What are the different delivery semantics in Kafka (at least-once, at-most-once, exactly-once)?

At-most-once: Producer fires and forgets. Messages may be lost. Use for metrics where loss is acceptable.

At-least-once: Producer retries until ack; consumer may see duplicates. Simpler; use with idempotent consumer.

Exactly-once: Transactional producer + transactional consumer (Kafka 0.11+). Producer commits atomically; consumer reads-processes-commits in transaction. No duplicates, no loss.

Why It Matters: Financial transactions, billing, inventory need exactly-once. Analytics can tolerate at-least-once with idempotent sink.

Scalability Trade-offs: Exactly-once adds latency; transactional commits are slower. Consumer must support transactions.

Cost Implications: Exactly-once requires more careful design; failures and retries can increase cost. At-least-once + idempotent sink often sufficient.