Is the anemic domain model an antipattern? It's a domain model without any behavior and only data properties.

Anemic domain models work great in simple applications, but they are difficult to maintain and evolve if you have rich business logic.

The important parts of your business logic and rules end up being scattered all over the application. It reduces cohesiveness and reusability, and makes adding new features more difficult.

Rich domain model attempts to solve this by encapsulating as much of the business logic as possible.

But how can you design a rich domain model?

This is a never-ending process of moving business logic into the domain and refining your domain model.

Let's see how to refactor from an anemic domain model to a rich domain model.

Working With Anemic Domain Model

To understand what working with an anemic domain model looks like, I'll use an example of handling a SendInvitationCommand.

I omitted the class and its dependencies so that we can focus on the Handle method. It loads some entities from the database, performs validation, executes the business logic, and finally persists the changes in the database and sends an email.

It already implements some good practices like using repositories and returning result objects.

However, it's working with an anemic domain model.

A few things indicating this:

Parameterless constructors
Public property setters
Exposed collections

In other words - the classes representing domain entities contain only data properties and no behavior.

The problems of an anemic domain model are:

Discoverability of operations
Potential code duplication
Lack of encapsulation

We'll apply a few techniques to push logic down into the domain, and try to make the model more domain-driven. I hope you'll be able to see the value and benefits this will bring.

public async Task<Result> Handle(SendInvitationCommand command)
{
    var member = await _memberRepository.GetByIdAsync(command.MemberId);

    var gathering = await _gatheringRepository.GetByIdAsync(command.GatheringId);

    if (member is null || gathering is null)
    {
        return Result.Failure(Error.NullValue);
    }

    if (gathering.Creator.Id == member.Id)
    {
        throw new Exception("Can't send invitation to the creator.");
    }

    if (gathering.ScheduledAtUtc < DateTime.UtcNow)
    {
        throw new Exception("Can't send invitation for the past.");
    }

    var invitation = new Invitation
    {
        Id = Guid.NewGuid(),
        Member = member,
        Gathering = gathering,
        Status = InvitationStatus.Pending,
        CreatedOnUtc = DateTime.UtcNow
    };

    gathering.Invitations.Add(invitation);

    _invitationRepository.Add(invitation);

    await _unitOfWork.SaveChangesAsync();

    await _emailService.SendInvitationSentEmailAsync(member, gathering);

    return Result.Success();
}

Moving Business Logic Into The Domain

The goal is to move as much of the business logic as possible into the domain.

Let's start with the Invitation entity and defining a constructor for it. I can simplify the design by setting the Status and CreatedOnUtc properties inside the constructor. I'm also going to make it internal so that an Invitation instance can only be created within the domain.

public sealed class Invitation
{
    internal Invitation(Guid id, Gathering gathering, Member member)
    {
        Id = id;
        Member = member;
        Gathering = gathering;
        Status = InvitationStatus.Pending;
        CreatedOnUtc = DateTime.Now;
    }

    // Data properties omitted for brevity.
}

The reason I made the Invitation constructor internal is so that I can introduce a new method on the Gathering entity. Let's call it SendInvitation and it will be responsible for instantiating a new Invitation instance and adding it to the internal collection.

Currently, the Gathering.Invitations collection is public, which means anyone can obtain a reference and modify the collection.

We don't want to allow this, so what we can do is encapsulate this collection behind a private field. This moves the responsibility for managing the _invitations collection to the Gathering class.

Here's how the Gathering class looks like now:

public sealed class Gathering
{
    private readonly List<Invitation> _invitations;

    // Other members omitted for brevity.

    public void SendInvitation(Member member)
    {
        var invitation = new Invitation(Guid.NewGuid(), gathering, member);

        _invitations.Add(invitation);
    }
}

Moving Validation Rules Into The Domain

The next thing we can do is move the validation rules into the SendInvitation method, further enriching the domain model.

Unfortunately, this is still a bad practice because of throwing "expected" exceptions when a validation fails. If you want to use exceptions to enforce your validation rules you should at least do it right, and use specific exceptions instead of generic ones.

But it would be even better to use a result object to express validation errors.

public sealed class Gathering
{
    // Other members omitted for brevity.

    public void SendInvitation(Member member)
    {
        if (gathering.Creator.Id == member.Id)
        {
            throw new Exception("Can't send invitation to the creator.");
        }

        if (gathering.ScheduledAtUtc < DateTime.UtcNow)
        {
            throw new Exception("Can't send invitation for the past.");
        }

        var invitation = new Invitation(Guid.NewGuid(), gathering, member);

        _invitations.Add(invitation);
    }
}

Here's how using result objects would look like:

public sealed class Gathering
{
    // Other members omitted for brevity.

    public Result SendInvitation(Member member)
    {
        if (gathering.Creator.Id == member.Id)
        {
            return Result.Failure(DomainErrors.Gathering.InvitingCreator);
        }

        if (gathering.ScheduledAtUtc < DateTime.UtcNow)
        {
            return Result.Failure(DomainErrors.Gathering.AlreadyPassed);
        }

        var invitation = new Invitation(Guid.NewGuid(), gathering, member);

        _invitations.Add(invitation);

        return Result.Success();
    }
}

The benefit of this approach is we can introduce constants for possible domain errors. The catalog of domain errors will act as documentation for your domain, and make it more expressive.

Finally, here's how the Handle method looks like with all the changes so far:

public async Task<Result> Handle(SendInvitationCommand command)
{
    var member = await _memberRepository.GetByIdAsync(command.MemberId);

    var gathering = await _gatheringRepository.GetByIdAsync(command.GatheringId);

    if (member is null || gathering is null)
    {
        return Result.Failure(Error.NullValue);
    }

    var result = gathering.SendInvitation(member);

    if (result.IsFailure)
    {
        return Result.Failure(result.Errors);
    }

    await _unitOfWork.SaveChangesAsync();

    await _emailService.SendInvitationSentEmailAsync(member, gathering);

    return Result.Success();
}

If you take a closer look at the Handle method you'll notice it's doing two things:

Persisting changes to the database
Sending an email

This means it's not atomic.

There's a potential for the database transaction to complete, and the email sending to fail. Also, sending the email will slow down the method which could affect performance.

How can make this method atomic?

By sending the email in the background. It's not important for our business logic, so this is safe to do.

Expressing Side Effects With Domain Events

You can use domain events to express that something occurred in your domain that might be interesting to other components in your system.

I often use domain events to trigger actions in the background, like sending a notification or email.

Let's introduce an InvitationSentDomainEvent:

public record InvitationSentDomainEvent(Invitation Invitation) : IDomainEvent;

We're going to raise this domain event inside the SendInvitation method:

public sealed class Gathering
{
    private readonly List<Invitation> _invitations;

    // Other members omitted for brevity.

    public Result SendInvitation(Member member)
    {
        if (gathering.Creator.Id == member.Id)
        {
            return Result.Failure(DomainErrors.Gathering.InvitingCreator);
        }

        if (gathering.ScheduledAtUtc < DateTime.UtcNow)
        {
            return Result.Failure(DomainErrors.Gathering.AlreadyPassed);
        }

        var invitation = new Invitation(Guid.NewGuid(), gathering, member);

        _invitations.Add(invitation);

        Raise(new InvitationSentDomainEvent(invitation));

        return Result.Success();
    }
}

The goal is to remove the code responsible for sending the email from the Handle method:

public async Task<Result> Handle(SendInvitationCommand command)
{
    var member = await _memberRepository.GetByIdAsync(command.MemberId);

    var gathering = await _gatheringRepository.GetByIdAsync(command.GatheringId);

    if (member is null || gathering is null)
    {
        return Result.Failure(Error.NullValue);
    }

    var result = gathering.SendInvitation(member);

    if (result.IsFailure)
    {
        return Result.Failure(result.Errors);
    }

    await _unitOfWork.SaveChangesAsync();

    return Result.Success();
}

We only want to worry about executing the business logic and persisting any changes to the database. Part of those changes will also be the domain event , which the system will publish in the background.

Of course, we need a respective handler for the domain event :

public sealed class InvitationSentDomainEventHandler
    : IDomainEventHandler<InvitationSentDomainEvent>
{
    private readonly IEmailService _emailService;

    public InvitationSentDomainEventHandler(IEmailService emailService)
    {
        _emailService = emailService;
    }

    public async Task Handle(InvitationSentDomainEvent domainEvent)
    {
        await _emailService.SendInvitationSentEmailAsync(
            domainEvent.Invitation.Member,
            domainEvent.Invitation.Gathering);
    }
}

We achieved two things:

Handling the SendInvitationCommand is now atomic
Email is sent in the background, and can be safely retried in case of an error

Takeaway

Designing a rich domain model is a gradual process, and you can slowly evolve the domain model over time.

The first step could be making your domain model more defensive:

Hiding constructors with the internal keyword
Encapsulating collection access

The benefit is your domain models will have a fine-grained public API (methods) which act as an entry point for executing the business logic.

It's easy to test behavior when it's encapsulated in a class without having to mock external dependencies.

You can raise domain events to notify the system that something of important occurred, and any interested components can subscribe to that domain event. Domain events allow you to develop a decoupled system, where you focus on the core domain logic, and don't have to worry about the side effects.

However, this doesn't mean that every system needs a rich domain model.

You should be pragmatic and decide when the complexity is worth it.

That's all for this week.

See you next Saturday.

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Refactoring From an Anemic Domain Model To a Rich Domain Model

Working With Anemic Domain Model

Moving Business Logic Into The Domain

Moving Validation Rules Into The Domain

Expressing Side Effects With Domain Events

Takeaway