The Project | About Us | Contribute | Donations | License
HOME My courses Completed
◄ Previous
◄

Domain-Driven Design

Next ►
►

Domain-Driven Design (DDD) is a software development approach that emphasizes collaboration between technical and domain experts to create a shared understanding of the problem space and to design software that accurately reflects the domain. In this module, we will explore the key concepts of DDD and how to implement them in F#.

Key Concepts of Domain-Driven Design

  1. Domain: The subject area to which the user applies a program is the domain. It is the sphere of knowledge and activity around which the application logic revolves.
  2. Ubiquitous Language: A common language shared by all team members (developers, domain experts, etc.) to ensure clear communication and understanding.
  3. Entities: Objects that have a distinct identity that runs through time and different states.
  4. Value Objects: Objects that describe some characteristic or attribute but have no conceptual identity.
  5. Aggregates: A cluster of domain objects that can be treated as a single unit.
  6. Repositories: Mechanisms for encapsulating storage, retrieval, and search behavior which emulates a collection of objects.
  7. Services: Operations that do not naturally fit within the domain objects.
  8. Bounded Contexts: Explicit boundaries within which a particular model is defined and applicable.

Implementing DDD in F#

Entities

Entities in F# can be represented using classes or records with mutable fields to maintain their identity over time.

type CustomerId = CustomerId of Guid

type Customer = {
    Id: CustomerId
    mutable Name: string
    mutable Email: string
}

let createCustomer name email =
    { Id = CustomerId(Guid.NewGuid()); Name = name; Email = email }

Value Objects

Value objects are immutable and can be represented using records or tuples.

type Address = {
    Street: string
    City: string
    ZipCode: string
}

let createAddress street city zipCode =
    { Street = street; City = city; ZipCode = zipCode }

Aggregates

Aggregates are clusters of entities and value objects. The root entity controls access to the aggregate.

type OrderId = OrderId of Guid

type OrderItem = {
    ProductId: Guid
    Quantity: int
}

type Order = {
    Id: OrderId
    Customer: Customer
    Items: OrderItem list
}

let createOrder customer items =
    { Id = OrderId(Guid.NewGuid()); Customer = customer; Items = items }

Repositories

Repositories abstract the data access layer. In F#, they can be represented using interfaces and implementations.

type ICustomerRepository =
    abstract member GetCustomerById: CustomerId -> Customer option
    abstract member SaveCustomer: Customer -> unit

type InMemoryCustomerRepository() =
    let mutable customers = Map.empty<CustomerId, Customer>
    
    interface ICustomerRepository with
        member _.GetCustomerById id =
            Map.tryFind id customers
        
        member _.SaveCustomer customer =
            customers <- Map.add customer.Id customer customers

Services

Services encapsulate domain logic that doesn't naturally fit within entities or value objects.

type OrderService(customerRepository: ICustomerRepository) =
    member _.PlaceOrder(customerId: CustomerId, items: OrderItem list) =
        match customerRepository.GetCustomerById customerId with
        | Some customer ->
            let order = createOrder customer items
            // Save order to repository (not shown)
            printfn "Order placed for customer %s" customer.Name
        | None ->
            printfn "Customer not found"

Bounded Contexts

Bounded contexts define clear boundaries within which a particular model is valid. In F#, this can be represented using modules and namespaces.

module CustomerContext =
    type CustomerId = CustomerId of Guid
    type Customer = { Id: CustomerId; Name: string; Email: string }
    // Other customer-related types and functions

module OrderContext =
    type OrderId = OrderId of Guid
    type OrderItem = { ProductId: Guid; Quantity: int }
    type Order = { Id: OrderId; Customer: CustomerContext.Customer; Items: OrderItem list }
    // Other order-related types and functions

Practical Exercise

Exercise: Implement a Simple DDD Model

  1. Define a Product entity with properties Id, Name, and Price.
  2. Define a Cart aggregate that contains a list of Product entities.
  3. Implement a repository interface ICartRepository with methods to add and retrieve carts.
  4. Create a service CartService that allows adding products to the cart and calculating the total price.

Solution

type ProductId = ProductId of Guid

type Product = {
    Id: ProductId
    Name: string
    Price: decimal
}

type Cart = {
    Id: Guid
    mutable Products: Product list
}

let createProduct name price =
    { Id = ProductId(Guid.NewGuid()); Name = name; Price = price }

let createCart () =
    { Id = Guid.NewGuid(); Products = [] }

type ICartRepository =
    abstract member AddCart: Cart -> unit
    abstract member GetCartById: Guid -> Cart option

type InMemoryCartRepository() =
    let mutable carts = Map.empty<Guid, Cart>
    
    interface ICartRepository with
        member _.AddCart cart =
            carts <- Map.add cart.Id cart carts
        
        member _.GetCartById id =
            Map.tryFind id carts

type CartService(cartRepository: ICartRepository) =
    member _.AddProductToCart(cartId: Guid, product: Product) =
        match cartRepository.GetCartById cartId with
        | Some cart ->
            cart.Products <- product :: cart.Products
            cartRepository.AddCart cart
        | None ->
            printfn "Cart not found"

    member _.CalculateTotalPrice(cartId: Guid) =
        match cartRepository.GetCartById cartId with
        | Some cart ->
            cart.Products |> List.sumBy (fun p -> p.Price)
        | None ->
            0M

Summary

In this module, we explored the key concepts of Domain-Driven Design and how to implement them in F#. We covered entities, value objects, aggregates, repositories, services, and bounded contexts. By following these principles, you can create software that accurately reflects the domain and is easier to maintain and extend.

In the next module, we will delve into practical applications of F# by building web applications with Giraffe.

◄ Previous
◄
Next ►
►
The Project | About Us | Contribute | Donations | License
© Copyright 2024. All rights reserved