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 a given topic, is there a way to determine which partition a key is send to?
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.
when you send message, Partitions are determined by the following classes
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
来源：https://stackoverflow.com/questions/50164566/how-to-check-which-partition-is-a-key-assign-to-in-kafka