How do you implement a custom partitioner in Kafka?

A custom partitioner overrides Kafka's default key-hash partitioning logic when the built-in behavior doesn't fit a specific routing requirement — for example, routing all records for a given tenant to a dedicated subset of partitions for isolation, rather than letting a generic hash spread them arbitrarily.

public class TenantPartitioner implements Partitioner {
    @Override
    public int partition(String topic, Object key, byte[] keyBytes,
                          Object value, byte[] valueBytes, Cluster cluster) {
        int numPartitions = cluster.partitionCountForTopic(topic);
        String tenantId = (String) key;
        return Math.abs(tenantId.hashCode()) % numPartitions;
    }
    @Override public void configure(Map<String, ?> configs) {}
    @Override public void close() {}
}

partitioner.class=com.example.TenantPartitioner

Custom partitioning is a powerful but sharp tool: it's easy to accidentally create a "hot partition" by routing disproportionately more traffic to one partition than the others, which then becomes a bottleneck regardless of how many total partitions or consumers exist. It's worth reaching for only when the default hash-based distribution genuinely can't express the routing requirement, since uneven partition load is a common and hard-to-diagnose consequence of a poorly-designed custom partitioner.

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