Saga

A saga: a processor whose OnEvent functions derive commands from committed events. It replaces 9.x's @StatelessSaga, and compensation that 9.x wrote with @OnError is written as a saga.

Each handler is compiled into its own subscription, named processor.function (or its Name). A handler takes the committed stream, or each of its events of a type, with the state after it, and optionally the me.ahoo.wow.api.subscribe.SagaContext, and returns the commands it derives as requests: one me.ahoo.wow.api.client.CommandRequest, or several (an Iterable, a Sequence, a Flow; empty or null for none). me.ahoo.wow.api.subscribe.SagaContext says how each becomes a command.

@Saga
class PaymentSaga {
    @OnEvent
    fun onOrderPlaced(event: StateTypedEvent<OrderState, OrderPlaced>): CommandRequest<RequestPayment> =
        CommandRequest.of(RequestPayment(event.event.amount), event.ref.id)
}

A handler may be suspend or return a Reactor Mono or Flux. Derived commands have deterministic request ids, so handling a commit again derives the same requests. The saga is done when its commands are committed (or, across contexts, durably accepted); a derived command that is rejected goes to the saga's OnRejected handler of its type, and parks the aggregate in the failure ledger only when the saga has none. A saga that must act on a command's success declares an OnCommitted handler of its type: the receipt of the commit comes back to it, from another context too. The processor's dependencies are its constructor's parameters.

A processor is of exactly one kind: Projection, Reactor or Saga.