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Integration / Apache Kafka Interview questions

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.

sequenceDiagram participant P as Producer participant L as Leader broker participant F as Follower broker P->>L: Produce request (new records) L->>L: Append records to local log L-->>P: Ack once acks condition satisfied F->>L: Fetch request (from follower's last known offset) L-->>F: Records since that offset F->>F: Append to local log F->>L: Next fetch request, now caught up to leader's latest offset L->>L: Advance follower's tracked position, update ISR membership if needed

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.

Is Kafka's replication protocol push-based (leader sends to followers) or pull-based?
What determines whether a follower is dropped from the ISR?

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