This page is meant as a template for writing a KIP. To create a KIP choose Tools->Copy on this page and modify with your content and replace the heading with the next KIP number and a description of your issue. Replace anything in italics with your own description.

Status

Current state: DraftUnder Discussion

Discussion thread: https://lists.apache.org/thread/7y7svyp3f560fzv1bgcr893vn258cn06

JIRA:

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:

• Binary log format

• The network protocol and api behavior

• 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)

• Monitoring

• Command line tools and arguments

• Anything else that will likely break existing users in some way when they upgrade

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.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Test Plan

Describe in few sentences how the KIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

...

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.

public interface StreamPartitioner<K, V> {

    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 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 headers the record headers
     * @param numPartitions the total number of partitions
     * @return an Optional of Set of integers between 0 and {@code numPartitions-1},
     * Empty optional means using default partitioner
     * Optional of an empty set means the record won't be sent to any partitions i.e drop it.
     * Optional of Set of integers means the partitions to which the record should be sent to.
     * */
    default Optional<Set<Integer>> partitions(String topic, K key, V value, Headers headers, int numPartitions) {
        return partitions(topic, key, value, numPartitions);
    }
}

    // already existing method
    public <K> KeyQueryMetadata queryMetadataForKey(final String storeName,
                                                    final K key,
                                                    final Serializer<K> keySerializer) {
        return queryMetadataForKey(storeName, key, new RecordHeaders(), keySerializer);
    }

    // already existing method 
    public <K> KeyQueryMetadata queryMetadataForKey(final String storeName,
                                                    final K key,
                                                    final StreamPartitioner<? super K, ?> partitioner) {
        return queryMetadataForKey(storeName, key, new RecordHeaders(), partitioner);
    }

    /**
     * Finds the metadata containing the active hosts and standby hosts where the key being queried would reside.
     *
     * @param storeName     the {@code storeName} to find metadata for
     * @param key           the key to find metadata for
     * @param headers       the record headers
     * @param keySerializer serializer for the key
     * @param <K>           key type
     * Returns {@link KeyQueryMetadata} containing all metadata about hosting the given key for the given store,
     * or {@code null} if no matching metadata could be found.
     */
    public <K> KeyQueryMetadata queryMetadataForKey(final String storeName,
                                                    final K key,
                                                    final Headers headers,
                                                    final Serializer<K> keySerializer) {
        validateIsRunningOrRebalancing();
        return streamsMetadataState.keyQueryMetadataForKey(storeName, key, headers, keySerializer);
    }

    /**
     * Finds the metadata containing the active hosts and standby hosts where the key being queried would reside.
     *
     * @param storeName     the {@code storeName} to find metadata for
     * @param key           the key to find metadata for
	 * @param headers       the record headers
     * @param partitioner   the partitioner to be used to locate the host for the key
     * @param <K>           key type
     * Returns {@link KeyQueryMetadata} containing all metadata about hosting the given key for the given store, using
     * the supplied partitioner, or {@code null} if no matching metadata could be found.
     */
    public <K> KeyQueryMetadata queryMetadataForKey(final String storeName,
                                                    final K key,
												    final Headers headers,
                                                    final StreamPartitioner<? super K, ?> partitioner) {
        validateIsRunningOrRebalancing();
        return streamsMetadataState.keyQueryMetadataForKey(storeName, key, headers, partitioner);
    }

Proposed Changes

1. Interface Enhancement: Add the headers-aware partitions method to StreamPartitioner. This is a default method to ensure binary compatibility.
2. Public API for Interactive Queries: Add new overloads to KafkaStreams#queryMetadataForKey that accept `Headers`. This allows users to perform metadata lookups for header-dependent partitioning strategies
3. No changes are planed for KafkaStreamsNamedTopologyWrapper. This class and entire feature are deprecated, so no changes are planed there
4. Internal Callers and Propagation:
   1. RecordCollectorImpl#send to pass record headers when calling the partitioner.
   2. StreamsMetadataState to support the new header-aware lookups.
5. Built-in Implementations:

   1. WindowedStreamPartitioner: Override the new method to propagate headers to the underlying WindowedSerializer#serializeBaseKey.

   2. DefaultStreamPartitioner: Override the new method to propagate headers to the keySerializer#serialize

Compatibility, Deprecation, and Migration Plan

• Backward Compatibility: change is fully backward compatible. Existing custom StreamPartitioner implementations will continue to work as they will use the default implementation of the new method.
• Deprecation: No methods are being deprecated in this KIP.
• Migration: Users wishing to leverage headers in their custom partitioning logic should migrate their StreamPartitioner implementations to override the new `partitions` method.

Test Plan

• Unit Tests and Integration tests:
  • Verify Partitioners correctly propagate headers.
  • Verify propagated headers are used to determine target partition

Rejected Alternatives

• Updated the existing method without default: Rejected as it would break all existing user implementations.