You can't have exactly-once in distributed system. Crashes happen, network or power goes down and your ack get's lost[0]. The best one can do is accept that an

You can't have exactly-once in distributed system. Crashes happen, network or power goes down and your ack get's lost[0]. The best one can do is accept that and make your processing idempotent. [0] The most interesting case I've witnessed was power going down because a hot air balloon crashed into power line.