A message broker tracks which stream offsets have been fully processed. Offsets arrive out of order, and the broker maintains a "committed offset" that represents the highest contiguous offset starting from 0.

An offset n can only be committed if every offset from 0 through n has been processed. After each incoming offset is recorded, the broker advances the committed offset as far as possible and reports its value. If no contiguous range starting from 0 exists yet, it reports -1. ...