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An in-sync replica (ISR) is a broker that has the LATEST DATA for a given partition.
A leader is always an in-sync replica.
A follower is an in-sync replica only if it has fully caught up to the partition it's following.
In other words, it can't be behind on the latest records for a given partition.
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In Kafka parlance, Kafka Replication means having multiple copies of the data, spread across multiple servers/brokers.
This helps in maintaining high availability in case one of the brokers goes down and is unavailable to serve the requests.