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

    private final byte id;

    RebalanceProtocol(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 protocol 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();
    }
}