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This KIP introduces org.apache.kafka.coordinator.group.share.SimpleAssignor . It attempts to balance the number of consumers assigned to the subscribed partitions, and assumes that all consumers are equally capable of consuming records.
The assignor hashes the member IDs of the members and maps the partitions assigned to the members based on the hash. This gives approximately even balance.
If any partitions
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were not assigned any members by (1), members are assigned round-robin until each partition has at least one member assigned to it.
When the number of members is greater than or equal to the number of partitions for a subscribed topic and every partition was assigned at least one member, adding more members to the group simply assigns more members to the partitions. If however, any partitions were not assigned any members by hashing, adding more members changes the distribution and will revoke some partitions previously assigned. Because assignments in share groups are looser than they are in consumer groups, the group coordinator doesn't require the members to confirm they have revoked partitions before they can be assigned to other members. For a brief period while the rebalancing is occurring, some partitions may have more members consuming than the assignment requires, but this situation soon resolves as the members receive their updated assignments and stop fetching from the revoked partitions.
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