Support nested sagas about eventuate-tram-sagas

[cer]

Builds on eventuate-tram/eventuate-tram-core#179

[cer]

Design overview

See NestedSagaTest and other classes in that package.

There are two sagas:

• OuterSaga - the saga, which has a participant that invokes InnerSaga
• InnerSaga - the saga that implements a step of OuterSaga

SagaCommandHandler (part of OuterSaga) that initiates InnerSaga

public class ParticipantCommandHandlers {

    public CommandHandlers commandHandlerDefinitions() {
        return SagaCommandHandlersBuilder
                .fromChannel("customerService")
                .onMessage(ReserveCreditCommand.class, this::reserveCredit)

    private void reserveCredit(CommandMessage<ReserveCreditCommand> cm, CommandReplyToken commandReplyToken) {
        InnerSagaData data = new InnerSagaData(commandReplyToken);
        sagaInstanceFactory.create(innerSaga, data);
    }

Key point: the command handler instantiates new InnerSagaData(commandReplyToken), which enables InnerSaga to send a reply to OuterSaga when it completes

Sending a reply when InnerSaga completes

public class InnerSaga implements SimpleSaga<InnerSagaData>  {

    @Override
    public void onSagaCompletedSuccessfully(String sagaId, InnerSagaData innerSagaData) {
        SimpleSaga.super.onSagaCompletedSuccessfully(sagaId, innerSagaData);
        commandReplyProducer.sendReplies(innerSagaData.getCommandReplyToken(), withSuccess());
    }