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Status

Current state: Under Discussion

Vote thread: https://lists.apache.org/thread/jsy1ld5f6b6k8qcjzpy5k08ropqd4dd1

Discussion thread: https://lists.apache.org/thread/n3k6vb4vddl1s5nopcyglnddtvzp4j63

JIRA: KAFKA-16796

Motivation

We are going to move all tools-related code from core module (scala) to tools module (java) in 4.0. kafka.serializer.Decoder is a public APIs and it is in core module. Hence we need to introduce replacement to deprecate it in 3.8.0

Public Interfaces

The new java interface org.apache.kafka.tools.api.Decoder is shown below. It is located at tools-api module, since it must be accessible by core module.

@FunctionalInterface
public interface Decoder<T> {
    T fromBytes(byte[] bytes);
}

The old scala trait will extend the new java interface with deprecated  annotation

@deprecated(since = "3.8.0")
trait Decoder[T] {
  def fromBytes(bytes: Array[Byte]): T
}

Proposed Changes

1. Introduce a new java interface org.apache.kafka.tools.api.Decoder to replace scala trait kafka.serializer.Decoder
   
2. deprecate all "class names" of kafka.serializer.Decoder implementations. That includes kafka.serializer.DefaultDecoder, kafka.serializer.StringDecoder, kafka.serializer.LongDecoder, and kafka.serializer.IntegerDecoder 
3. remove the VerifiableProperties from constructor. We "always" pass empty VerifiableProperties before (known issue: ), and hence using the VerifiableProperties to create new decoder does not make sense. If users require configurable decoder, we can make decoder extend Configurable in the future.

Compatibility, Deprecation, and Migration Plan

• before 4.0, users can keep using the deprecated kafka.serializer.Decoder
• after 4.0, all users need to update superclass from kafka.serializer.Decoder to org.apache.kafka.tools.api.Decoder. They have same method, but the new constructor does not accept VerifiableProperties. Hence, users' custom decoders have to offer default constructor
  
• before 4.0, users can access both interfaces by depending on core module.
• after 4.0, users need to import tools-api module if core module is gone

Test Plan

The new unit test used to verify the "warning messages" about deprecated kafka.serializer.Decoder will be added. The messages are used to encourage users to adopt the new interface

Rejected Alternatives

N/A