Modular Monolith Architecture in Go: Scaling Without Microservices
Go Architecture Patterns Series: ← Previous: Domain-Driven Design | Series Overview | Next: Microservices Architecture → What is Modular Monolith Architecture? Modular Monolith Architecture is an approach that combines the simplicity of monolithic deployment with the modularity of microservices. It organizes code into independent, loosely coupled modules with well-defined boundaries, all deployed as a single application. Key Principles: Module Independence: Each module is self-contained with its own domain logic Clear Boundaries: Modules communicate through well-defined interfaces Shared Deployment: All modules deployed together in a single process Domain Alignment: Modules organized around business capabilities Internal APIs: Modules expose APIs for inter-module communication Data Ownership: Each module owns its data and database schema Architecture Overview graph TD subgraph "Modular Monolith Application" API[API Gateway/Router] subgraph "User Module" U1[User Service] U2[User Repository] U3[(User DB Schema)] end subgraph "Order Module" O1[Order Service] O2[Order Repository] O3[(Order DB Schema)] end subgraph "Product Module" P1[Product Service] P2[Product Repository] P3[(Product DB Schema)] end subgraph "Payment Module" PA1[Payment Service] PA2[Payment Repository] PA3[(Payment DB Schema)] end API --> U1 API --> O1 API --> P1 API --> PA1 U1 --> U2 O1 --> O2 P1 --> P2 PA1 --> PA2 U2 --> U3 O2 --> O3 P2 --> P3 PA2 --> PA3 O1 -.->|Module API| U1 O1 -.->|Module API| P1 O1 -.->|Module API| PA1 end style U1 fill:#e1f5ff style O1 fill:#fff4e1 style P1 fill:#e8f5e9 style PA1 fill:#f3e5f5 Module Communication Patterns sequenceDiagram participant Client participant OrderModule participant UserModule participant ProductModule participant PaymentModule Client->>OrderModule: Create Order OrderModule->>UserModule: Validate User UserModule-->>OrderModule: User Valid OrderModule->>ProductModule: Check Stock ProductModule-->>OrderModule: Stock Available OrderModule->>ProductModule: Reserve Items ProductModule-->>OrderModule: Items Reserved OrderModule->>PaymentModule: Process Payment PaymentModule-->>OrderModule: Payment Success OrderModule->>ProductModule: Confirm Reservation ProductModule-->>OrderModule: Confirmed OrderModule-->>Client: Order Created Real-World Use Cases E-commerce Platforms: Product, order, inventory, and payment management SaaS Applications: Multi-tenant applications with distinct features Content Management Systems: Content, media, user, and workflow modules Banking Systems: Account, transaction, loan, and reporting modules Healthcare Systems: Patient, appointment, billing, and medical records Enterprise Applications: HR, finance, inventory, and CRM modules Modular Monolith Implementation Project Structure ├── cmd/ │ └── app/ │ └── main.go ├── internal/ │ ├── modules/ │ │ ├── user/ │ │ │ ├── domain/ │ │ │ │ ├── user.go │ │ │ │ └── repository.go │ │ │ ├── application/ │ │ │ │ └── service.go │ │ │ ├── infrastructure/ │ │ │ │ └── postgres_repository.go │ │ │ ├── api/ │ │ │ │ └── http_handler.go │ │ │ └── module.go │ │ ├── order/ │ │ │ ├── domain/ │ │ │ │ ├── order.go │ │ │ │ └── repository.go │ │ │ ├── application/ │ │ │ │ └── service.go │ │ │ ├── infrastructure/ │ │ │ │ └── postgres_repository.go │ │ │ ├── api/ │ │ │ │ └── http_handler.go │ │ │ └── module.go │ │ ├── product/ │ │ │ ├── domain/ │ │ │ │ ├── product.go │ │ │ │ └── repository.go │ │ │ ├── application/ │ │ │ │ └── service.go │ │ │ ├── infrastructure/ │ │ │ │ └── postgres_repository.go │ │ │ ├── api/ │ │ │ │ └── http_handler.go │ │ │ └── module.go │ │ └── payment/ │ │ ├── domain/ │ │ │ ├── payment.go │ │ │ └── repository.go │ │ ├── application/ │ │ │ └── service.go │ │ ├── infrastructure/ │ │ │ └── postgres_repository.go │ │ ├── api/ │ │ │ └── http_handler.go │ │ └── module.go │ └── shared/ │ ├── database/ │ │ └── postgres.go │ └── events/ │ └── event_bus.go └── go.mod Module 1: User Module // internal/modules/user/domain/user.go package domain import ( "context" "errors" "time" ) type UserID string type User struct { ID UserID Email string Name string Active bool CreatedAt time.Time UpdatedAt time.Time } var ( ErrUserNotFound = errors.New("user not found") ErrUserAlreadyExists = errors.New("user already exists") ErrInvalidEmail = errors.New("invalid email") ) // Repository defines the interface for user storage type Repository interface { Create(ctx context.Context, user *User) error GetByID(ctx context.Context, id UserID) (*User, error) GetByEmail(ctx context.Context, email string) (*User, error) Update(ctx context.Context, user *User) error Delete(ctx context.Context, id UserID) error } // internal/modules/user/application/service.go package application import ( "context" "fmt" "regexp" "app/internal/modules/user/domain" ) type Service struct { repo domain.Repository } func NewService(repo domain.Repository) *Service { return &Service{repo: repo} } func (s *Service) CreateUser(ctx context.Context, email, name string) (*domain.User, error) { if !isValidEmail(email) { return nil, domain.ErrInvalidEmail } // Check if user exists existing, _ := s.repo.GetByEmail(ctx, email) if existing != nil { return nil, domain.ErrUserAlreadyExists } user := &domain.User{ ID: domain.UserID(generateID()), Email: email, Name: name, Active: true, CreatedAt: time.Now(), UpdatedAt: time.Now(), } if err := s.repo.Create(ctx, user); err != nil { return nil, fmt.Errorf("failed to create user: %w", err) } return user, nil } func (s *Service) GetUser(ctx context.Context, id domain.UserID) (*domain.User, error) { return s.repo.GetByID(ctx, id) } func (s *Service) ValidateUser(ctx context.Context, id domain.UserID) (bool, error) { user, err := s.repo.GetByID(ctx, id) if err != nil { return false, err } return user.Active, nil } func isValidEmail(email string) bool { emailRegex := regexp.MustCompile(`^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$`) return emailRegex.MatchString(email) } func generateID() string { return fmt.Sprintf("user_%d", time.Now().UnixNano()) } // internal/modules/user/infrastructure/postgres_repository.go package infrastructure import ( "context" "database/sql" "fmt" "app/internal/modules/user/domain" ) type PostgresRepository struct { db *sql.DB } func NewPostgresRepository(db *sql.DB) *PostgresRepository { return &PostgresRepository{db: db} } func (r *PostgresRepository) Create(ctx context.Context, user *domain.User) error { query := ` INSERT INTO users.users (id, email, name, active, created_at, updated_at) VALUES ($1, $2, $3, $4, $5, $6) ` _, err := r.db.ExecContext(ctx, query, user.ID, user.Email, user.Name, user.Active, user.CreatedAt, user.UpdatedAt) return err } func (r *PostgresRepository) GetByID(ctx context.Context, id domain.UserID) (*domain.User, error) { query := ` SELECT id, email, name, active, created_at, updated_at FROM users.users WHERE id = $1 ` user := &domain.User{} err := r.db.QueryRowContext(ctx, query, id).Scan( &user.ID, &user.Email, &user.Name, &user.Active, &user.CreatedAt, &user.UpdatedAt, ) if err == sql.ErrNoRows { return nil, domain.ErrUserNotFound } return user, err } func (r *PostgresRepository) GetByEmail(ctx context.Context, email string) (*domain.User, error) { query := ` SELECT id, email, name, active, created_at, updated_at FROM users.users WHERE email = $1 ` user := &domain.User{} err := r.db.QueryRowContext(ctx, query, email).Scan( &user.ID, &user.Email, &user.Name, &user.Active, &user.CreatedAt, &user.UpdatedAt, ) if err == sql.ErrNoRows { return nil, domain.ErrUserNotFound } return user, err } func (r *PostgresRepository) Update(ctx context.Context, user *domain.User) error { query := ` UPDATE users.users SET email = $2, name = $3, active = $4, updated_at = $5 WHERE id = $1 ` _, err := r.db.ExecContext(ctx, query, user.ID, user.Email, user.Name, user.Active, user.UpdatedAt) return err } func (r *PostgresRepository) Delete(ctx context.Context, id domain.UserID) error { query := `DELETE FROM users.users WHERE id = $1` _, err := r.db.ExecContext(ctx, query, id) return err } // internal/modules/user/module.go package user import ( "database/sql" "app/internal/modules/user/application" "app/internal/modules/user/infrastructure" ) type Module struct { Service *application.Service } func NewModule(db *sql.DB) *Module { repo := infrastructure.NewPostgresRepository(db) service := application.NewService(repo) return &Module{ Service: service, } } Module 2: Product Module // internal/modules/product/domain/product.go package domain import ( "context" "errors" "time" ) type ProductID string type Product struct { ID ProductID Name string Description string Price float64 Stock int CreatedAt time.Time UpdatedAt time.Time } var ( ErrProductNotFound = errors.New("product not found") ErrInsufficientStock = errors.New("insufficient stock") ErrInvalidPrice = errors.New("invalid price") ) type Repository interface { Create(ctx context.Context, product *Product) error GetByID(ctx context.Context, id ProductID) (*Product, error) Update(ctx context.Context, product *Product) error ReserveStock(ctx context.Context, id ProductID, quantity int) error ReleaseStock(ctx context.Context, id ProductID, quantity int) error } // internal/modules/product/application/service.go package application import ( "context" "fmt" "app/internal/modules/product/domain" ) type Service struct { repo domain.Repository } func NewService(repo domain.Repository) *Service { return &Service{repo: repo} } func (s *Service) CreateProduct(ctx context.Context, name, description string, price float64, stock int) (*domain.Product, error) { if price <= 0 { return nil, domain.ErrInvalidPrice } product := &domain.Product{ ID: domain.ProductID(generateID()), Name: name, Description: description, Price: price, Stock: stock, CreatedAt: time.Now(), UpdatedAt: time.Now(), } if err := s.repo.Create(ctx, product); err != nil { return nil, fmt.Errorf("failed to create product: %w", err) } return product, nil } func (s *Service) GetProduct(ctx context.Context, id domain.ProductID) (*domain.Product, error) { return s.repo.GetByID(ctx, id) } func (s *Service) CheckStock(ctx context.Context, id domain.ProductID, quantity int) (bool, error) { product, err := s.repo.GetByID(ctx, id) if err != nil { return false, err } return product.Stock >= quantity, nil } func (s *Service) ReserveStock(ctx context.Context, id domain.ProductID, quantity int) error { available, err := s.CheckStock(ctx, id, quantity) if err != nil { return err } if !available { return domain.ErrInsufficientStock } return s.repo.ReserveStock(ctx, id, quantity) } func (s *Service) ConfirmReservation(ctx context.Context, id domain.ProductID, quantity int) error { // In a real implementation, this would mark the reservation as confirmed return nil } func generateID() string { return fmt.Sprintf("product_%d", time.Now().UnixNano()) } // internal/modules/product/infrastructure/postgres_repository.go package infrastructure import ( "context" "database/sql" "app/internal/modules/product/domain" ) type PostgresRepository struct { db *sql.DB } func NewPostgresRepository(db *sql.DB) *PostgresRepository { return &PostgresRepository{db: db} } func (r *PostgresRepository) Create(ctx context.Context, product *domain.Product) error { query := ` INSERT INTO products.products (id, name, description, price, stock, created_at, updated_at) VALUES ($1, $2, $3, $4, $5, $6, $7) ` _, err := r.db.ExecContext(ctx, query, product.ID, product.Name, product.Description, product.Price, product.Stock, product.CreatedAt, product.UpdatedAt) return err } func (r *PostgresRepository) GetByID(ctx context.Context, id domain.ProductID) (*domain.Product, error) { query := ` SELECT id, name, description, price, stock, created_at, updated_at FROM products.products WHERE id = $1 ` product := &domain.Product{} err := r.db.QueryRowContext(ctx, query, id).Scan( &product.ID, &product.Name, &product.Description, &product.Price, &product.Stock, &product.CreatedAt, &product.UpdatedAt, ) if err == sql.ErrNoRows { return nil, domain.ErrProductNotFound } return product, err } func (r *PostgresRepository) Update(ctx context.Context, product *domain.Product) error { query := ` UPDATE products.products SET name = $2, description = $3, price = $4, stock = $5, updated_at = $6 WHERE id = $1 ` _, err := r.db.ExecContext(ctx, query, product.ID, product.Name, product.Description, product.Price, product.Stock, product.UpdatedAt) return err } func (r *PostgresRepository) ReserveStock(ctx context.Context, id domain.ProductID, quantity int) error { query := ` UPDATE products.products SET stock = stock - $2 WHERE id = $1 AND stock >= $2 ` result, err := r.db.ExecContext(ctx, query, id, quantity) if err != nil { return err } rows, err := result.RowsAffected() if err != nil { return err } if rows == 0 { return domain.ErrInsufficientStock } return nil } func (r *PostgresRepository) ReleaseStock(ctx context.Context, id domain.ProductID, quantity int) error { query := ` UPDATE products.products SET stock = stock + $2 WHERE id = $1 ` _, err := r.db.ExecContext(ctx, query, id, quantity) return err } // internal/modules/product/module.go package product import ( "database/sql" "app/internal/modules/product/application" "app/internal/modules/product/infrastructure" ) type Module struct { Service *application.Service } func NewModule(db *sql.DB) *Module { repo := infrastructure.NewPostgresRepository(db) service := application.NewService(repo) return &Module{ Service: service, } } Module 3: Order Module (Coordinates Other Modules) // internal/modules/order/domain/order.go package domain import ( "context" "errors" "time" "app/internal/modules/user/domain" "app/internal/modules/product/domain" ) type OrderID string type OrderStatus string const ( OrderStatusPending OrderStatus = "pending" OrderStatusConfirmed OrderStatus = "confirmed" OrderStatusCancelled OrderStatus = "cancelled" ) type OrderItem struct { ProductID domain.ProductID Quantity int Price float64 } type Order struct { ID OrderID UserID domain.UserID Items []OrderItem Total float64 Status OrderStatus CreatedAt time.Time UpdatedAt time.Time } var ( ErrOrderNotFound = errors.New("order not found") ErrInvalidOrder = errors.New("invalid order") ) type Repository interface { Create(ctx context.Context, order *Order) error GetByID(ctx context.Context, id OrderID) (*Order, error) Update(ctx context.Context, order *Order) error } // internal/modules/order/application/service.go package application import ( "context" "fmt" "time" orderdomain "app/internal/modules/order/domain" productapp "app/internal/modules/product/application" userapp "app/internal/modules/user/application" ) // Service coordinates between modules type Service struct { repo orderdomain.Repository userService *userapp.Service productService *productapp.Service } func NewService( repo orderdomain.Repository, userService *userapp.Service, productService *productapp.Service, ) *Service { return &Service{ repo: repo, userService: userService, productService: productService, } } func (s *Service) CreateOrder(ctx context.Context, userID domain.UserID, items []orderdomain.OrderItem) (*orderdomain.Order, error) { // Validate user through User module valid, err := s.userService.ValidateUser(ctx, userID) if err != nil { return nil, fmt.Errorf("failed to validate user: %w", err) } if !valid { return nil, fmt.Errorf("user is not active") } // Calculate total and validate products var total float64 for i, item := range items { product, err := s.productService.GetProduct(ctx, item.ProductID) if err != nil { return nil, fmt.Errorf("failed to get product: %w", err) } // Check stock availability available, err := s.productService.CheckStock(ctx, item.ProductID, item.Quantity) if err != nil { return nil, fmt.Errorf("failed to check stock: %w", err) } if !available { return nil, fmt.Errorf("insufficient stock for product %s", item.ProductID) } items[i].Price = product.Price total += product.Price * float64(item.Quantity) } // Reserve stock for all items for _, item := range items { if err := s.productService.ReserveStock(ctx, item.ProductID, item.Quantity); err != nil { // Rollback reservations on failure return nil, fmt.Errorf("failed to reserve stock: %w", err) } } order := &orderdomain.Order{ ID: orderdomain.OrderID(generateID()), UserID: userID, Items: items, Total: total, Status: orderdomain.OrderStatusPending, CreatedAt: time.Now(), UpdatedAt: time.Now(), } if err := s.repo.Create(ctx, order); err != nil { return nil, fmt.Errorf("failed to create order: %w", err) } return order, nil } func (s *Service) GetOrder(ctx context.Context, id orderdomain.OrderID) (*orderdomain.Order, error) { return s.repo.GetByID(ctx, id) } func (s *Service) ConfirmOrder(ctx context.Context, id orderdomain.OrderID) error { order, err := s.repo.GetByID(ctx, id) if err != nil { return err } // Confirm stock reservations for _, item := range order.Items { if err := s.productService.ConfirmReservation(ctx, item.ProductID, item.Quantity); err != nil { return fmt.Errorf("failed to confirm reservation: %w", err) } } order.Status = orderdomain.OrderStatusConfirmed order.UpdatedAt = time.Now() return s.repo.Update(ctx, order) } func generateID() string { return fmt.Sprintf("order_%d", time.Now().UnixNano()) } // internal/modules/order/module.go package order import ( "database/sql" "app/internal/modules/order/application" "app/internal/modules/order/infrastructure" productapp "app/internal/modules/product/application" userapp "app/internal/modules/user/application" ) type Module struct { Service *application.Service } func NewModule(db *sql.DB, userService *userapp.Service, productService *productapp.Service) *Module { repo := infrastructure.NewPostgresRepository(db) service := application.NewService(repo, userService, productService) return &Module{ Service: service, } } Main Application // cmd/app/main.go package main import ( "database/sql" "log" "net/http" _ "github.com/lib/pq" "app/internal/modules/user" "app/internal/modules/product" "app/internal/modules/order" ) func main() { // Initialize database db, err := sql.Open("postgres", "postgres://user:pass@localhost/modular_monolith?sslmode=disable") if err != nil { log.Fatal(err) } defer db.Close() // Initialize modules userModule := user.NewModule(db) productModule := product.NewModule(db) orderModule := order.NewModule(db, userModule.Service, productModule.Service) // Setup HTTP routes mux := http.NewServeMux() // User endpoints mux.HandleFunc("POST /users", func(w http.ResponseWriter, r *http.Request) { // Handle user creation }) // Product endpoints mux.HandleFunc("POST /products", func(w http.ResponseWriter, r *http.Request) { // Handle product creation }) // Order endpoints mux.HandleFunc("POST /orders", func(w http.ResponseWriter, r *http.Request) { // Handle order creation using orderModule.Service }) log.Println("Server starting on :8080") if err := http.ListenAndServe(":8080", mux); err != nil { log.Fatal(err) } } Best Practices Module Boundaries: Keep modules independent with clear interfaces Shared Database: Use schemas or table prefixes to separate module data Module APIs: Define explicit APIs for inter-module communication Dependency Direction: Modules should depend on interfaces, not implementations Event-Driven Communication: Use events for async inter-module communication Transaction Management: Handle cross-module transactions carefully Testing: Test modules independently with mocked dependencies Documentation: Document module APIs and boundaries clearly Common Pitfalls Shared Models: Sharing domain models between modules creates tight coupling Direct Database Access: Modules accessing other modules’ database tables Circular Dependencies: Modules depending on each other directly Anemic Modules: Modules with no business logic, just CRUD operations God Modules: Modules that know too much about other modules Ignoring Boundaries: Calling internal implementations instead of module APIs Synchronous Coupling: Over-reliance on synchronous inter-module calls When to Use Modular Monolith Use When: ...