How does a Kafka stream-processing pipeline achieve exactly-once semantics end to end?

At-least-once duplicates and at-most-once loses data; everyone wants exactly-once and few can explain how Kafka delivers it. The signal is idempotent producers plus transactional read-process-write plus the read-committed isolation that ties them together.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.