code: func (s *CassandraSource) Generate(f *flow.Flow) *flow.Dataset {
  	return s.genShardInfos(f).RoundRobin(s.prefix, s.Concurrency).Map(s.prefix+".Read", MapperReadShard)
  }
docs: // Generate generates data shard info,
  // partitions them via round robin,
  // and reads each shard on each executor