public List<ProducerBatch> inFlightBatches(TopicPartition tp) {
        return inFlightBatches.containsKey(tp) ? inFlightBatches.get(tp) : new ArrayList<>();
    }
The background thread that handles the sending of produce requests to the Kafka cluster. This thread makes metadata
requests to renew its view of the cluster and then sends produce requests to the appropriate nodes.
clients/src/main/java/org/apache/kafka/clients/producer/internals/Sender.java