Parallel messages are sent through the eInvoicing and the DRR channels. The flow in case of an "exception" has to be clarified. What happens if the eInvoice is rejected? Is there a rollback mechanism for DRR? What happens if there is a credit note? Is there a "negative DRR" message and proper treatment available? What happens if the DRR infrastructure is temporarily unavailable on MS level ? Is this blocking for eInvoices to be sent? (This is a recipe for disaster!) What happens if the DRR infrastructure is temporarily unavailable on Central VIES level ? Is this blocking for eInvoices to be sent? (This is a recipe for even bigger disaster!) -> it seems clear that messaging will have to be asynchronous and that messages should conform to ACID properties (atomicity, consistency, isolation, durability). Is an eInvoice and the DRR to be considered as one transaction ? Is a two-phase commit protocol for distributed transaction management needed ? Does the DRR message have to wait for the eInvoice response before being sent ?