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This KIP could be seen as an spin-off of KIP-740: Clean up public API in TaskId and fix TaskMetadata#taskId(). During the one of the Pull Requests that were created to implement the aforementioned KIP, it became apparent that TaskMetadata should have was never been a class available for the general public, but more of an internal classthat the user would ever need to instantiate, but rather an API for exposing metadata. For this reason, it would make sense to separate the public interface from the internal implementation, hence this proposal to create the TaskMetadata interface. See https://github.com/apache/kafka/pull/10755#discussion_r639338584 for further context on this topic.

Also it is important to notice that the goal of this KIP needs to still be consistent with that of KIP-740, which was to not expose TaskID as a String in the public APIs.

For reasons described in further detail under "Public Interfaces" this KIP should be part of release 3.0.0, so we can keep the APIs consistent without the need to wait for a full deprecation cycle.

Public Interfaces

Create a new Interface named TaskMetadata under package org.apache.kafka.streams, which will contain all currently non deprecated methods:

...

/**
 * Represents the state of a single task running within a {@link KafkaStreams} application.
 * @deprecated since 3.0, use {@link org.apache.kafka.streams.TaskMetadata} instead
 */
@Deprecated
public class TaskMetadata {
...
}

Furthermore, the method getTaskId newly introduced in KIP-740 (but not yet released) will be removed, as it won't be needed any more. For this reason, it makes sense to include this KIP in release 3.0.0 so we can avoid a full cycle of deprecation for this newly introduced method.

// This method will be removed in this KIP
/**
 * @return the basic task metadata such as subtopology and partition id
*/
public TaskId getTaskId() {
   return taskId;
}

Proposed Changes

By introducing a new interface on a different package, we are able to keep the same name (no need to come up with some forced name), and also, we are freed from carrying over changes like the one introduced in https://github.com/apache/kafka/pull/10755 where a method getTaskId was introduced (going against Kafka naming convention) because taskId was already taken and needed to be deprecated.
Together with this change, a new class TaskMetadataImpl will be created under org.apache.kafka.streams.processor.internals, which will implement the aforementioned interface with the implementation present on the org.apache.kafka.streams.processor.TaskMetadata. The rest of the Kafka code base using the newly deprecated TaskMetadata will be migrated to use the new classes instead.

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