private ConsumerGroupMetadata initializeConsumerGroupMetadata(final String groupId,
                                                                  final Optional<String> groupInstanceId) {
        return new ConsumerGroupMetadata(
            groupId,
            JoinGroupRequest.UNKNOWN_GENERATION_ID,
            JoinGroupRequest.UNKNOWN_MEMBER_ID,
            groupInstanceId
        );
    }
A thread-safe {@link FetchBuffer fetch buffer} for the results that are populated in the
{@link ConsumerNetworkThread network thread} when the results are available. Because of the interaction
of the fetch buffer in the application thread and the network I/O thread, this is shared between the
two threads and is thus designed to be thread-safe.
clients/src/main/java/org/apache/kafka/clients/consumer/internals/AsyncKafkaConsumer.java