There's some excellent choices in this design, but using Kafka in the middle makes no sense. The Cassandra change-data segments are written in batches, and BigQuery loads in batches. So the author is splitting up change-data segments into individual rows, feeding those rows into Kafka, and then re-batching them on the BQ side for ingestion.

The use of a streaming system adds a tremendous amount of complexity for no benefit, because the original data source is batch. A simpler solution would be to process the data in 5-minute batches and use a BigQuery MERGE statement to apply that "patch" to the existing table.

You really shouldn't use Kafka unless you have a true streaming data source and a hard latency requirement of < 5 minutes. It's much easier to work with data in small batches, using local disk and blob stores like S3 as your queue.