Go slowly   About  Contact  Archives

Domain-Driven Design in Action

1. DDD

Domain-driven design (DDD) is a software design approach focusing on modelling software to match a domain according to input from that domain’s experts.

1.1 Domain

Domain is the subject area to which the user applies a program.

1.2 Model

A domain contains one or many domain models which are systems of abstractions that describes selected aspects of a domain and can be used to solve problems related to that domain.

Domain models can be:

1.3 Entity

Now to begin the “in action” part, let’s say we have to implement an e-commerce system with Products, buyers can create Orders to buy things which are those Products.

We define 3 entities here to hold corresponding model data:

type Product struct {
    ID    string
    Name  string

type Order struct {
    ID    string
    Email string
    Name  string

type OrderItem struct {
    ID        string
    OrderID   string
    ProductID string

Those entities may reside on the entity subfolder or on the root of your current service.

2. N-tier architecture

DDD is all about making the business domain a part of your code, we’ve done just that using entities. For better unit-testing and seperation of concerns, we will organize our code using 3-tier architecture.

2.1 Data layer

This is where we store our data. Repositories only know about persist and retrieve entities from storage, simple as that, no business logic here.

type ProductRepository interface {
    Create(ctx context.Context, prd entity.Product) error
    Retrieve(ctx context.Context, id string) (prd entity.Product, error)

type OrderRepository interface {
    Create(ctx context.Context, ord entity.Order) error
    Retrieve(ctx context.Context, id string) (entity.Order, error)

The storage may be a SQL or a NoSQL database, it’s not the responsibility of interfaces. That’s the implementation detail.

Repositories could be put on the data or store folder.

2.2 Application layer

This is where the business logic happens. We can list some of the logic for our e-commerce use case here:

We can see 2 kinds of action here: read-only actions and write (and/or read) actions. We can separate them into 2 interfaces, or just combine them into one.

type ProductService interface {
    Create(ctx context.Context, prs entity.ProductParams) error
    Retrieve(ctx context.Context, id string) (entity.Product, error)

type OrderService interface {
    Create(ctx context.Context, ord entity.OrderParams) error
    Retrieve(ctx context.Context, id string) (entity.Order, error)
    Pay(ctx context.Context, id string) error

Services will call to Repositories or other Services to do their job. They don’t talk directly with store layer.

Services should be on the app folder.

2.3 Presentation layer

This is the window from our application to the outside world and vice versa.

For http web applications, this place is where we define routes, validate authentication, map from http.Request to internal parameters which are used to call the Services, then get the results and map them back to http.Response.

For task workers, this is where we define worker pool size and fire up child processes to do the work. They all call to Services, no funny logic here.

Let’s define the API interface for our e-commerce website:

type ProductAPI interface {
    Create(w http.ResponseWriter, r *http.Request) error
    Retrieve(w http.ResponseWriter, r *http.Request) error

type OrderAPI interface {
    Create(w http.ResponseWriter, r *http.Request) error
    Retrieve(w http.ResponseWriter, r *http.Request) error
    Pay(w http.ResponseWriter, r *http.Request) error

And then maker some routes from them, we use chi router here for simple routing:

func NewProductRouter(api ProductAPI) mux.Router {
    r := chi.NewRouter()

    r.Post("/", api.Create)
    r.Get("/{id}", api.Retrieve)

    return r

func NewOrderRouter(api OrderAPI) mux.Router {
    r := chi.NewRouter()

    r.Post("/", api.Create)
    r.Get("/{id}", api.Retrieve)
    r.Post("/{id}", api.Pay)

    return r

APIs should go to api folder, workers, well, the worker one.

2.4 Wire them all together

After defining all the necessary interfaces, our Go code will compile just fine. And we can use Go Swagger to generate the Swagger specification first. And then work on the implementation later.

Let’s wire our APIs together in the main package:

func main() {
    postgresPool := NewPostgresPool(...)

    productRepo := NewProductRepository(postgresPool)
    orderRepo := NewOrderRepository(postgresPool)

    productSvc := NewProductService(productRepo)
    orderSvc := NewOrderService(productRepo, orderRepo)

    productAPI := NewProductAPI(productSvc)
    orderAPI := NewOrderAPI(orderSvc)

    productRouter := NewProductRouter(productAPI)
    orderRouter := NewOrderRouter(orderAPI)

    router := chi.NewRouter()

    router.Mount("/products", productRouter)
    router.Mount("/orders", orderRouter)

    http.ListenAndServe("localhost:8080", router)

That’s it! The most simple full-fledged DDD program ever! It may look tedious for simple program, but when things get big, DDD with separated layers really help.

Written on July 28, 2022.