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Current state: Under DiscussionAccepted

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JIRA: KAFKA-2273

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Motivation

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.

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• the numbers of topic partitions assigned to consumers differ by at most one; or
• if a consumer A has 2+ fewer topic partitions assigned to it compared to another consumer B, none of the topic partitions assigned to B A can be assigned to AB.

When starting a fresh assignment, the Sticky Assignor would distribute the partitions over consumers as evenly as possible. Even though this may sound similar to how round robin assignor works, the second example below shows that it results in a more balanced assignment.

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Suppose there are three consumers C₀, C₁, C₂, four topics t₀, t₁, t₂, t₃, and each topic has 2 partitions, resulting in partitions t₀p₀, t₀p₁, t₁p₀, t₁p₁, t₂p₀, t₂p₁, t₃p₀, t₃p₁. Each consumer is subscribed to all three four topics.

The assignment with both sticky and round robin assignors results in

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The round robin assignor would produce this (which is not the stickies stickiest assignment):

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