If enable.auto.commit is true and you do not use synchronous processing, what offset commit semantics do you get?

• A. No commits
• B. At-least-once
• C. Exactly-once
• D. At-most-once

Answer: (D)

With auto-commit enabled, the consumer advances and stores its committed offsets on a timer, independently of how you process messages. If you’re not using synchronous (manual) commits after your processing finishes, the commit can occur while you’re still working on a batch or even after an error, meaning some messages may be considered processed and their offsets committed before you’ve actually completed handling them. If a crash happens after that commit, those messages won’t be reprocessed, so you can lose data. That combination yields at-most-once semantics. Exactly-once would require coordinated transactional guarantees, and at-least-once would come from committing only after successful processing, which isn’t the case here.