How to check which partition is a key assign to in kafka?

Question

I am trying to debug a issue for which I am trying to prove that each distinct key only goes to 1 partition if the cluster is not rebalancing.

So I was wondering for

Answer 1

when you send message, Partitions are determined by the following classes

https://github.com/apache/kafka/blob/trunk/clients/src/main/java/org/apache/kafka/clients/producer/internals/DefaultPartitioner.java

If you want change logics, implement org.apache.kafka.clients.producer.Partitioner interface and, set ProduceConfig's 'partitioner.class'

reference docuement : https://kafka.apache.org/documentation/#producerconfigs

Answer 2

As explained here or also in the source code

You need the byte[] keyBytes assuming it isn't null, then using org.apache.kafka.common.utils.Utils, you can run the following.

Utils.toPositive(Utils.murmur2(keyBytes)) % numPartitions;

For strings or JSON, it's UTF8 encoded, and the Utils class has helper functions to get that.
For Avro, such as Confluent serialized values, it's a bit more complicated (a magic byte, then a schema ID, then the data). See Wire format

only goes to 1 partition

This isn't a guarantee. Hashes can collide.

It makes more sense to say that a given key isn't in more than one partition.

if the cluster is not rebalancing

Rebalancing will still preserve a partition value.