Apache Kafka Practice Exam 2025 – The Complete All-in-One Guide for Exam Success!

When an out-of-sync replica becomes the leader in Kafka, messages that were not synced to the new leader are lost. This happens because Kafka ensures that only in-sync replicas (ISRs) can be elected as leaders for a given partition. If a replica that has fallen out of sync is promoted to leader, it may not have the full set of messages that were present on the previous leader. As a result, any messages that were produced to the partition while the replica was out of sync will not be available after it becomes the leader. This emphasizes the importance of maintaining synchronization among replicas to ensure data integrity and consistency in the Kafka cluster.

The other options do not accurately describe the behavior of Kafka in this scenario. For instance, all messages are not preserved when a leader is elected from an out-of-sync replica; consequently, the option stating that messages are preserved during the transition is incorrect. The processes in Kafka do not halt for synchronization completion in this case; instead, the system moves on with the new leader elected. Finally, while backups can be a part of a Kafka deployment strategy, automatic backup triggers are not a standard behavior resulting from an out-of-sync replica becoming leader.