Aggregate Polymorphism

In certain cases it is beneficial to have a polymorphic hierarchy in aggregate structure. Subtypes in polymorphic aggregate hierarchy inherit @CommandHandlers, @EventSourcingHandlers and @CommandHandlerInterceptors from the super aggregates. Based on @AggregateIdentifier the correct aggregate type is loaded and command is executed on it. Let's take a look at the following example:

public abstract class Card {}

public class GiftCard extends Card {}

public class ClosedLoopGiftCard extends GiftCard {}

public class OpenLoopGiftCard extends GiftCard {}

public class RechargeableGiftCard extends ClosedLoopGiftCard {}

We can define this structure as Polymorphic Aggregate of type GiftCard and subtypes of ClosedLoopGiftCard, OpenLoopGiftCard, and RechargeableGiftCard. If there are handlers present on Card class, those will be present on all aggregates as well.

While modeling a polymorphic aggregate hierarchy it is important to keep these constraints in mind:

• It is not allowed to have a constructor annotated with @CommandHandler on abstract aggregate. The rationale for this is that an abstract aggregate can never be created.

• Having creational command handlers of the same command name on different aggregates in the same hierarchy is forbidden too, since Axon cannot derive which one to invoke.

• In a polymorphic aggregate hierarchy it is not allowed to have multiple @AggregateIdentifier and @AggregateVersion annotated fields.

Registering aggregate subtypes

A polymorphic aggregate hierarchy can be registered via the AggregateConfigurer by invoking AggregateConfigurer#withSubtypes(Set<Class<? extends A>>).

Do note that children of the parent aggregate that you do not register as a subtype will be automatically registered by the framework as a subtype. So, in the following example, the ClosedLoopGiftCard is transitively registered as a subtype of GiftCard. However, if there is a LimitedRechargeableGiftCard extends RechargeableGiftCard defined, the framework will not pick it up automatically since it is not a direct child of the parent aggregate.

public class AxonConfig {
    // omitting other configuration methods...
    public AggregateConfigurer<GiftCard> giftCardConfigurer() {
        Set<Class<? extends GiftCard>> subtypes = new HashSet<>();
        subtypes.add(OpenLoopGiftCard.class);
        subtypes.add(RechargeableGiftCard.class);

        return AggregateConfigurer.defaultConfiguration(GiftCard.class)
                                  .withSubtypes(subtypes);
    }

    // ...
}

class GiftCard {
    // omitted implementation for brevity
}

class OpenLoopGiftCard extends GiftCard {
    // omitted implementation for brevity
}

class RechargeableGiftCard extends GiftCard {
    // omitted implementation for brevity
}

Polymorphic Aggregates in Spring

If you are using Spring, Axon will automatically detect polymorphic aggregates based on the @Aggregate annotations and class hierarchy. The @Aggregate annotation needs to be put on the shared parent class that contains the aggregate identifier, as well as every subclass that is a potential instance type of that shared parent class.