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Table of Contents

Status

Current state: "Under Discussion"Implemented and merged

Discussion thread: here [Change the link from the KIP proposal email archive to your own email thread]

JIRA: here

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

Motivation

KIP-478, a the new strongly-typed Processor API, brings the option to reconsider the abstractions for around custom processing in the Kafka Streams DSL.

Currently, multiple Transformers and a final Processor operation are the options available to implement custom processing with access to the Record context; including record metadata (incle.g. topic, partition, offset), timestamp, and headers; and reference to state stores.

There have been discussions on how to refactor these APIs to minimize/simplify into a single processor:

Transformers are currently limited to the old ProcessorContext, and there have been multiple extensions to support value processing without repartitioningand one-to-many record processing. With the addition of the new Processor API, KStream can be extended to offer access to the newnewer, chainable API that can typed API and effectively replace most of the operations transformers  *Transforms offer at the moment, with more open control to forward records.

This KIP could be considered as a step towards deprecating Transforms, though that should be discussed in a follow-up KIP.

Public Interfaces

...

Modified methods

  • KStream<KOut,VOut> KStream#process(ProcessorSupplier<K, V, KOut, VOut> processorSupplier, String... stateStoreNames)
    • from void KStream#process(ProcessorSupplier<K, V, Void, Void> processorSupplier, ...)
  • KStream<KOut,VOut> KStream#process(ProcessorSupplier<K, V, KOut, VOut> processorSupplier, Named named, String... stateStoreNames)
    • from void KStream#process(ProcessorSupplier<K, V, Void, Void> processorSupplier, ...)

New methods

Processors without forcing repartitioning:

  • KStream<K,VOut> KStream#processValues(FixedKeyProcessorSupplier<K, V, K, VOut> processorSupplier, String... stateStoreNames)
  • KStream<K,VOut> KStream#processValues(ProcessorSupplier<KFixedKeyProcessorSupplier<K, V, K, VOut> processorSupplier, Named named, String... stateStoreNames)

Internal changes

  • Infrastructure for Fixed Key Records:
    • FixedKeyRecord:
      • Not a Record sub/superclass to casting to Record.
      • Private constructor to avoid reconstructing record and change key.
    • FixedKeyProcessor(Supplier) and FixedKeyProcessorContext interfaces for FixedKeyRecord.
      • FixedKeyContextualProcessor abstract class, similar to ContextualProcessor.

Deprecated methods

All transform operations on the KStream will be deprecated in favor of process  and processValues  operations:

  • KStream#transform
  • KStream#flatTransform
  • KStream#transformValues
  • KStream#flatTransformValues

Proposed Changes

...

KStream#process and 

...

KStream#processValues 

...

to replacing most Transformers

With the ability to manage  forward  calls as part of the  Processor  itself, transform,  flatTransform  valueTransform, flatTransform, and flatValueTransform can be replaced by a  Processor process/processValues:

Code Block
languagejava
  private Topology topology() {
    final var builder = new StreamsBuilder();
    final var input = builder.stream("words", Consumed.with(Serdes.String(), Serdes.String()));
    input.processValues(    .processValues(() -> new FixedKeyContextualProcessor<String, String, String>() {
            @Override
            public void process(FixedKeyRecord<String, String> record) {
                for (final var word : record.value().split(",")) {
                    context().forward(record.withValue("Hello " + word));
                }
            }}, Named.as("process-values-without-repartitioning"))
        .process(() -> new ContextualProcessor<String, String, String, String>() {
            @Override
            public void process(Record<String, String> record) {
                for (final var sword : record.value().split(",")) {
                    context().forward(record.withKey(word).withValue("Hello " + sword));
                }
      }
      }}, Named.as("testprocess-with-partitioning"))

        .to("output", Produced.with(Serdes.String(), Serdes.String()));
    return builder.build();
  }

New ValueProcessorContext to enforce only Value changes

Processors that require enforcing the same key to avoid repartitioning at the moment trust internal implementations. As a result the KStream API added *Values methods to signal when key is not changed, and repartitioning can be avoided.

At the moment record keys can be changed or records recreated between the record entering Processor#process(Record) and ProcessorContext#forward(Record).

As an alternative to enforce the same key, ProcessorContext wrapper could be implemented to define an initial key:

Infrastructure for Fixed Key Records
FixedKeyRecord
Record with immutable key.
 Code Block
public final class FixedKeyRecord<K, V> {

 Code Block
languagejava
public class ValueProcessorContext<KForward, VForward> implements ProcessorContext<KForward, VForward> {
//...
    private final KForwardK key;
    
private final V value;
 public void setRecordKey(KForward initialKey)private {
final long timestamp;
    private final this.key = initialKey;Headers headers;

    }
FixedKeyRecord(final K key, final 
V value, final long timestamp, publicfinal voidHeaders clearRecordKey(headers) {
        this.key = nullkey;
    }

    @Override
this.value = value;
  public <K extends KForward, V extends VForward> voidif forward(Record<K,timestamp V>< record0) {
            throw ifnew StreamsException(key
 != null) {
            if (!record.key().equals(key)) {
  "Malformed Record",
               throw new IllegalArgumentException("KeyTimestamp hasmay changednot whilebe processingnegative. theGot: record" and requires enforcing repartitioning.");+ timestamp)
            });
        }
        delegate.forward(record)this.timestamp = timestamp;
      }
//...
}
This ValueProcessorContext could be used internally at the Processor level, and the method setRecordKey(K) must be used before calling user-defined Processor.
 Code Block
public class KStreamValueProcessorSupplier<KIn, VIn, VOut> implements ProcessorSupplier<KIn, VIn, KIn, VOut> {
// get ...  this.headers = new RecordHeaders(headers);
    }

    public K key() { return key; }

    public static class KStreamValueProcessor<KIn, VIn, VOut> extends ContextualProcessor<KIn, VIn, KIn, VOut> { V value() { return value; }

    public long timestamp() { return timestamp; }

    public Headers headers()  private final Processor<KIn, VIn, KIn, VOut> processor;

        private ValueProcessorContext<KIn, VOut> processorContext;

        public KStreamValueProcessor(Processor<KIn, VIn, KIn, VOut> transformer) {{ return headers; }

    public <NewV> FixedKeyRecord<K, NewV> withValue(final NewV value) { return new FixedKeyRecord<>(key, value, timestamp, headers); }

    public FixedKeyRecord<K, V> withTimestamp(final long timestamp) { return new FixedKeyRecord<>(key, value, timestamp, headers); }

    public FixedKeyRecord<K, V> withHeaders(final Headers headers) { return new  this.processor = transformer;
        }

FixedKeyRecord<>(key, value, timestamp, headers); }
}

FixedKeyProcessorSupplier
 Code Block
@FunctionalInterface
public interface FixedKeyProcessorSupplier<KIn, VIn, VOut> extends ConnectedStoreProvider, Supplier<FixedKeyProcessor<KIn, VIn, VOut>> {
    FixedKeyProcessor<KIn, VIn, VOut>  @Override
    get();
}


FixedKeyProcessor

 Code Block
public interface FixedKeyProcessor<KIn, VIn, VOut> {

    publicdefault void init(final ProcessorContext<KInFixedKeyProcessorContext<KIn, VOut> context) {}

    void process(FixedKeyRecord<KIn,       super.init(contextVIn> record);

    default void close() {}
}


FixedKeyContextualProcessor
Helper, same as ContextualProcessor.
 Code Block
public abstract class FixedKeyContextualProcessor<KIn, VIn, this.processorContextVOut> =implements new ValueProcessorContext<>(context);
  FixedKeyProcessor<KIn, VIn, VOut> {

    private FixedKeyProcessorContext<KIn,     processor.init(processorContext);VOut> context;

    protected FixedKeyContextualProcessor()   {}

        @Override
        public void processinit(final Record<KInFixedKeyProcessorContext<KIn, VIn>VOut> recordcontext) {
        this.context    processorContext.setRecordKey(record.key())= context;
    }

    protected final FixedKeyProcessorContext<KIn, VOut> processor.processcontext(record); {
            processorContext.clearRecordKey();
    return context;
    }
    }

}

ProcessingContext
To be extended by FixedKeyProcessorContext  and ProcessorContext Based on the previous example, if key is changed then processing will fail :
 Code Block
interface ProcessingContext {

   private TopologyString topologyapplicationId();

 {
    final var builder = new StreamsBuilderTaskId taskId();

    Optional<RecordMetadata> recordMetadata();

    final var input = builder.stream("words", Consumed.with(Serdes.String(), Serdes.String()));
    input.processValues(() -> new ContextualProcessor<String, String, String, String>() {
      @Override
Serde<?> keySerde();

    Serde<?> valueSerde();

    File stateDir();

    StreamsMetrics metrics();

    <S extends StateStore> S getStateStore(final String name);

    Cancellable schedule(final Duration interval,
        public   void process(Record<String, String> record) {
        for (var sfinal : record.value().split(",")) {PunctuationType type,
          context().forward(record.withKey("FAIL").withValue("Hello " + s));
        }
    final Punctuator }callback);

    }, Named.as("test"))void commit();

    .to("output"Map<String, Produced.with(Serdes.String(), Serdes.String()));
    return builder.build();
  } Object> appConfigsWithPrefix(final String prefix);
}

FixedKeyProcessorContext
 Code Block
public interface FixedKeyProcessorContext<KForward, VForward> extends ProcessingContext {

    <K extends KForward, V extends VForward> void forward(FixedKeyRecord<K, V> record);

    <K extends KForward, V extends VForward> void forward(FixedKeyRecord<K, V> record, final String childName);
}
 Code Block
 12:08:19.125 [new-process-c2e58cba-3c47-41a9-b221-adea4cd38c87-StreamThread-1] ERROR org.apache.kafka.streams.processor.internals.TaskManager - stream-thread [new-process-c2e58cba-3c47-41a9-b221-adea4cd38c87-StreamThread-1] Failed to process stream task 0_0 due to the following error:
org.apache.kafka.streams.errors.StreamsException: Exception caught in process. taskId=0_0, processor=KSTREAM-SOURCE-0000000000, topic=words, partition=0, offset=6, stacktrace=java.lang.IllegalArgumentException: Key has changed while processing the record and requires enforcing repartitioning. 

Compatibility, Deprecation, and Migration Plan
Modified method KStream#process should be compatible with previous version, that at the moment is fixed to a Void return type.
Leaving Transform API with older ProcessorContext could be considered a driver to adopt the methods proposed in this KIP.
KStreams#process Return type change
Changing return type from void to KStream<KOut, VOut> is source-compatible, but not binary-compatible. It will require users to recompile the application to use the latest version of the library.
Though users will not require to change their code as the current returned value is void, and the input processor supplier types will include Void  that is the current type for output key and value.
KStream#*transform* deprecations
This KIP is including the deprecation of the transform operations on KStream to propose using the latest Processor API operations.
Transform API is not marked as deprecated yet, as it requires additional considerationsEven though, the new Processor API could potentially replace most Transformer implementations, it is not yet possible to deprecate Transformer APIs:
  • Transform API is broadly adopted for custom processing. Even though most functionality is possible to implement with new Processor, this migration is not straightforward.
  • KTable#transformValues is an interesting use-case for ValueTransformerWithKey as the internal value is Change<V> — an internal type to handle record changes on KTable — not exposed as public API. An approach would be to deprecate this method in favor of .toStream().processValues().toTable().
A new KIP should be proposed to continue the deprecation of Transformer  APIs.
Rejected Alternatives
...
Migrate Transform APIs 
...
to the latest ProcessContext
This would involve extending KStream/KTable APIs even more with specific Transform variants. If a new Processor is able to can support most Transform in the long-term becoming the adopted option to transform values, flat transform, etc.; then we should start getting the new Processor properly adopted in the DSL and let the usage drive the next steps.
Runtime key validation on KStream#processValues and changing key output type to Void
On one hand, with Void type for key output, we force the users to cast to Void and change the key to null, though this can be documented on the API, so the users are aware of the peculiarity of forwarding within processValues. 
On the other hand, keeping the key type as output doesn't require to do any change of keys, but this could lead to key-checking runtime exceptions when changing keys.
This KIP proposes adding a new Record type for Fixed Keys, to remove the need for runtime checked and reduce as much as possible the ability to change the key when implementing the processor.
References