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Status

Current state: Under Discussion

Discussion thread: [DISCUSS] KIP-251: Allow timestamp manipulation in Processor API

JIRA:  Unable to render Jira issues macro, execution error.

Released: 1.2

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

Motivation

 

Kafka Streams has a defined "contract" about timestamp propagation at the Processor API level: all processors within a sub-topology, see the timestamp from the input topic record that is currently processed and this timestamp will be used for all result records when writing them to a topic, too. For the DSL and also for custom operators, it would be desirable to allow timestamp manipulation at Processor API level for individual records that are forwarded. This allows to support a larger scope of possible semantics for stream processing.

Public Interfaces

Using Processor API, users use ProcessorContext to forward record to downstream operators by calling forward(). Thus, we need to add overloads for forward() to allow user to pass in a timestamp for the output record.

package org.apache.kafka.streams.processor;


public interface ProcessorContext {
  // existing overloads of forward()
  <K, V> void forward(K key, V value);
  <K, V> void forward(K key, V value, int childIndex);
  <K, V> void forward(K key, V value, String childName);


  // new overloads of forward()
  <K, V> void forward(K key, V value, long timestamp);
  <K, V> void forward(K key, V value, long timestamp, int childIndex);
  <K, V> void forward(K key, V value, long timestamp, String childName);

  // other existing methods omitted for brevity
}

 

Proposed Changes

We add three new overloads of ProcessorContext#forward() that take an additional timestamp parameter of type long. If users call those new overloads, the output record gets the specified timestamp assigned. For the existing methods, the default contract using the input record timestamp for the output record will be used.

Compatibility, Deprecation, and Migration Plan

This change is backward compatible, as we don't alter the contract of existing methods, and only add new method with new functionality.

Test Plan

We can test this feature with unit tests, by implementing test Processors that manipulate the output record timestamp using the new API, and by verifying that the output record have the assigned timestamps. Existing test ensure, that the old behavior is preserved.

Rejected Alternatives

None.

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