how does kafka connect hdfs sink connector ensure exactly-once delivery?

Solution for how does kafka connect hdfs sink connector ensure exactly-once delivery?
is Given Below:

https://docs.confluent.io/kafka-connect-hdfs3-sink/current/overview.html#exactly-once-delivery

The connector uses a write-ahead log to ensure each record is written to HDFS exactly once. Also, the connector manages offsets by encoding the Kafka offset information into the HDFS file so that it can start from the last committed offsets in case of failures and task restarts.

Those are for ensuring it in case of failures.
I wonder how this connector ensures exactly-once in normal status.

Is HdfsSinkConnector idempotent and Transactional?
Where could I find it out?