golang-design-patterns

Adapted from samber/cc-skills-golang@golang-design-patterns. Pair with golang-common-practice for shared verification guidance.

Persona: You are a Go architect who values simplicity and explicitness. You apply patterns only when they solve a real problem — not to demonstrate sophistication — and you push back on premature abstraction.

Modes:

• Design mode — creating new APIs, packages, or application structure: ask the developer about their architecture preference before proposing patterns; favor the smallest pattern that satisfies the requirement.
• Review mode — auditing existing code for design issues: scan for init() abuse, unbounded resources, missing timeouts, and implicit global state; report findings before suggesting refactors.

Go Design Patterns & Idioms

Idiomatic Go patterns for production-ready code. For error handling details see the golang-error-handling skill; for context propagation see golang-context skill; for struct/interface design see golang-structs-interfaces skill.

Best Practices Summary

1. Constructors SHOULD use functional options — they scale better as APIs evolve (one function per option, no breaking changes)
2. Functional options MUST return an error if validation can fail — catch bad config at construction, not at runtime
3. Avoid `init()` — runs implicitly, cannot return errors, makes testing unpredictable. Use explicit constructors
4. Enums SHOULD start at 1 (or Unknown sentinel at 0) — Go's zero value silently passes as the first enum member
5. Error cases MUST be handled first with early return — keep happy path flat
6. Panic is for bugs, not expected errors — callers can handle returned errors; panics crash the process
7. `defer Close()` immediately after opening — later code changes can accidentally skip cleanup
8. `runtime.AddCleanup` over runtime.SetFinalizer — finalizers are unpredictable and can resurrect objects
9. Every external call SHOULD have a timeout — a slow upstream hangs your goroutine indefinitely
10. Limit everything (pool sizes, queue depths, buffers) — unbounded resources grow until they crash
11. Retry logic MUST check context cancellation between attempts
12. Use `strings.Builder` for concatenation in loops → see golang-code-style
13. string vs []byte: use `[]byte` for mutation and I/O, string for display and keys — conversions allocate
14. Iterators (Go 1.23+): use for lazy evaluation — avoid loading everything into memory
15. Stream large transfers — loading millions of rows causes OOM; stream keeps memory constant
16. //go:embed for static assets — embeds at compile time, eliminates runtime file I/O errors
17. Use `crypto/rand` for keys/tokens — math/rand is predictable → see golang-security
18. Regexp MUST be compiled once at package level — compilation is O(n) and allocates
19. Compile-time interface checks: *`var _ Interface = (Type)(nil)`**
20. A little recode > a big dependency — each dep adds attack surface and maintenance burden
21. Design for testability — accept interfaces, inject dependencies

Constructor Patterns: Functional Options vs Builder

Functional Options (Preferred)

type Server struct {
    addr         string
    readTimeout  time.Duration
    writeTimeout time.Duration
    maxConns     int
}

type Option func(*Server)

func WithReadTimeout(d time.Duration) Option {
    return func(s *Server) { s.readTimeout = d }
}

func WithWriteTimeout(d time.Duration) Option {
    return func(s *Server) { s.writeTimeout = d }
}

func WithMaxConns(n int) Option {
    return func(s *Server) { s.maxConns = n }
}

func NewServer(addr string, opts ...Option) *Server {
    // Default options
    s := &Server{
        addr:         addr,
        readTimeout:  5 * time.Second,
        writeTimeout: 10 * time.Second,
        maxConns:     100,
    }
    for _, opt := range opts {
        opt(s)
    }
    return s
}

// Usage
srv := NewServer(":8080",
    WithReadTimeout(30*time.Second),
    WithMaxConns(500),
)

Constructors SHOULD use functional options — they scale better with API evolution and require less code. Use builder pattern only if you need complex validation between configuration steps.

Constructors & Initialization

Avoid init() and Mutable Globals

init() runs implicitly, makes testing harder, and creates hidden dependencies:

• Multiple init() functions run in declaration order, across files in filename alphabetical order — fragile
• Cannot return errors — failures must panic or log.Fatal
• Runs before main() and tests — side effects make tests unpredictable

// Bad — hidden global state
var db *sql.DB

func init() {
    var err error
    db, err = sql.Open("postgres", os.Getenv("DATABASE_URL"))
    if err != nil {
        log.Fatal(err)
    }
}

// Good — explicit initialization, injectable
func NewUserRepository(db *sql.DB) *UserRepository {
    return &UserRepository{db: db}
}

Enums: Start at 1

Zero values should represent invalid/unset state:

type Status int

const (
    StatusUnknown Status = iota // 0 = invalid/unset
    StatusActive                // 1
    StatusInactive              // 2
    StatusSuspended             // 3
)

Compile Regexp Once

// Good — compiled once at package level
var emailRegex = regexp.MustCompile(`^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`)

func ValidateEmail(email string) bool {
    return emailRegex.MatchString(email)
}

Use //go:embed for Static Assets

import "embed"

//go:embed templates/*
var templateFS embed.FS

//go:embed version.txt
var version string

Compile-Time Interface Checks

→ See golang-structs-interfaces for the var _ Interface = (*Type)(nil) pattern.

Error Flow Patterns

Error cases MUST be handled first with early return — keep the happy path at minimal indentation. → See golang-code-style for the full pattern and examples.

When to Panic vs Return Error

• Return error: network failures, file not found, invalid input — anything a caller can handle
• Panic: nil pointer in a place that should be impossible, violated invariant, Must* constructors used at init time
• `.Close()` / `Flush()` errors: read-only cleanup can often use defer f.Close(), but write/flush resources must report close or flush errors when durability matters

Data Handling

string vs []byte vs []rune

Avoid repeated conversions — each one allocates. Stay in one type until you need the other.

Iterators & Streaming for Large Data

Use iterators (Go 1.23+) and streaming patterns to process large datasets without loading everything into memory. For large transfers between services (e.g., 1M rows DB to HTTP), stream to prevent OOM.

For code examples, see Data Handling Patterns.

Resource Management

defer Close() immediately after opening — don't wait, don't forget:

f, err := os.Open(path)
if err != nil {
    return err
}
defer f.Close() // right here, not 50 lines later

rows, err := db.QueryContext(ctx, query)
if err != nil {
    return err
}
defer rows.Close()

For graceful shutdown, resource pools, and runtime.AddCleanup, see Resource Management.

Resilience & Limits

Timeout Every External Call

ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
defer cancel()

resp, err := httpClient.Do(req.WithContext(ctx))

Retry & Context Checks

Retry logic MUST check ctx.Err() between attempts and use exponential/linear backoff via select on ctx.Done(). Long loops MUST check ctx.Err() periodically. → See golang-context skill.

Database Patterns

→ See golang-database skill for sqlx/pgx, transactions, nullable columns, connection pools, repository interfaces, testing.

Architecture

Ask the developer which architecture they prefer: clean architecture, hexagonal, DDD, or flat layout. Don't impose complex architecture on a small project.

Architecture style and navigability: Consider vertical slices when they keep related feature behavior easier to discover, but preserve useful boundaries and existing repository conventions. Agent involvement alone does not justify changing architecture. Review cohesion as files grow; roughly 500 lines is a heuristic, not a universal limit. The references below describe alternatives and their trade-offs.

Core principles regardless of architecture:

• Keep domain pure — no framework dependencies in the domain layer
• Fail fast — validate at boundaries, trust internal code
• Make illegal states unrepresentable — prefer type-level enforcement where it can prevent incorrect usage; document and validate the constraints that types cannot express. This helps human and agent callers without assuming a measured distribution of failures.
• Respect 12-factor app principles

Detailed Guides

Code Philosophy

• Locality and DRY — weigh discoverability against duplication. Extract when copies represent the same rule and should change together; preserve separate code when similar shapes have different responsibilities. Copy count alone does not establish the right abstraction.
• Ceremony-to-logic ratio — avoid structural boilerplate that adds navigation without hiding useful complexity. Judge the pattern against representative project changes; a language-wide ratio or file-count threshold does not establish agent usability.
• Minimize dependencies — a little recode > a big dependency
• Design for testability — accept interfaces, inject dependencies, keep functions pure. Caveat: extract an interface only when there are 2+ implementations or a genuine testability requirement; a single-implementation interface is a zero-information token that increases ceremony without adding navigational value (→ see golang-structs-interfaces for the full rule).

Cross-References

• → See golang-data-structures skill for data structure selection, internals, and container/ packages
• → See golang-error-handling skill for error wrapping, sentinel errors, and the single handling rule
• → See golang-structs-interfaces skill for interface design and composition
• → See golang-concurrency skill for goroutine lifecycle and graceful shutdown
• → See golang-context skill for timeout and cancellation patterns