CQRS Pattern An Example

Let's consider a simple application that manages user profiles. In this application, you can create a new user (command), update their email (command), and view user details (query).

1. Command Model (Write Model)

1. Command Class: Defines an action to be performed. For instance, CreateUserCommand with fields like userId, name, and email.
2. Command Handler: Processes the commands. It would take CreateUserCommand, perform business logic, and persist the changes.
3. Domain Model: Represents the state of the data (e.g., a User class).
4. Repository: Handles data persistence.

2. Query Model (Read Model)

1. DTO (Data Transfer Object): Represents the data we want to read, like UserProfileDTO.
2. Query Class: Defines a query, for instance, GetUserProfileQuery with a userId.
3. Query Handler: Handles the query and returns the requested data, often using a read-optimized storage.

3. Infrastructure

1. Command Bus: Routes commands to their respective handlers.
2. Query Bus: Routes queries to their respective handlers.
3. Event Store: In a more advanced CQRS setup, this would store events resulting from commands.

Here is the sample code for the aforementioned components:

Command Model (Write Model)

1. Command Class

public class CreateUserCommand {
    private String userId;
    private String name;
    private String email;

    // Constructor, Getters, and Setters
}

public class UpdateUserEmailCommand {
    private String userId;
    private String newEmail;

    // Constructor, Getters, and Setters
}

2. Command Handler

public class CreateUserCommandHandler {
    private UserRepository userRepository;

    public CreateUserCommandHandler(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public void handle(CreateUserCommand command) {
        User user = new User(command.getUserId(), command.getName(), command.getEmail());
        userRepository.save(user);
    }
}

public class UpdateUserEmailCommandHandler {
    private UserRepository userRepository;

    public UpdateUserEmailCommandHandler(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public void handle(UpdateUserEmailCommand command) {
        if (!isValidEmail(command.getNewEmail())) {
            throw new IllegalArgumentException("Invalid email format");
        }
        User user = userRepository.findById(command.getUserId());
        if (user != null) {
            user.setEmail(command.getNewEmail());
            userRepository.save(user);
        }
    }

    private boolean isValidEmail(String email) {
        // Simple email validation logic
        return email != null && email.contains("@");
    }
}

3. Domain Model

public class User {
    private String id;
    private String name;
    private String email;

    // Constructor, Getters, and Setters
}

4. Repository

public interface UserRepository {
    void save(User user);
    User findById(String id);
}

public class UserRepositoryImpl implements UserRepository {
    // Implementation of save and findById methods
}

Query Model (Read Model)

1. DTO (Data Transfer Object)

public class UserProfileDTO {
    private String id;
    private String name;
    private String email;

    // Constructor, Getters, and Setters
}

2. Query Class

public class GetUserProfileQuery {
    private String userId;

    // Constructor, Getters, and Setters
}

// Read Email Query
public class ReadUserEmailQuery {
    private String userId;

    // Constructor, Getters, and Setters
}

3. Query Handler

public class GetUserProfileQueryHandler {
    private UserRepository userRepository;

    public GetUserProfileQueryHandler(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public UserProfileDTO handle(GetUserProfileQuery query) {
        User user = userRepository.findById(query.getUserId());
        return new UserProfileDTO(user.getId(), user.getName(), user.getEmail());
    }
}

// Read Email Query Handler
public class ReadUserEmailQueryHandler {
    private UserRepository userRepository;

    public ReadUserEmailQueryHandler(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public String handle(ReadUserEmailQuery query) {
        User user = userRepository.findById(query.getUserId());
        return user != null ? user.getEmail() : null;
    }
}

Infrastructure

1. Command Bus

public interface CommandBus {
    void dispatch(CreateUserCommand command);
}

public class SimpleCommandBus implements CommandBus {
    private CreateUserCommandHandler handler;

    public SimpleCommandBus(CreateUserCommandHandler handler) {
        this.handler = handler;
    }

    @Override
    public void dispatch(CreateUserCommand command) {
        handler.handle(command);
    }
}

2. Query Bus

public interface QueryBus {
    UserProfileDTO dispatch(GetUserProfileQuery query);
}

public class SimpleQueryBus implements QueryBus {
    private GetUserProfileQueryHandler handler;

    public SimpleQueryBus(GetUserProfileQueryHandler handler) {
        this.handler = handler;
    }

    @Override
    public UserProfileDTO dispatch(GetUserProfileQuery query) {
        return handler.handle(query);
    }
}

3. Event Store (Simplified Example)

public interface EventStore {
    void storeEvent(UserEvent event);
}

public class SimpleEventStore implements EventStore {
    private List<UserEvent> events = new ArrayList<>();

    @Override
    public void storeEvent(UserEvent event) {
        events.add(event);
    }
}

Putting It All Together

This example includes a basic implementation of a command bus, query bus, and a simplified event store. In a full-fledged application, an event store would be used for event sourcing, which is often paired with CQRS. The event store captures changes as a series of events, which can be replayed to rebuild the state of an entity.