private void sendProduceRequests(Map<Integer, List<ProducerBatch>> collated, long now) {
        for (Map.Entry<Integer, List<ProducerBatch>> entry : collated.entrySet())
            sendProduceRequest(now, entry.getKey(), acks, requestTimeoutMs, entry.getValue());
    }
Transfer the record batches into a list of produce requests on a per-node basis
clients/src/main/java/org/apache/kafka/clients/producer/internals/Sender.java