq { ... } where seq is iter.Seq[T].
• Return `iter.Seq[T]` when: the sequence is large/unknown-size, consumer might short-circuit, or it's backed by an underlying cursor (DB pagination, HTTP pages).
• Return `[]T` when: the result is small, bounded, and the caller almost always wants the whole thing.
• Anti-pattern: wrapping a []T in slices.Values just to look modern. Round-tripping back to slices.Collect is wasted ceremony.

7. Concurrency

• State: Embed sync.RWMutex directly above the fields it protects. sync/atomic for simple counters.
• WaitGroup: Use sync.WaitGroup.Go(fn) (Go 1.25+) — replaces Add(1) / go func() { defer Done(); ... }() boilerplate.
• Context: Pass context.Context as the first parameter for any blocking or I/O. Use signal.NotifyContext for shutdown; the cancellation cause now records which signal fired (Go 1.26).
• Containers: GOMAXPROCS respects cgroup CPU limits since Go 1.25 — don't hardcode in containerized deploys.

8. Errors & Panics

• Checking: errors.As for custom types; errors.Is for sentinels.
• Aggregation: errors.Join(errs...) when collecting multiple errors (validation, parallel fan-out).
• Wrapping: fmt.Errorf("op: %w", err) to add context while preserving the chain.
• Panics: Restrict to initialization (MustCompile style). Never as control flow.

9. Testing

• Patterns: Table-driven tests and black-box testing (package mypkg_test).
• Helpers: t.Helper() as the first line in any custom assertion.
• Concurrent code: Use testing/synctest (GA in Go 1.25) — virtualizes time, waits for goroutines to quiesce, makes async tests deterministic.
• Library: stretchr/testify for assertions/mocks/suites when stdlib alone is awkward.

10. Packages

• Naming: Short, lowercase, single-word. Never util, common, helpers.
• Layout: Organize by domain/feature, not by technical layer.

11. Dependencies & Logging

• Injection: Pass dependencies via constructors. No global state, no package init() setups. For larger graphs use uber-go/fx or google/wire.
• Config: spf13/viper for layered env + file + flag configuration.
• Logging: log/slog (stdlib). Structured, context-aware. zap/zerolog only when slog is provably the bottleneck — start with slog.

12. Database access — SQL files + //go:embed

Strong preference: raw SQL in `.sql` files, embedded at compile time, executed via jmoiron/sqlx. Avoids ORM magic, keeps queries auditable in git, gives editors full SQL syntax highlighting and linting.

//go:embed queries/get_user_by_id.sql
var getUserByIDSQL string

func (r *UserRepo) GetByID(ctx context.Context, id int64) (User, error) {
    var u User
    err := r.db.GetContext(ctx, &u, getUserByIDSQL, id)
    return u, err
}

Layout:

internal/userrepo/
├── repo.go
└── queries/
    ├── get_user_by_id.sql
    ├── insert_user.sql
    └── list_users.sql

• Migrations: golang-migrate — versioned up/down pairs in migrations/.
• Dynamic queries: If a query needs runtime composition (optional filters), still write the static fragments in .sql files and join them in Go. Avoid building SQL by string concatenation against user input — use parameter binding.

13. Generics

• Usage: Data structures and utility functions only. If an interface would serve, prefer it.
• Self-referential constraints (Go 1.26): type Adder[A Adder[A]] interface { Add(A) A } is valid — useful for fluent-style domain types.
• Type aliases with type params (Go 1.24): type IntMap[V any] = map[int]V.

14. Tooling

• Lint: golangci-lint v2 — a single binary that aggregates staticcheck, govet, gofmt, goimports, revive, and dozens more. Pin via .golangci.yml (v2 config schema). Run on every commit and in CI. Treat warnings as errors. Drop-in template: assets/golangci.yml — copy to your project root as .golangci.yml and set goimports.local-prefixes to your module path.
• Modernization: Run go fix periodically (revamped in Go 1.26 as push-button modernizers). It rewrites legacy patterns toward current stdlib APIs and respects //go:fix inline directives for local refactors. Ongoing hygiene, not a one-off.
• Format: gofmt (built-in) plus goimports (via golangci-lint) — no other formatter, no debate.
• Build/run: go build, go test, go vet, go work for multi-module repos. Stdlib only — avoid Make/Task wrappers unless multi-language CI demands it.

15. Documentation

• Format: Exported names get full-sentence comments starting with the identifier name. Include a package-level comment.
• Style: Weave parameter and return names into prose (no @param). Document specific error conditions.
• Focus: Explain why (business rules, edge cases) — code already shows what.

Canonical libraries

See STACK.md for the full pinned list — viper, validator, cobra/pflag, gin, sqlx, grpc, protobuf-go, testify, fx, golang-migrate, buf.