Projection Processor

The projection processor is a mechanism that organizes and stores the state of the domain model in a form that meets query requirements.

Since the EventStore stores aggregate root domain event streams, which are very unfriendly for queries. By combining the CQRS architectural pattern, separating the read model and write model, allowing specialized optimization for query requirements.

In the Wow framework, read model projections are implemented by defining projection processors. Projection processors are responsible for processing domain events and updating the index state of the read model to reflect the latest data state.

TIP

It should be particularly noted that when the snapshot mode is set to all, projections are not necessary.

In general scenarios, the latest state snapshot of the aggregate root can be used as the read model, for example, the event compensation console does not define a projection processor and directly uses the latest state snapshot as the read model.

• Projection processors need to be marked with the @ProjectionProcessor annotation so that the framework can automatically discover them.
• Domain event handler functions need to add the @OnEvent annotation, but this annotation is not required. By default, naming it onEvent indicates that the function is a domain event handler function.
• Domain event handler functions accept parameters of: specific domain events (OrderCreated), domain events (DomainEvent<OrderCreated>).

@ProjectionProcessor
class OrderProjector {

    fun onEvent(orderCreated: OrderCreated) {
        // Update read model based on domain events
    }
}