private Optional<ConsumerGroupMetadata> initializeGroupMetadata(final ConsumerConfig config,
                                                                    final GroupRebalanceConfig groupRebalanceConfig) {
        final Optional<ConsumerGroupMetadata> groupMetadata = initializeGroupMetadata(
            groupRebalanceConfig.groupId,
            groupRebalanceConfig.groupInstanceId
        );
        if (!groupMetadata.isPresent()) {
            config.ignore(ConsumerConfig.AUTO_COMMIT_INTERVAL_MS_CONFIG);
            config.ignore(THROW_ON_FETCH_STABLE_OFFSET_UNSUPPORTED);
        }
        return groupMetadata;
    }
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