public Deque<ProducerBatch> getDeque(TopicPartition tp) {
        TopicInfo topicInfo = topicInfoMap.get(tp.topic());
        if (topicInfo == null)
            return null;
        return topicInfo.batches.get(tp.partition());
    }
The earliest absolute time a batch will expire (in milliseconds)
clients/src/main/java/org/apache/kafka/clients/producer/internals/RecordAccumulator.java