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Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

The motivation for KIP-1321 emerged during the implementation of KAFKA-20173, which is part of a larger initiative to provide comprehensive header support in Kafka Streams state stores, as outlined in KIP-1271: Allow to Store Record Headers in State Stores.

The StreamPartitioner interface is used to determine the destination partition for records sent to sink topics or internal repartition topics. Currently, the partitions method only receives the topic, key, value, and the number of partitions.

By adding header support to StreamPartitioner, we ensure that headers are consistently available throughout the Kafka Streams processing pipeline—from the source, through transformations and state stores (as per KIP-1271), to final partitioning and serializationDescribe the problems you are trying to solve.

Public Interfaces

Briefly list any new interfaces that will be introduced as part of this proposal or any existing interfaces that will be removed or changed. The purpose of this section is to concisely call out the public contract that will come along with this feature.

A public interface is any change to the following:

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Binary log format

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The network protocol and api behavior

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Any class in the public packages under clientsConfiguration, especially client configuration

• org/apache/kafka/common/serialization

• org/apache/kafka/common

• org/apache/kafka/common/errors

• org/apache/kafka/clients/producer

• org/apache/kafka/clients/consumer (eventually, once stable)

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Monitoring

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Command line tools and arguments

We propose adding a new method to the StreamPartitioner interface. To maintain backward compatibility, this will be a default method that delegates to the existing partitions method.

org.apache.kafka.streams.processor.StreamPartitioner

```java
@FunctionalInterface
public interface StreamPartitioner<K, V> {

    /**
     * Determine the number(s) of the partition(s) to which a record with the given key and value should be sent,
     * for the given topic and current partition count.
     * 
     * @param topic the topic name this record is sent to
     * @param key the key of the record
     * @param value the value of the record
     * @param numPartitions the total number of partitions
     * @return an Optional of Set of integers between 0 and {@code numPartitions-1}
     */
    Optional<Set<Integer>> partitions(String topic, K key, V value, int numPartitions);

    /**
     * Determine the number(s) of the partition(s) to which a record with the given key, value, and headers should be sent,
     * for the given topic and current partition count.
     * 
     * @param topic the topic name this record is sent to
     * @param key the key of the record
     * @param value the value of the record
     * @param headers the headers of the record
     * @param numPartitions the total number of partitions
     * @return an Optional of Set of integers between 0 and {@code numPartitions-1}
     */
    default Optional<Set<Integer>> partitions(String topic, K key, V value, Headers headers, int numPartitions) {
        return partitions(topic, key, value, numPartitions);
    }
}
```

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Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

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