Explain the internal working of Kafka's replication protocol between leader and follower brokers?

Replication in Kafka is pull-based: follower brokers actively fetch new records from the partition's leader, rather than the leader pushing data out to followers, which reuses the exact same fetch-request mechanism consumers use.

The leader tracks, per follower, the highest offset it has confirmed fetching, which is what determines ISR membership — a follower whose fetch position falls too far behind (beyond replica.lag.time.max.ms) is dropped from the ISR until it catches back up. Because followers pull rather than the leader pushing, a slow follower naturally just falls further behind on its own fetch cadence rather than blocking or slowing down the leader's ability to serve producers, which keeps replication lag isolated to the affected follower instead of degrading write latency for everyone.

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