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This is with respect to Consume Group Leader and Consumer Group Coordinator.I went though the documentation to understand the responsibilities of "Group Leader" and "Group Coordinator" and especially the significant reason for having other role such as ""Group Leader"

It looks like "Group Coordinator" is primarily responsible for group management be it in terms of managing consumers membership, offsets , re balancing etc. The only time "Group Leader" is needed for "Calculating partition assignments" from

After deciding on the partition assignment, the consumer group leader sends the list of assignments to the Group Coordinator, which sends this information to all the consumers. Each consumer only sees his own assignment

Does the "group leader" play any other "significant role" ? Is it something really needed that adds the value ?

Nag
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The significant role played by the group leader has to do with the distributed design that Kafka follows. While the group coordinator is one of the brokers the group leader is one of the consumers. This means that whatever processing the group leader needs to execute in order to come up with the partition assignment is going to burn CPU clocks from one of the consumers rather than one of the Kafka brokers. This approach offloads CPU consumption from Kafka brokers especially because partition assignment happens very frequently since this process repeats every time a rebalance happens.

So yes, group leaders are just responsible for partition assignment but the fact that this is executed by one of the consumers means a lot in terms of:

  • Performance: Kafka brokers are supposed to be more I/O bound than CPU bound. Offloading CPU usage as much as possible in this case is just smart.

  • Scalability: Conceptually a given Kafka architecture will be composed by few brokers and lots of consumers. Keeping the partition assignment in the consumer's lap ensures better horizontal scalability since there will be different consumer groups to be managed and with lots of consumers executing in the edge this workload can be spread evenly.

Additionally, delegating partition assignment to the group leader allows different consumer groups to utilize different partition assignment strategies. A single group coordinator could be coordinating many consumer groups, so performing partition assignment there would require all groups managed by that coordinator to use the same strategy.

Ricardo Ferreira
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  • Awesome . Could you also please share some use cases where different consumer groups might require different strategies which is one of the reason that supports the need for group leader – Nag Jul 23 '20 at 16:55
  • Different strategies are useful to give consumers the ability to control how partitions are going to land on each consumer thread. Range allows co-partitioning which is useful to ensure that the same thread will read from topics with keyed records, but it may lead to idle threads. RoundRobin fixes that and tries to evenly distributed partitions among threads, with no preference dictated. Sticky as the name implies tries to sticky partitions into threads to eliminate the need for recomputation every time a rebalance occurs. Different strategies for each consumer group is a great thing :-) – Ricardo Ferreira Jul 23 '20 at 17:31
  • Thanks . Could you also please review my other questions esp https://stackoverflow.com/questions/63012732/kafka-controller-broker – Nag Jul 23 '20 at 18:15
  • Just replied there! – Ricardo Ferreira Jul 23 '20 at 19:31