org.apache.kafka.streams.kstream.ValueTransformerWithKey Maven / Gradle / Ivy
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* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
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*
* http://www.apache.org/licenses/LICENSE-2.0
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* Unless required by applicable law or agreed to in writing, software
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package org.apache.kafka.streams.kstream;
import java.time.Duration;
import org.apache.kafka.streams.KeyValue;
import org.apache.kafka.streams.errors.StreamsException;
import org.apache.kafka.streams.processor.ProcessorContext;
import org.apache.kafka.streams.processor.PunctuationType;
import org.apache.kafka.streams.processor.Punctuator;
import org.apache.kafka.streams.processor.StateStore;
import org.apache.kafka.streams.processor.To;
/**
* The {@code ValueTransformerWithKey} interface for stateful mapping of a value to a new value (with possible new type).
* This is a stateful record-by-record operation, i.e, {@link #transform(Object, Object)} is invoked individually for each
* record of a stream and can access and modify a state that is available beyond a single call of
* {@link #transform(Object, Object)} (cf. {@link ValueMapper} for stateless value transformation).
* Additionally, this {@code ValueTransformerWithKey} can
* {@link ProcessorContext#schedule(Duration, PunctuationType, Punctuator) schedule} a method to be
* {@link Punctuator#punctuate(long) called periodically} with the provided context.
* Note that the key is read-only and should not be modified, as this can lead to corrupt partitioning.
* If {@code ValueTransformerWithKey} is applied to a {@link KeyValue} pair record the record's key is preserved.
*
* Use {@link ValueTransformerWithKeySupplier} to provide new instances of {@link ValueTransformerWithKey} to
* Kafka Stream's runtime.
*
* If a record's key and value should be modified {@link Transformer} can be used.
*
* @param key type
* @param value type
* @param transformed value type
* @see ValueTransformer
* @see ValueTransformerWithKeySupplier
* @see KStream#transformValues(ValueTransformerSupplier, String...)
* @see KStream#transformValues(ValueTransformerWithKeySupplier, String...)
* @see Transformer
*/
public interface ValueTransformerWithKey {
/**
* Initialize this transformer.
* This is called once per instance when the topology gets initialized.
*
* The provided {@link ProcessorContext context} can be used to access topology and record meta data, to
* {@link ProcessorContext#schedule(Duration, PunctuationType, Punctuator) schedule} a method to be
* {@link Punctuator#punctuate(long) called periodically} and to access attached {@link StateStore}s.
*
* Note that {@link ProcessorContext} is updated in the background with the current record's meta data.
* Thus, it only contains valid record meta data when accessed within {@link #transform(Object, Object)}.
*
* Note that using {@link ProcessorContext#forward(Object, Object)} or
* {@link ProcessorContext#forward(Object, Object, To)} is not allowed within any method of
* {@code ValueTransformerWithKey} and will result in an {@link StreamsException exception}.
*
* @param context the context
* @throws IllegalStateException If store gets registered after initialization is already finished
* @throws StreamsException if the store's change log does not contain the partition
*/
void init(final ProcessorContext context);
/**
* Transform the given [key and] value to a new value.
* Additionally, any {@link StateStore} that is {@link KStream#transformValues(ValueTransformerWithKeySupplier, String...)
* attached} to this operator can be accessed and modified arbitrarily (cf.
* {@link ProcessorContext#getStateStore(String)}).
*
* Note that using {@link ProcessorContext#forward(Object, Object)} or
* {@link ProcessorContext#forward(Object, Object, To)} is not allowed within {@code transform} and
* will result in an {@link StreamsException exception}.
*
* Note that if a {@code ValueTransformerWithKey} is used in a {@link KTable#transformValues(ValueTransformerWithKeySupplier, String...)}
* (or any other overload of {@code KTable#transformValues(...)}) operation,
* then the provided {@link ProcessorContext} from {@link #init(ProcessorContext)}
* does not guarantee that all context information will be available when {@code transform()}
* is executed, as it might be executed "out-of-band" due to some internal optimizations
* applied by the Kafka Streams DSL.
*
* @param readOnlyKey the read-only key
* @param value the value to be transformed
* @return the new value
*/
VR transform(final K readOnlyKey, final V value);
/**
* Close this processor and clean up any resources.
*
* It is not possible to return any new output records within {@code close()}.
* Using {@link ProcessorContext#forward(Object, Object)} or {@link ProcessorContext#forward(Object, Object, To)},
* will result in an {@link StreamsException exception}.
*/
void close();
}