Indeed, either Siberite is a queue system which purpose is to dispatch each message to one and only one consumer for further processing and which requires the consumers to acknowledge fully processed messages ;

or Siberite is a journal system (in the spirit of Kafka) which purpose is to replay the full log to any consumer asking for it and which offers the consumer a watermark mechanism to keep track of their progress.

In the former case, the queue system is responsible of what to do in case of a missing or late acknowledgement (choosing between "at least once" or "at most once" message delivering). In the later case, the consumers are responsible of how to maintain an atomic view of message consumption and message processing (for instance using a transaction to persist an offset with a state).