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

A follower replica in Apache Kafka may be marked as out of sync primarily when it is unable to fetch messages from the leader for a specified duration, which is 10 seconds in this context. This situation indicates that the follower is lagging behind in the replication process, failing to keep up with the latest messages being produced to the leader.

When a follower replica cannot fetch messages, it either means there are network issues impacting the follower's ability to connect with the leader or the follower is processing messages slowly. Whichever the cause, prolonged inability to replicate messages results in the follower being flagged as out of sync, creating a risk for data consistency and availability.

Other scenarios related to network conditions or heartbeat signals do not directly lead to an out-of-sync state as they pertain to connectivity and the general health of the replica rather than actual data replication delays.