Exactly once semantics are guaranteed by storing offsets(by topic, partition) in a separate clickhouse table. The table will be ordered by time and a materialized view will be created as a cache to avoid expensive table scans.
Topic management will be disabled in Kafka connect and the precommit() function in
SinkTask would return the offsets stored in clickhouse.
References: [1] https://stackoverflow.com/questions/55608325/clickhouse-select-last-record-without-max-on-all-table