Go Advanced Patterns

This skill extends go-patterns with architecture, struct design, memory optimization, and tooling.

When to Activate

• Designing interface hierarchies or choosing between small and large interfaces
• Organizing packages for a Go service (hexagonal layout, naming, dependency direction)
• Designing hexagonal/clean architecture in Go
• Optimizing memory or performance
• Configuring Go tooling (golangci-lint, etc.)
• Using Go 1.21+ slices/maps stdlib
• Reviewing struct design or avoiding anti-patterns
• Structuring a new Go service with clear domain, application, and adapter layers following ports-and-adapters
• Applying functional options or embedding to avoid constructor explosion and over-specified structs
• Replacing hand-rolled slice or map utilities with the Go 1.21+ slices and maps standard library packages

Hexagonal in Go (Ports & Adapters)

Go's core idioms ARE the hexagonal pattern — no ceremony required:

Go Idiom	Hexagonal Concept
Accept interfaces, return structs	Output Port: the interface is the port
Define interfaces where they're used	Port defined in domain/app, not in repository
Constructor injection (NewService(store Store))	Adapter injection
Small, focused interfaces	One interface per capability (not one giant Repository)
internal/ package boundary	Enforced architectural boundary

Domain Model — Zero External Imports

// internal/domain/market.go
package domain

import (
    "errors"
    "strings"
    "time"
)

// Market is the aggregate root.
type Market struct {
    ID        string
    Name      string
    Slug      string
    Status    MarketStatus
    CreatedAt time.Time
}

type MarketStatus string

const (
    MarketStatusDraft  MarketStatus = "DRAFT"
    MarketStatusActive MarketStatus = "ACTIVE"
)

var (
    ErrInvalidMarket      = errors.New("invalid market")
    ErrMarketAlreadyLive  = errors.New("market already published")
)

// NewMarket is the factory — enforces creation invariants.
func NewMarket(name, slug string) (Market, error) {
    if strings.TrimSpace(name) == "" {
        return Market{}, fmt.Errorf("%w: name required", ErrInvalidMarket)
    }
    return Market{Name: name, Slug: slug, Status: MarketStatusDraft, CreatedAt: time.Now()}, nil
}

// Publish is a behavior method — domain logic, not a setter.
func (m Market) Publish() (Market, error) {
    if m.Status != MarketStatusDraft {
        return Market{}, fmt.Errorf("%w: %s", ErrMarketAlreadyLive, m.Slug)
    }
    m.Status = MarketStatusActive
    return m, nil
}

Port Interface — Defined in the Consuming Package

// internal/app/market_service.go
package app

import (
    "context"
    "myproject/internal/domain"
)

// MarketStore is the output port — defined HERE in app/, not in repository/.
// The concrete Postgres implementation satisfies this interface implicitly.
type MarketStore interface {
    Save(ctx context.Context, market domain.Market) (domain.Market, error)
    FindBySlug(ctx context.Context, slug string) (domain.Market, error)
}

// MarketService is the use case — depends on the port interface, not the adapter.
type MarketService struct {
    store MarketStore
}

func NewMarketService(store MarketStore) *MarketService {
    return &MarketService{store: store}
}

func (s *MarketService) Create(ctx context.Context, name, slug string) (domain.Market, error) {
    market, err := domain.NewMarket(name, slug)  // domain logic
    if err != nil {
        return domain.Market{}, fmt.Errorf("create market: %w", err)
    }
    return s.store.Save(ctx, market)
}

func (s *MarketService) Publish(ctx context.Context, slug string) (domain.Market, error) {
    market, err := s.store.FindBySlug(ctx, slug)
    if err != nil {
        return domain.Market{}, fmt.Errorf("publish market: %w", err)
    }
    published, err := market.Publish()  // domain logic
    if err != nil {
        return domain.Market{}, err
    }
    return s.store.Save(ctx, published)
}

Inbound Adapter — HTTP Handler

// internal/handler/market_handler.go
package handler

import (
    "context"
    "encoding/json"
    "net/http"
    "myproject/internal/domain"
)

// MarketUseCase is the input port — handler depends on this interface, not *app.MarketService.
type MarketUseCase interface {
    Create(ctx context.Context, name, slug string) (domain.Market, error)
}

type MarketHandler struct {
    useCase MarketUseCase
}

func NewMarketHandler(useCase MarketUseCase) *MarketHandler {
    return &MarketHandler{useCase: useCase}
}

// ProblemDetails is the RFC 7807 / RFC 9457 error response struct.
// Always use Content-Type: application/problem+json for error responses.
type ProblemDetails struct {
    Type     string `json:"type"`
    Title    string `json:"title"`
    Status   int    `json:"status"`
    Detail   string `json:"detail,omitempty"`
    Instance string `json:"instance,omitempty"`
}

func writeProblem(w http.ResponseWriter, r *http.Request, status int, problemType, title, detail string) {
    p := ProblemDetails{Type: problemType, Title: title, Status: status, Detail: detail, Instance: r.RequestURI}
    w.Header().Set("Content-Type", "application/problem+json")
    w.WriteHeader(status)
    _ = json.NewEncoder(w).Encode(p)
}

func (h *MarketHandler) Create(w http.ResponseWriter, r *http.Request) {
    var req struct {
        Name string `json:"name"`
        Slug string `json:"slug"`
    }
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        writeProblem(w, r, http.StatusBadRequest,
            "https://api.example.com/problems/bad-request", "Bad Request",
            "Request body could not be parsed.")
        return
    }

    market, err := h.useCase.Create(r.Context(), req.Name, req.Slug)
    if errors.Is(err, domain.ErrInvalidMarket) {
        writeProblem(w, r, http.StatusUnprocessableEntity,
            "https://api.example.com/problems/validation-failed", "Validation Failed",
            err.Error())
        return
    }
    if err != nil {
        writeProblem(w, r, http.StatusInternalServerError, "about:blank", "Internal Server Error", "")
        return
    }

    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(http.StatusCreated)
    json.NewEncoder(w).Encode(market)
}

Outbound Adapter — Postgres Repository

// internal/repository/market_repo.go
package repository

import (
    "context"
    "database/sql"
    "myproject/internal/domain"
)

// PostgresMarketRepo satisfies app.MarketStore implicitly — no 'implements' declaration.
type PostgresMarketRepo struct {
    db *sql.DB
}

func NewPostgresMarketRepo(db *sql.DB) *PostgresMarketRepo {
    return &PostgresMarketRepo{db: db}
}

func (r *PostgresMarketRepo) Save(ctx context.Context, market domain.Market) (domain.Market, error) {
    _, err := r.db.ExecContext(ctx,
        `INSERT INTO markets (id, name, slug, status) VALUES ($1, $2, $3, $4)
         ON CONFLICT (slug) DO UPDATE SET name=$2, status=$4`,
        market.ID, market.Name, market.Slug, market.Status,
    )
    if err != nil {
        return domain.Market{}, fmt.Errorf("save market: %w", err)
    }
    return market, nil
}

func (r *PostgresMarketRepo) FindBySlug(ctx context.Context, slug string) (domain.Market, error) {
    var m domain.Market
    err := r.db.QueryRowContext(ctx,
        `SELECT id, name, slug, status FROM markets WHERE slug = $1`, slug,
    ).Scan(&m.ID, &m.Name, &m.Slug, &m.Status)
    if err == sql.ErrNoRows {
        return domain.Market{}, fmt.Errorf("market %s: %w", slug, domain.ErrNotFound)
    }
    if err != nil {
        return domain.Market{}, fmt.Errorf("find market: %w", err)
    }
    return m, nil
}

DI Wiring in main.go

// cmd/myapp/main.go
package main

func main() {
    db := mustOpenDB(os.Getenv("DATABASE_URL"))

    // Outbound adapters (implement port interfaces defined in app/)
    marketRepo := repository.NewPostgresMarketRepo(db)

    // Use cases (depend on port interfaces, not concrete adapters)
    marketSvc := app.NewMarketService(marketRepo)

    // Inbound adapters (depend on use case interfaces)
    marketHandler := handler.NewMarketHandler(marketSvc)

    mux := http.NewServeMux()
    mux.HandleFunc("POST /api/markets", marketHandler.Create)

    log.Fatal(http.ListenAndServe(":8080", mux))
}

Testing Use Cases (Mock the Port Interface)

// internal/app/market_service_test.go
package app_test

import (
    "context"
    "testing"
    "myproject/internal/app"
    "myproject/internal/domain"
)

// Inline mock — no mocking library needed thanks to Go interfaces
type mockMarketStore struct {
    saved []domain.Market
}

func (m *mockMarketStore) Save(ctx context.Context, market domain.Market) (domain.Market, error) {
    m.saved = append(m.saved, market)
    return market, nil
}

func (m *mockMarketStore) FindBySlug(ctx context.Context, slug string) (domain.Market, error) {
    return domain.Market{}, domain.ErrNotFound
}

func TestMarketService_Create(t *testing.T) {
    store := &mockMarketStore{}
    svc := app.NewMarketService(store)

    market, err := svc.Create(context.Background(), "Test Market", "test-market")
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
    if market.Name != "Test Market" {
        t.Errorf("got name %q, want %q", market.Name, "Test Market")
    }
    if len(store.saved) != 1 {
        t.Errorf("expected 1 saved market, got %d", len(store.saved))
    }
}

func TestMarketService_Create_BlankName(t *testing.T) {
    svc := app.NewMarketService(&mockMarketStore{})
    _, err := svc.Create(context.Background(), "", "slug")
    if !errors.Is(err, domain.ErrInvalidMarket) {
        t.Errorf("expected ErrInvalidMarket, got %v", err)
    }
}

Struct Design

Functional Options Pattern

type Server struct {
    addr    string
    timeout time.Duration
    logger  *log.Logger
}

type Option func(*Server)

func WithTimeout(d time.Duration) Option {
    return func(s *Server) {
        s.timeout = d
    }
}

func WithLogger(l *log.Logger) Option {
    return func(s *Server) {
        s.logger = l
    }
}

func NewServer(addr string, opts ...Option) *Server {
    s := &Server{
        addr:    addr,
        timeout: 30 * time.Second, // default
        logger:  log.Default(),    // default
    }
    for _, opt := range opts {
        opt(s)
    }
    return s
}

// Usage
server := NewServer(":8080",
    WithTimeout(60*time.Second),
    WithLogger(customLogger),
)

Embedding for Composition

type Logger struct {
    prefix string
}

func (l *Logger) Log(msg string) {
    fmt.Printf("[%s] %s\n", l.prefix, msg)
}

type Server struct {
    *Logger // Embedding - Server gets Log method
    addr    string
}

func NewServer(addr string) *Server {
    return &Server{
        Logger: &Logger{prefix: "SERVER"},
        addr:   addr,
    }
}

// Usage
s := NewServer(":8080")
s.Log("Starting...") // Calls embedded Logger.Log

Memory and Performance

Preallocate Slices When Size is Known

// Bad: Grows slice multiple times
func processItems(items []Item) []Result {
    var results []Result
    for _, item := range items {
        results = append(results, process(item))
    }
    return results
}

// Good: Single allocation
func processItems(items []Item) []Result {
    results := make([]Result, 0, len(items))
    for _, item := range items {
        results = append(results, process(item))
    }
    return results
}

Use sync.Pool for Frequent Allocations

var bufferPool = sync.Pool{
    New: func() interface{} {
        return new(bytes.Buffer)
    },
}

func ProcessRequest(data []byte) []byte {
    buf := bufferPool.Get().(*bytes.Buffer)
    defer func() {
        buf.Reset()
        bufferPool.Put(buf)
    }()

    buf.Write(data)
    // Process...
    return buf.Bytes()
}

Avoid String Concatenation in Loops

// Bad: Creates many string allocations
func join(parts []string) string {
    var result string
    for _, p := range parts {
        result += p + ","
    }
    return result
}

// Good: Single allocation with strings.Builder
func join(parts []string) string {
    var sb strings.Builder
    for i, p := range parts {
        if i > 0 {
            sb.WriteString(",")
        }
        sb.WriteString(p)
    }
    return sb.String()
}

// Best: Use standard library
func join(parts []string) string {
    return strings.Join(parts, ",")
}

Go Tooling Integration

Essential Commands

# Build and run
go build ./...
go run ./cmd/myapp

# Testing
go test ./...
go test -race ./...
go test -cover ./...

# Static analysis
go vet ./...
staticcheck ./...
golangci-lint run

# Module management
go mod tidy
go mod verify

# Formatting
gofmt -w .
goimports -w .

Recommended Linter Configuration (.golangci.yml)

linters:
  enable:
    - errcheck
    - gosimple
    - govet
    - ineffassign
    - staticcheck
    - unused
    - gofmt
    - goimports
    - misspell
    - unconvert
    - unparam

linters-settings:
  errcheck:
    check-type-assertions: true
  govet:
    check-shadowing: true

issues:
  exclude-use-default: false

Standard Library: slices and maps (Go 1.21+)

The slices and maps packages replace most hand-rolled slice/map utilities. Prefer these over manual loops.

import (
    "cmp"
    "maps"
    "slices"
)

// --- slices ---

// Contains (replaces manual loop)
if slices.Contains(ids, targetID) { ... }

// Sort (type-safe, no less-func boilerplate)
slices.Sort(names)                              // ordered types
slices.SortFunc(users, func(a, b User) int {   // custom comparator
    return cmp.Compare(a.Name, b.Name)
})

// Binary search on sorted slice
i, found := slices.BinarySearch(sorted, "target")

// Deduplicate (requires sorted input)
unique := slices.Compact(slices.Clone(items))

// Filter in-place (Go 1.23+)
items = slices.DeleteFunc(items, func(x Item) bool { return x.Expired() })

// Reverse
slices.Reverse(items)

// Max / Min
max := slices.Max(scores)

// --- maps ---

// Collect all keys or values
keys := slices.Collect(maps.Keys(m))     // order not guaranteed — sort if needed
vals := slices.Collect(maps.Values(m))

// Shallow clone
clone := maps.Clone(original)

// Delete matching entries
maps.DeleteFunc(m, func(k string, v int) bool { return v == 0 })

Note: maps.Keys / maps.Values return iterators (Go 1.23 range-over-func). Use slices.Collect to materialise them into a slice, or range over them directly:

for k := range maps.Keys(m) { ... }

Interface Design

Small, Focused Interfaces

// Good: Single-method interfaces
type Reader interface {
    Read(p []byte) (n int, err error)
}

type Writer interface {
    Write(p []byte) (n int, err error)
}

// Compose interfaces as needed
type ReadWriteCloser interface {
    Reader
    Writer
    io.Closer
}

Define Interfaces Where They're Used

// In the consumer package, not the provider
package service

// UserStore defines what this service needs
type UserStore interface {
    GetUser(id string) (*User, error)
    SaveUser(user *User) error
}

type Service struct {
    store UserStore
}
// Concrete implementation lives in another package — it doesn't know about this interface

Optional Behavior with Type Assertions

type Flusher interface {
    Flush() error
}

func WriteAndFlush(w io.Writer, data []byte) error {
    if _, err := w.Write(data); err != nil {
        return err
    }
    if f, ok := w.(Flusher); ok {
        return f.Flush()
    }
    return nil
}

Package Organization

Hexagonal Project Layout

Go's idioms naturally align with hexagonal (ports & adapters) architecture:

myproject/
├── cmd/
│   └── myapp/
│       └── main.go           # Entry point + DI wiring
├── internal/
│   ├── domain/               # Pure Go types + behavior — zero external imports
│   ├── app/                  # Use cases: orchestrate domain + call port interfaces
│   ├── handler/              # Inbound adapters: HTTP, gRPC, CLI
│   ├── repository/           # Outbound adapters: Postgres, Redis, external APIs
│   └── config/               # Configuration structs, no business logic
├── pkg/
│   └── client/               # Public API client (if library)
├── api/
│   └── v1/                   # API definitions (proto, OpenAPI)
├── go.mod
└── Makefile

Package Naming

// Good: Short, lowercase, no underscores
package user

// Bad: Verbose, mixed case, or redundant
package httpHandler
package json_parser
package userService // Redundant 'Service' suffix

Avoid Package-Level State

// Bad: Global mutable state in init()
var db *sql.DB
func init() { db, _ = sql.Open("postgres", os.Getenv("DATABASE_URL")) }

// Good: Dependency injection via constructor
type Server struct { db *sql.DB }
func NewServer(db *sql.DB) *Server { return &Server{db: db} }

Quick Reference: Go Idioms

Idiom	Rule
Accept interfaces, return structs	Functions accept interface params, return concrete types
Errors are values	Treat errors as first-class values, not exceptions
Don't communicate by sharing memory	Use channels for goroutine coordination
Make the zero value useful	Types should work without explicit initialization
Clear is better than clever	Prioritize readability over cleverness
Return early	Handle errors first, keep happy path unindented

For anti-patterns (naked returns, panic for control flow, mixed receivers, context in struct), see the core go-patterns skill.