Go: Goプロジェクトのアーキテクチャ：標準レイアウト、クリーンアーキテクチャの4層モデル、依存性注入

// Bad code: everything piled into main.go
package main

var db *sql.DB

func main() {
    // Database connection
    // Route registration
    // HTML templates
    // User handler
    // Order handler
    // Product handler
    // All mixed together!
}

// To find "user" related code: Ctrl+F search for "user", scattered across 50 places

myapp/
├── cmd/
│   └── server/
│       └── main.go         # Entry → dependency injection + start service
├── internal/
│   ├── handler/            # Layer 1: HTTP handlers (parse request / return response)
│   ├── service/            # Layer 2: Business logic (domain rules)
│   └── repository/         # Layer 3: Data access (database / external API)
├── pkg/
│   └── model/              # Layer 4: Domain models (data structures)
└── go.mod

myproject/
├── cmd/                    # Executable entry points
│   ├── server/             #   server binary
│   │   └── main.go
│   └── migrate/            #   database migration tool
│       └── main.go
├── internal/               # Private packages (cannot be imported externally)
│   ├── handler/            #   HTTP handlers
│   ├── service/            #   Business logic
│   └── repository/         #   Data access
├── pkg/                    # Exportable public packages
│   └── model/              #   Domain models
├── migrations/             # SQL migration files
├── config/                 # Configuration files
├── scripts/                # Helper scripts
├── go.mod
└── go.sum

handler (HTTP) → service (business) → repository (data)
     ↓                ↓                ↓
   Request parsing   Domain rules     Database/API
   Response return   Transaction mgmt CRUD operations
    Param validation  Multi-step orchestration  Cache access

// ---------- Layer 4: Model (domain models) ----------
// pkg/model/user.go
package model

type User struct {
    ID        int
    Name      string
    Email     string
    CreatedAt time.Time
}

type CreateUserRequest struct {
    Name  string
    Email string
}

// ---------- Layer 3: Repository (data access) ----------
// internal/repository/user.go
package repository

import (
    "database/sql"
    "myapp/pkg/model"
)

type UserRepository interface {
    FindByID(id int) (*model.User, error)
    Create(req model.CreateUserRequest) (*model.User, error)
    List() ([]*model.User, error)
    Delete(id int) error
}

type userRepository struct {
    db *sql.DB
}

func NewUserRepository(db *sql.DB) UserRepository {
    return &userRepository{db: db}
}

func (r *userRepository) FindByID(id int) (*model.User, error) {
    row := r.db.QueryRow("SELECT id, name, email, created_at FROM users WHERE id = ?", id)
    user := &model.User{}
    err := row.Scan(&user.ID, &user.Name, &user.Email, &user.CreatedAt)
    if err == sql.ErrNoRows {
        return nil, nil
    }
    return user, err
}

func (r *userRepository) Create(req model.CreateUserRequest) (*model.User, error) {
    result, err := r.db.Exec("INSERT INTO users (name, email) VALUES (?, ?)", req.Name, req.Email)
    if err != nil {
        return nil, err
    }
    id, _ := result.LastInsertId()
    return r.FindByID(int(id))
}

// ---------- Layer 2: Service (business logic) ----------
// internal/service/user.go
package service

import (
    "errors"
    "myapp/internal/repository"
    "myapp/pkg/model"
    "strings"
)

var (
    ErrUserNotFound    = errors.New("user not found")
    ErrInvalidName     = errors.New("name is required")
    ErrInvalidEmail    = errors.New("invalid email format")
)

type UserService struct {
    repo repository.UserRepository
}

func NewUserService(repo repository.UserRepository) *UserService {
    return &UserService{repo: repo}
}

func (s *UserService) Create(req model.CreateUserRequest) (*model.User, error) {
    if strings.TrimSpace(req.Name) == "" {
        return nil, ErrInvalidName
    }
    if !strings.Contains(req.Email, "@") {
        return nil, ErrInvalidEmail
    }
    return s.repo.Create(req)
}

func (s *UserService) Get(id int) (*model.User, error) {
    user, err := s.repo.FindByID(id)
    if err != nil {
        return nil, err
    }
    if user == nil {
        return nil, ErrUserNotFound
    }
    return user, nil
}

// ---------- Layer 1: Handler (HTTP handler) ----------
// internal/handler/user.go
package handler

import (
    "encoding/json"
    "errors"
    "net/http"
    "strconv"

    "myapp/internal/service"
    "myapp/pkg/model"
)

type UserHandler struct {
    svc *service.UserService
}

func NewUserHandler(svc *service.UserService) *UserHandler {
    return &UserHandler{svc: svc}
}

func (h *UserHandler) CreateUser(w http.ResponseWriter, r *http.Request) {
    var req model.CreateUserRequest
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        writeError(w, http.StatusBadRequest, "invalid JSON")
        return
    }

    user, err := h.svc.Create(req)
    if err != nil {
        writeError(w, http.StatusBadRequest, err.Error())
        return
    }

    writeJSON(w, http.StatusCreated, user)
}

func (h *UserHandler) GetUser(w http.ResponseWriter, r *http.Request) {
    id, _ := strconv.Atoi(r.PathValue("id"))

    user, err := h.svc.Get(id)
    if errors.Is(err, service.ErrUserNotFound) {
        writeError(w, http.StatusNotFound, err.Error())
        return
    }
    if err != nil {
        writeError(w, http.StatusInternalServerError, "internal error")
        return
    }

    writeJSON(w, http.StatusOK, user)
}

// ---------- Utility functions ----------

func writeJSON(w http.ResponseWriter, status int, data interface{}) {
    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(status)
    json.NewEncoder(w).Encode(data)
}

func writeError(w http.ResponseWriter, status int, msg string) {
    writeJSON(w, status, map[string]string{"error": msg})
}

Handler → Service → Repository (→ DB)
   |          |           |
   ↓          ↓           ↓
  Model     Model       Model

// cmd/server/main.go
package main

import (
    "database/sql"
    "log"
    "net/http"

    "myapp/internal/handler"
    "myapp/internal/repository"
    "myapp/internal/service"
)

func main() {
    // ---- Dependency injection (assemble all layers) ----

    // 1. Database connection
    db, err := sql.Open("sqlite3", "./app.db")
    if err != nil {
        log.Fatal(err)
    }
    defer db.Close()

    // 2. Repository layer
    userRepo := repository.NewUserRepository(db)
    orderRepo := repository.NewOrderRepository(db)

    // 3. Service layer (depends on Repository)
    userSvc := service.NewUserService(userRepo)
    orderSvc := service.NewOrderService(orderRepo, userRepo)

    // 4. Handler layer (depends on Service)
    userHandler := handler.NewUserHandler(userSvc)
    orderHandler := handler.NewOrderHandler(orderSvc)

    // 5. Route registration
    mux := http.NewServeMux()
    userHandler.Register(mux, "/api/v1/users")
    orderHandler.Register(mux, "/api/v1/orders")

    log.Println("Service listening on :8080")
    log.Fatal(http.ListenAndServe(":8080", mux))
}

// cmd/server/main.go
package main

import (
    "context"
    "database/sql"
    "encoding/json"
    "log"
    "net/http"
    "os"
    "os/signal"
    "syscall"
    "time"

    _ "github.com/mattn/go-sqlite3"
)

// ---------- Model (pkg/model) ----------

type Product struct {
    ID    int     `json:"id"`
    Name  string  `json:"name"`
    Price float64 `json:"price"`
}

type Order struct {
    ID        int       `json:"id"`
    UserID    int       `json:"user_id"`
    ProductID int       `json:"product_id"`
    Quantity  int       `json:"quantity"`
    Total     float64   `json:"total"`
    Status    string    `json:"status"`
    CreatedAt time.Time `json:"created_at"`
}

// ---------- Repository (internal/repository) ----------

type ProductRepository struct {
    db *sql.DB
}

func NewProductRepository(db *sql.DB) *ProductRepository {
    return &ProductRepository{db: db}
}

func (r *ProductRepository) FindByID(id int) (*Product, error) {
    p := &Product{}
    err := r.db.QueryRow("SELECT id, name, price FROM products WHERE id = ?", id).
        Scan(&p.ID, &p.Name, &p.Price)
    if err == sql.ErrNoRows {
        return nil, nil
    }
    return p, err
}

func (r *ProductRepository) List() ([]*Product, error) {
    rows, err := r.db.Query("SELECT id, name, price FROM products")
    if err != nil {
        return nil, err
    }
    defer rows.Close()

    var products []*Product
    for rows.Next() {
        p := &Product{}
        if err := rows.Scan(&p.ID, &p.Name, &p.Price); err != nil {
            return nil, err
        }
        products = append(products, p)
    }
    return products, rows.Err()
}

type OrderRepository struct {
    db *sql.DB
}

func NewOrderRepository(db *sql.DB) *OrderRepository {
    return &OrderRepository{db: db}
}

func (r *OrderRepository) Create(order *Order) error {
    result, err := r.db.Exec(
        "INSERT INTO orders (user_id, product_id, quantity, total, status) VALUES (?, ?, ?, ?, ?)",
        order.UserID, order.ProductID, order.Quantity, order.Total, order.Status,
    )
    if err != nil {
        return err
    }
    id, _ := result.LastInsertId()
    order.ID = int(id)
    return nil
}

func (r *OrderRepository) FindByID(id int) (*Order, error) {
    o := &Order{}
    err := r.db.QueryRow(
        "SELECT id, user_id, product_id, quantity, total, status, created_at FROM orders WHERE id = ?", id,
    ).Scan(&o.ID, &o.UserID, &o.ProductID, &o.Quantity, &o.Total, &o.Status, &o.CreatedAt)
    if err == sql.ErrNoRows {
        return nil, nil
    }
    return o, err
}

// ---------- Service (internal/service) ----------

type OrderService struct {
    productRepo *ProductRepository
    orderRepo   *OrderRepository
}

func NewOrderService(productRepo *ProductRepository, orderRepo *OrderRepository) *OrderService {
    return &OrderService{
        productRepo: productRepo,
        orderRepo:   orderRepo,
    }
}

type PlaceOrderInput struct {
    UserID    int
    ProductID int
    Quantity  int
}

var (
    ErrProductNotFound  = &AppError{Code: "PRODUCT_NOT_FOUND", Message: "product not found", HTTPStatus: 404}
    ErrInsufficientStock = &AppError{Code: "INSUFFICIENT_STOCK", Message: "insufficient stock", HTTPStatus: 409}
)

type AppError struct {
    Code       string `json:"code"`
    Message    string `json:"message"`
    HTTPStatus int    `json:"-"`
}

func (e *AppError) Error() string {
    return e.Message
}

func (s *OrderService) PlaceOrder(input PlaceOrderInput) (*Order, error) {
    product, err := s.productRepo.FindByID(input.ProductID)
    if err != nil {
        return nil, err
    }
    if product == nil {
        return nil, ErrProductNotFound
    }

    total := product.Price * float64(input.Quantity)

    order := &Order{
        UserID:    input.UserID,
        ProductID: input.ProductID,
        Quantity:  input.Quantity,
        Total:     total,
        Status:    "created",
    }

    if err := s.orderRepo.Create(order); err != nil {
        return nil, err
    }

    return order, nil
}

// ---------- Handler (internal/handler) ----------

type OrderHandler struct {
    svc *OrderService
}

func NewOrderHandler(svc *OrderService) *OrderHandler {
    return &OrderHandler{svc: svc}
}

func (h *OrderHandler) Register(mux *http.ServeMux, basePath string) {
    mux.HandleFunc("POST "+basePath, h.PlaceOrder)
    mux.HandleFunc("GET "+basePath+"/{id}", h.GetOrder)
}

type placeOrderRequest struct {
    ProductID int `json:"product_id"`
    Quantity  int `json:"quantity"`
}

func (h *OrderHandler) PlaceOrder(w http.ResponseWriter, r *http.Request) {
    // Get user ID from Context (injected by auth middleware)
    userID, ok := r.Context().Value("user_id").(int)
    if !ok {
        writeError(w, http.StatusUnauthorized, "not authenticated")
        return
    }

    var req placeOrderRequest
    if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
        writeError(w, http.StatusBadRequest, "invalid JSON")
        return
    }

    order, err := h.svc.PlaceOrder(PlaceOrderInput{
        UserID:    userID,
        ProductID: req.ProductID,
        Quantity:  req.Quantity,
    })

    if appErr, ok := err.(*AppError); ok {
        writeError(w, appErr.HTTPStatus, appErr.Message)
        return
    }
    if err != nil {
        writeError(w, http.StatusInternalServerError, "internal error")
        return
    }

    writeJSON(w, http.StatusCreated, order)
}

func (h *OrderHandler) GetOrder(w http.ResponseWriter, r *http.Request) {
    // ... business logic
}

func writeJSON(w http.ResponseWriter, status int, data interface{}) {
    w.Header().Set("Content-Type", "application/json")
    w.WriteHeader(status)
    json.NewEncoder(w).Encode(data)
}

func writeError(w http.ResponseWriter, status int, msg string) {
    writeJSON(w, status, map[string]string{"error": msg})
}

// ---------- Main (dependency injection entry) ----------

func main() {
    db, err := sql.Open("sqlite3", "./shop.db")
    if err != nil {
        log.Fatal(err)
    }
    defer db.Close()

    // Initialize tables
    db.Exec(`CREATE TABLE IF NOT EXISTS products (
        id INTEGER PRIMARY KEY AUTOINCREMENT, name TEXT, price REAL
    )`)
    db.Exec(`CREATE TABLE IF NOT EXISTS orders (
        id INTEGER PRIMARY KEY AUTOINCREMENT, user_id INTEGER,
        product_id INTEGER, quantity INTEGER, total REAL,
        status TEXT, created_at DATETIME DEFAULT CURRENT_TIMESTAMP
    )`)

    // Dependency injection
    productRepo := NewProductRepository(db)
    orderRepo := NewOrderRepository(db)
    orderSvc := NewOrderService(productRepo, orderRepo)
    orderHandler := NewOrderHandler(orderSvc)

    mux := http.NewServeMux()
    orderHandler.Register(mux, "/api/v1/orders")

    // Health check
    mux.HandleFunc("GET /health", func(w http.ResponseWriter, r *http.Request) {
        writeJSON(w, http.StatusOK, map[string]string{"status": "ok"})
    })

    server := &http.Server{
        Addr:    ":8080",
        Handler: mux,
    }

    // Graceful shutdown
    go func() {
        sigCh := make(chan os.Signal, 1)
        signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
        <-sigCh
        log.Println("Shutting down...")
        ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
        defer cancel()
        server.Shutdown(ctx)
    }()

    log.Println("E-commerce service listening on :8080")
    if err := server.ListenAndServe(); err != http.ErrServerClosed {
        log.Fatal(err)
    }
}