Graduate Program KB

Decoupling the domain layer from persistence concerns can be achieved by implementing the Repository pattern, Dependency Injection, and using interfaces. These techniques ensure that the domain layer remains agnostic to the underlying storage mechanism, allowing for more maintainability and testability.

1. Repository pattern: A Repository is an abstraction that encapsulates the storage, retrieval, and search behavior for domain entities. This allows the domain layer to remain unaware of the data source.

Let's consider a simple example with a User domain entity and a UserRepository interface:

// user.js
class User {
  constructor(id, name, email) {
    this.id = id;
    this.name = name;
    this.email = email;
  }
}

// userRepository.js
class UserRepository {
  getById(id) {}
  save(user) {}
}

Now, let's implement a concrete UserRepository for a specific storage mechanism, such as an in-memory storage:

// inMemoryUserRepository.js
class InMemoryUserRepository extends UserRepository {
  constructor() {
    super();
    this.users = new Map();
  }

  getById(id) {
    return this.users.get(id);
  }

  save(user) {
    this.users.set(user.id, user);
  }
}

2. Dependency Injection: Dependency Injection (DI) is a technique that allows us to pass dependencies to an object, instead of having the object create or manage those dependencies itself. This further decouples the domain layer from persistence concerns.

Consider a UserService that depends on the UserRepository:

// userService.js
class UserService {
  constructor(userRepository) {
    this.userRepository = userRepository;
  }

  createUser(id, name, email) {
    const user = new User(id, name, email);
    this.userRepository.save(user);
    return user;
  }

  findUserById(id) {
    return this.userRepository.getById(id);
  }
}

3. Using the decoupled components:

Now that we have a decoupled domain layer and persistence, we can use them together in the application layer:

// main.js
const userRepository = new InMemoryUserRepository();
const userService = new UserService(userRepository);

const user = userService.createUser(1, 'John Doe', 'john.doe@example.com');
const foundUser = userService.findUserById(1);
console.log(foundUser);

By implementing the Repository pattern and using Dependency Injection, the domain layer remains decoupled from persistence concerns. This makes it easier to swap out or add new persistence mechanisms, and simplifies testing by allowing you to inject mock repositories during testing.