How does kafka decides the partition if I don't mention any

Question

This is how I produce message :

String json = gson.toJson(msg);

ProducerRecord<String, String> record = new ProducerRecord<>(kafkaProducerConfig.getTopic(), json);
long startTime = System.currentTimeMillis();

try {
    RecordMetadata meta = producer.send(record).get(5, TimeUnit.SECONDS);
} catch (InterruptedException e) {
    e.printStackTrace();
} catch (ExecutionException e) {
    e.printStackTrace();
} catch (TimeoutException e) {
    e.printStackTrace();
}

I have 15 partitions for this topic, I did not mention the partition key while producing, what will be the default partition assigned ?

Answer 1

Since you're sending no key as part of the record, it is null.

Kafka has a DefaultPartitioner that will round-robin any null keys over each partition. As of Kafka 2.4, a StickyPartitioner was introduced, so partition is only computed per batch rather than per record.

For non-null keys, a Murmur2 hash is computed, then modulo'd by the number of partitions for the topic

Answer 2

If you are not defined any custom partition it will use the default partitioner as per the below rule

1. If a partition is specified in the record, use it that partition to publish.
2. If no partition is specified but a key is present choose a partition based on a hash of the key
3. If no partition or key is present choose a partition in a round-robin fashion

Below default, Partition implementation to get a better understanding

public int partition(String topic, Object key, byte[] keyBytes, Object value, byte[] valueBytes, Cluster cluster) {
        List<PartitionInfo> partitions = cluster.partitionsForTopic(topic);
        int numPartitions = partitions.size();
        if (keyBytes == null) {
            int nextValue = nextValue(topic);
            List<PartitionInfo> availablePartitions = cluster.availablePartitionsForTopic(topic);
            if (availablePartitions.size() > 0) {
                int part = Utils.toPositive(nextValue) % availablePartitions.size();
                return availablePartitions.get(part).partition();
            } else {
                // no partitions are available, give a non-available partition
                return Utils.toPositive(nextValue) % numPartitions;
            }
        } else {
            // hash the keyBytes to choose a partition
            return Utils.toPositive(Utils.murmur2(keyBytes)) % numPartitions;
        }
    }