Conquer Apache Kafka 2025 – Dive into Data Streaming Dominance!

In Kafka, a replica is considered in-sync when it aligns closely with the leader's log, which is essential for maintaining data consistency and availability. One of the criteria for a replica to be classified as in-sync is that it must have fetched the latest messages from the leader. This ensures that all replicas have up-to-date information and can serve read requests if the leader becomes unavailable.

The ability to contact Zookeeper is also crucial, as it helps manage broker coordination and metadata but does not directly influence the in-sync status of a replica. However, regarding the option provided in the answer, it is accurate to state that while a replica can indeed lag behind the leader, it must frequently update to remain in-sync. If a replica lags significantly without fetching updates, it could be marked as out-of-sync. Therefore, the idea that a replica can lag behind while maintaining its in-sync status is misleading when it implies that no updates are necessary for a healthy and consistently synced replica.

Furthermore, having the replica hosted on a different broker than the leader is a common practice to ensure fault tolerance and reliability. This separation helps prevent data loss if the leader fails, as there would still be copies of the data available on separate brokers.

Overall, the correct answer reflects