Improve exception handling for local saga steps

[cer]

Current behavior

If a local step throws an exception the saga is rolled back.

Application code, e.g. onSagaRolledBack() does not have access to the exception and so cannot doesn't know the reason.

Also, the rollback occurs for any exception, not just 'business-related' exceptions. Consequently, a transient technical failure will result in a rollback

Proposal

            .step()
                  .invokeLocal(steps::approveOrder)
                    .onException(InvalidOrderException.class, LocalExceptionCreateOrderSagaData::saveInvalidOrder)
                    .onExceptionRollback(InvalidOrderException.class)

• onException() is the equivalent of a try-catch that updates the saga data with the reason for the error. Note: an exception can't be simply stored in the saga data because of the requirement to (de)serialize to/from JSON
• onExceptionRollback()
  • specifies those exceptions that should cause a rollback.
  • other exceptions are assumed to indicate a transient technical failure and are rethrown by the saga logic - saga creation or reply handling will fail
  • if onExceptionRollback() is not used, then all exceptions cause a rollback (original behavior)

[cer]

See LocalExceptionCreateOrderSaga for example usage.

[cer]

Proposal: 'Global' exception handlers

Define Spring @Beans that implement the following interface:

interface LocalStepExceptionHandler<E extends Exception, SagaData> extends Ordered   {
     void handleException(E e, SagaData sagaData);
}

If a local step throws an exception, the first (sorted by Ordered) applicable - to exception and saga data - is invoked to update the SagaData prior to starting compensation.