Status

Current state: "Under Discussion"

Discussion thread: here 

JIRA: KAFKA-15187 

Motivation

Current partition method only accepts key and value. Wanted to provide support for selecting partition based on headers too. This lets users pass values to partitioner method without tampering the key and value. 

Few of the use cases where this can be helpful :

1. The user has a header which provides priority of the message [lets say 0 to 9]. You have 10 partitions which you want to use for each priority. With headers available in partition method the partition can be chosen based on the header value. 
   1. Currently, there is an implementation which achieve the functionality by appending the header value to the key and stripping it off in the keySerializer when creating the keyBytes. This is a hacky solution.
2. The user wants to have a decorator partitioner which they want to use to select from a different set of partitioners based some params but do not want to tamper with the key and value. The user can pass the params in headers, evaluate it in the headers and call partitioner of interest.

Public Interfaces

Partitioner interface in org/apache/kafka/clients/producer will have the following update :

• A new overloaded method partition(String topic, Object key, byte[] keyBytes, Object value, byte[] valueBytes, Cluster cluster, Headers headers) with default implementation to call the existing method.

Proposed Changes

• Add a new overloaded method partition(String topic, Object key, byte[] keyBytes, Object value, byte[] valueBytes, Cluster cluster, Headers headers) with default implementation to call the existing method in the Partitioner interface in org/apache/kafka/clients/producer.
  • This lets the users to implement the partitioner with ability to choose partition based on header values.
  • If a partitioner implements both the methods, the override of new method (one which accepts headers) will be in effect.
• Make the call to partition method in KafkaProducer to use the new method.
  • Since it is defaulted to use the existing method, this should not cause any behaviour change.
• Mark headers as readonly before the first call for
  • Make sure the client cannot update the headers.
  • Ensure consistency as it get marked before the second call.

Total changes : https://github.com/apache/kafka/pull/13981

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
  • None
• If we are changing behavior how will we phase out the older behavior?
  • NA
• If we need special migration tools, describe them here.
  • NA
• When will we remove the existing behavior?
  • NA

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?

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Rejected Alternatives

None