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In certain circumstances the round robin assignor, which produces better assignments compared to range assignor, fails to produce an optimal and balanced assignment of topic partitions to consumers. KIP-49 touches on some of these circumstances. In addition, when a reassignment occurs, none of the existing strategies consider what topic partition assignments were before reassignment, as if they are about to perform a fresh assignment. Preserving the existing assignments could reduce some of the overheads of a reassignment. For example, Kafka consumers retain pre-fetched messages for partitions assigned to them before a reassignment. Therefore, preserving the partition assignment could save on the number of messages delivered to consumers after a reassignment.  Another advantage would be reducing the need to cleanup local partition state between rebalances.

Public Interfaces

This assignment strategy, which is implemented for the new consumer, would add a StickyAssignor class that can be used as org.apache.kafka.clients.consumer.StickyAssignor for the value of the consumer property partition.assignment.strategy. It would not affect the default value of this consumer property.

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