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public enum RebalanceProtocol {
    EAGER((byte) 0), COOPERATIVE((byte) 1);

    private final byte id;

    RebalanceStrategyRebalanceProtocol(byte id) {
        this.id = id;
    }

    public byte id() {
        return id;
    }

    public static RebalanceProtocol forId(byte id) {
        switch (id) {
            case 0:
                return EAGER;
            case 1:
                return COOPERATIVE;
            default:
                throw new IllegalArgumentException("Unknown rebalance strategyprotocol id: " + id);
        }
    }
}

interface PartitionAssignor {

    // existing interfaces

    short version();                                         // new API, the version of the assignor which indicate the user metadata / algorithmic difference.

    String name();

    List<RebalanceProtocol> supportedProtocols();            // new API, indicate which rebalance strategy it would work with;
                                                             // and associate the protocol with a unique name of the assignor.

    class Subscription {
        public List<String> topics();

        public List<TopicPartition> ownedPartitions();       // new API, on older version 1 should always be empty

        public ByteBuffer userData();
    }

    class Assignment {
        public List<TopicPartition> partitions();

        public ConsumerProtocol.Errors error();             // new API, on older version 1 should always be NONE

        public ByteBuffer userData();
    }
}

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With the existing built-in Assignor implementations, they will be updated accordingly:

The reason we make "range" and "round-robin" to not support cooperative rebalance is that, this protocol implicitly relies on the assignor to be somewhat sticky to make benefits by trading an extra rebalance. However, for these two assignors, they would not be sticky (although sometimes range may luckily reassign partitions back to old owners, it is not best-effort) and hence we've decided to not make them be selected for cooperative protocol.

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From the user's perspective, the upgrade path of leveraging new protocols is just the same as 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. 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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The existing built-in Assignor implementations will then be updated to:

Although it makes the upgrade path simpler since we would no longer need the "rebalance.protocol" config on consumer anymore, while just encoding multiple assignors during the first rolling bounce of the upgrade path, it requires duplicated assignor class (of course, the new class could just be extending from the old one and there's not much LOC duplicated) which is a bit cumbersome.

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