OnSourcing

Marks a function as a state sourcing handler.

Functions annotated with @OnSourcing are responsible for rebuilding aggregate state from historical events during event sourcing. They transform event streams into current aggregate state, enabling state recovery and reconstruction.

State sourcing handlers:

• Are called during aggregate initialization from event store

• Must be deterministic (same events always produce same state)

• Should not have side effects (no external system calls)

• Are applied in event order to build current state

• Enable state snapshots and performance optimization

Example usage:

class OrderState{

    @OnSourcing
    fun onOrderCreated(event: OrderCreated) {
        this.status = OrderStatus.CREATED
        this.items.addAll(event.items)
        this.totalAmount = event.totalAmount
    }

    @OnSourcing
    fun onOrderPaid(event: OrderPaid) {
        this.status = OrderStatus.PAID
        this.paidAt = event.paidAt
    }
}

See also