IIRC the HW of the followers is updated after it writes the message to memory meaning that if the leader failed that instant, it would have writes which weren't

IIRC the HW of the followers is updated after it writes the message to memory meaning that if the leader failed that instant, it would have writes which weren't acknowledged to the client. Kafka is a at-least-once system, not transactional. It's been a while since I've read the code though, so it might have changed.