code: func (c *CassandraStore) insertChunk(key string, t0, ttl uint32, data []byte) error {
  	// for unit tests
  	if c.Session == nil {
  		return nil
  	}
  
  	table, ok := c.TTLTables[ttl]
  	if !ok {
  		return errTableNotFound
  	}
  
  	row_key := fmt.Sprintf("%s_%d", key, t0/Month_sec) // "month number" based on unix timestamp (rounded down)
  	pre := time.Now()
  	ctx, cancel := context.WithTimeout(context.Background(), c.timeout)
  	ret := c.Session.Query(table.QueryWrite, row_key, t0, data, ttl).WithContext(ctx).Exec()
  	cancel()
  	cassPutExecDuration.Value(time.Now().Sub(pre))
  	return ret
  }
docs: // Insert Chunks into Cassandra.
  //
  // key: is the metric_id
  // ts: is the start of the aggregated time range.
  // data: is the payload as bytes.