Which statement about high watermark is false?

• A. It marks the offset of the last message that was successfully copied to all replicas.
• B. It bounds reads to the high watermark offset.
• C. It is the offset of the oldest message in the log.
• D. It is updated only when new messages are produced.

Answer: (C)

High watermark is the boundary that defines what has been safely replicated across the cluster and is thus readable by consumers. It is the offset of the last message that has been replicated to all in-sync replicas, so reads are bounded up to this point. This makes reads deterministic across replicas and protects against reading data that isn’t durably replicated yet. The boundary can advance not only when producers add new messages but also as in-sync followers catch up to the current log, so it moves forward with replication progress, not solely on new production.

The statement that is false is the idea that the high watermark is the offset of the oldest message in the log. The oldest message is simply the earliest offset in the log, not the committed boundary.