Domain building blocks

The domain model is one of the integral parts of applications practicing domain-driven design and also the place where most of their business logic resides. Because the implementation of the DDD is not a trivial task, the framework includes a number of basic building blocks for the model implementation.

In most of the cases where working with entity identifiers, Revo framework takes an opinionated approach and chooses to use globally-unique GUIDs.

Entities

An entity can be any object that has a (be it local or global) ID and is implemented using IEntity interface. There are two predefined base classes for entities – BasicEntity and EventSourcedEntity (see below for more information). Entities are always a member of an aggregate.

Aggregates

Aggregates are one of the most important concepts in DDD defining the consistency and transaction boundaries. They are implemented by aggregate roots as an IAggregateRoot (which itself is also an entity) that provide a public interface for working with the aggregate.

Basic aggregates and entities

BasicAggregateRoot and BasicEntity define base classes for aggregate roots and entities which will be persisted using their visible state (typically perstisting their public properties) to an RDBMS using an ORM or to a document-oriented database. Alone deriving from these base classes will be enough to be picked-up be Revo's selected persistence implentation (e.g. EF Core).

Any class annotated withDatabaseEntityAttribute (just like these basic aggregate base classes) will be automatically be mapped to database using their public state (e.g. using EF Core if you are using Revo.EFCore as your primary data repository). For more see Data persistence.

These base classes also implement IQueryableEntity which means it will be possible to query them as IQueryable with repositories. By default, they will be automatically row-versioned to implement optimistic concurrency when saving them to a database (increasing their version number with each save).

For compatibility with ORMs like EF Core, these basic entities will usually need to define a parameterless constructor (which may be protected ; this means it should not limit your from exposing another proper public constructor taking parameters and ensuring entity invariants).

Event sourced aggregates

Event sourced aggregates implemented using EventSourcedAggregateRoot and EventSourcedEntity will use events to progress their state. Unlike the basic entities, they should only modify their state upon emitting a new event. Aggregates can internally publish new events using the Publish<TEvent> method. This pushes the event to the internal queue of uncommitted events that will get persisted once the repository is saved and invokes an event handler for the specific event type that actually produces the effect of modifying the internal state of the aggregate (e.g. changes the values of its fields). The same event handlers get also invoked when the aggregate gets loaded from a repository.

When loading an event-sourced aggregate, the repository first creates a blank instance of the aggregate and then replays all the events that the aggregate published previously – effectively reconstructing its complete state. It is obvious that it is vital for event-sourced aggregates to only use event for progressing their state as change made in any other way will get lost upon the next loading.

For the repository to be able to create new instances of event sourced aggregate roots (e.g. when loading them from the event store), the aggregate roots always need to define a constructor accepting a single Guid id parameter. However, similarly to (non-event-sourced) basic aggregates, this constructor can be kept protected or private (being used only for the needs of deserialization) and the aggregate can define other public constructor(s) that will normally enforce its invariants by requiring more parameters. See also the example below.

EventSourcedAggregateRoot and EventSourcedEntity use a convention-based event handling by default and will invoke any methods with the following signature:

void Apply(TEvent);

(considering TEvent is the type of the event handled).

These methods need to be either private, protected or internal (non-public in general). Furthermore, all events published by an aggregate need to derive from DomainAggregateEvent. This domain type includes the aggregate ID which gets automatically injected when publishing the event within an aggregate.

Example

Here is a contrived example of a simple event sourced aggregate publishing and handling events:

[DomainClassId("061ABAC8-7BE0-46D6-A6DB-BA8593027EC9")]
public class Ticket : EventSourcedAggregateRoot
{
    public Ticket(Guid id, string subject)
        : base(id)
    {
        Publish(new TicketStateChangedEvent(TicketState.Open));
        Rename(subject);
    }

    protected Ticket(Guid id) : base(id)
    {
    }

    public string Subject { get; private set; }
    public TicketState State { get; private set; }

    public void Rename(string subject)
    {
       if (Subject != subject)
       {
           Publish(new TicketRenamedEvent(subject));
       }
    }

    public void Resolve()
    {
        if (State == TicketState.Resolved)
        {
            throw new InvalidOperationException(
                $"{this} has already been resolved");
        }

        Publish(new TicketStateChangedEvent(TicketState.Resolved));
    }

    private void Apply(TicketRenamedEvent ev)
    {
        Subject = ev.Subject;
    }
    
    private void Apply(TicketStateChangedEvent ev)
    {
        State = ev.State;
    }
}

public enum TicketState
{
    Open, Resolved
}

public class TicketStateChangedEvent : DomainAggregateEvent
{
	public TicketStateChangedEvent(TicketState state)
	{
		State = state;
	}

	public TicketState State { get; }
}

public class TicketRenamedEvent : DomainAggregateEvent
{
	public TicketRenamedEvent(string subject)
	{
		Subject = subject;
	}
	
	public string Subject { get; }
}

Event routing inside an aggregate

Entities that are part of an event-sourced aggregate can also participate in the even-driven architecture. Any event-sourced entity can react to the publishing of an event inside an aggregate (i.e. not just event published by itself, but also events published by other entities or the aggregate root itself). For that, it is enough to simply derived from EventSourcedEntity or directly EventSourcedComponent, passing in the event router from the aggregate root (its EventRouter). The event router takes care of routing the events inside the aggregate, delivering them to all registered components. With that, the event sourced entities and components can use the same Apply method convention for handling events as with aggregate root and use the event router for publishing new events.

As already mentioned, it is also possible to compose the aggregate roots and entities of several smaller components that are however not entities of their own (i.e. they have no distinct identity). To do that, all that is needed is to derive from EventSourcedComponent instead of EventSourcedEntity instead.

Domain events

A basic insight on how to implement domain events in aggregates was provided in previous parts named Event sourced aggregates and Event routing inside an aggregate.

Although events are mostly discussed in connection to event-sourced aggregates, the framework has a notable feature which allows you to publish even in basic aggregates while still providing the same transactional delivery guarantees (when saving a basic aggregate to an RDBMS, both the aggregate and the events published by it get saved inside one transaction and the events get processed only after successfully persisting them).

For more info on events and their processing, see chapter Events.

This means you can use events as a powerful means of inter-aggregate communication (or even cross-system communication) or to simply implement an event-driven driven architecture (e.g. you can asynchronously trigger actions using an event listener anytime a modified aggregate publishes an event).

You can publish events from inside an aggregate using its protected Publish method:

protected virtual void Publish(T evt) where T : DomainAggregateEvent

Value objects

Value objects represent another option how to implements objects in an aggregate. In domain-driven design, value objects have no identity and should usually be immutable (this is different from entities which have an identity and a mutable state, which also implies having a clearly defined life time and scope, contrary to value objects). As such, an instance of a value object would usually be treated as a single value that is fully defined by the set of the properties it encapsulates.

The framework provides a basic support for easier implementation of these objects. This is primarily achieved with the ValueObject<T> abstract base class that implements basic value-like semantics. An example follows.

public class Reward : ValueObject<Reward>
{
    public Reward(string awardedTitle, ImmutableDictionary<string, decimal> tokens)
    {
        AwardedTitle = awardedTitle;
        Tokens = tokens;
    }

    public string AwardedTitle { get; }
    public ImmutableDictionary<string, decimal> Tokens { get; }

    protected override IEnumerable<(string Name, object Value)> GetValueComponents()
    {
        yield return (nameof(AwardedTitle), AwardedTitle);
        yield return (nameof(Tokens), Tokens.AsValueObject());
    }
}

Out-of-the box, ValueObject<T> (with T being a self-reference to the value object type implemented) overrides object.Equals, IEquatable<T>.Equals, GetHashCode and ToString methods making it possible to treat the instances of the object as a single value - e.g. they can be correctly used in HashSets, as Dictionary keys, compared between each other, etc. (with objects having the same values considered to be equal, very much unlike with the default behavior of C# classes which considered references only), e.g.:

Reward first = new Reward("Hero", new Dictionary<string, decimal>() { { "TKN", 1000 } });
Reward second = new Reward("Hero", new Dictionary<string, decimal>() { { "TKN", 1000 } });
Assert.Equals(first, second); // passes

To be able to do so, it only needs to know what the components making up the object value are. These are defined by overriding GetValueComponents method as shown previously.

Please be aware that because the hash codes should usually be constant for an object instance, the GetValueComponents method should not return values of any mutable fields. As previously stated, it is also generally a good idea to keep value objects fully immutable.

Collection value helpers

Note that the GetValueComponents method is expected to return objects which correctly implement their Equals/GetHashCode/ToString methods. While this will be true by default for most of the C# language primitives (e.g. number types, strings, etc.), it does not hold up for the collections types (e.g. lists, dictionaries or sets) which use the default class by-reference comparisons. To be able to correctly use them in value objects, the framework defines a number of helpers which wrap them as a value. CollectionAsValueExtensions implements the following extension methods (abridged code snippet):

public static class CollectionAsValueExtensions
{
	public static IEnumerable<T> AsValueObject<T>(this IImmutableList<T> list);
	public static IEnumerable<T> AsValueObject<T>(this ImmutableArray<T> array);
	public static IEnumerable<T> AsValueObject<T>(this IImmutableSet<T> set);
	public static IReadOnlyDictionary<TKey, TValue> AsValueObject<TKey, TValue>(this IImmutableDictionary<TKey, TValue> dictionary);
}

As shown in the previous value objects example, these methods can be used in the GetValueComponents method to wrap lists, arrays, sets and dictionaries with correct value-object-like semantics.

Sagas

The framework implements a comprehensive support for long-running process coordination using sagas. Thanks to this support, sagas can be considered first-class citizens of the domain model. Because of complexity of this topic, there is a separate chapter dedicated to them.

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