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

An in-sync replica (ISR) in Apache Kafka is defined as a replica that has caught up with the leader replica in terms of the messages it contains. Specifically, it is a replica that is synchronized with the leader and has all the latest messages within a certain acknowledgment threshold. This definition is essential for ensuring high availability and durability of messages in a Kafka topic.

The correct answer emphasizes that an in-sync replica is one that has the potential to become a leader if the current leader fails. It means that this replica has been receiving and acknowledging messages from the leader within an acceptable timeframe, ensuring that it has the most recent state of the data. The ability to become a leader is crucial for the resilience of Kafka, enabling seamless failover and continued operation in case of broker failures.

This concept also ties into Kafka's design of retaining message consistency and durability, making sure that there are always replicas ready to take over leadership without data loss.

In contrast, the other interpretations do not accurately capture the essence of an in-sync replica. For instance, a replica that contains all messages is not a requirement for being in-sync, as it could be lagging behind due to various factors. Similarly, while a replica must be operational (not failed) to be considered