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Now that 3.0 is coming up, we can change the default ConsumerPartitionAssignor to something better than the RangeAssignor. The original plan was to switch over to the StickyAssignor, but now that we have incremental cooperative rebalancing, we should consider using the new "CooperativeStickyAssignor, RangeAssignor" instead: this will enable the consumer group to follow the COOPERATIVE protocol, improving the rebalancing experience OOTB, if all consumers upgraded to the new byte-code. Once KAFKA-12477 is completed, we can ensure that all consumers will use CooperativeStickyAssignor after they all upgraded, or fall back to RangeAssignor if some consumers are in old version.
In Kafka, we currently support the following assignors:
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this will require users to follow the upgrade path laid out in KIP-429 to safely perform a rolling upgrade. Please note: KAFKA-12477 is doing the upgrade experience improvement to reduce the safe upgrade path to just a single rolling bounce. Also, KAFKA-12477 will also make sure it will fall back to "RangeAssignor" if user doesn't follow the recommended upgrade path. It's still in progress so far. Before it completed, we still need to follow the following upgrade path:
<Copy from KIP-429>
From the user's perspective, the upgrade path of leveraging new protocols is similar to switching to a new assignor. For example, assuming the current version of Kafka consumer is 2.2 and "range" assignor is specified in the config (or no assignor is configured, which is identical as the RangeAssignor is the default below 3.0). The upgrade path would be:
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The key point behind this two rolling bounce is that, we want to avoid the situation where leader is on old byte-code and only recognize "eager", but due to compatibility would still be able to deserialize the new protocol data from newer versioned members, and hence just go ahead and do the assignment while new versioned members did not revoke their partitions before joining the group. Note the difference with KIP-415 here: since on consumer we do not have the luxury to leverage on list of built-in assignors since it is user-customizable and hence would be black box to the consumer coordinator, we'd need two rolling bounces instead of one rolling bounce to complete the upgrade, whereas Connect only need one rolling bounce.
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