Which of the following best describes the high watermark's role?

• A. It marks the offset of the last message replicated to all replicas
• B. It marks the offset of the earliest message in the log
• C. It marks the offset of the last committed transaction
• D. It marks the offset for the next produced message

Answer: (A)

The high watermark represents the boundary of what’s safely replicated and considered committed for a partition. It marks the offset of the last message that has been replicated to all in-sync replicas, so any consumer reading up to this offset can trust those messages are durable across the cluster. As replicas catch up, the high watermark moves forward, expanding the range of committed messages.

This isn’t the offset of the earliest message in the log, which is about where the log starts. It isn’t the offset of the last committed transaction, which is a transactional concern rather than replication-wide commit boundaries. And it isn’t the offset for the next produced message, which would be the current end of the log being written to, not the replication-safe boundary.