org.apache.kafka.streams.kstream.ValueTransformer Maven / Gradle / Ivy
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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 ValueTransformer} 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)} 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)} (cf. {@link ValueMapper} for stateless value transformation).
* Additionally, this {@code ValueTransformer} can {@link ProcessorContext#schedule(Duration, PunctuationType, Punctuator) schedule}
* a method to be {@link Punctuator#punctuate(long) called periodically} with the provided context.
* If {@code ValueTransformer} is applied to a {@link KeyValue} pair record the record's key is preserved.
*
* Use {@link ValueTransformerSupplier} to provide new instances of {@code ValueTransformer} to Kafka Stream's runtime.
*
* If a record's key and value should be modified {@link Transformer} can be used.
*
* @param value type
* @param transformed value type
* @see ValueTransformerSupplier
* @see ValueTransformerWithKeySupplier
* @see KStream#transformValues(ValueTransformerSupplier, String...)
* @see KStream#transformValues(ValueTransformerWithKeySupplier, String...)
* @see Transformer
*/
public interface ValueTransformer {
/**
* Initialize this transformer.
* This is called once per instance when the topology gets initialized.
* When the framework is done with the transformer, {@link #close()} will be called on it; the
* framework may later re-use the transformer by calling {@link #init(ProcessorContext)} again.
*
* 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)}.
*
* Note that using {@link ProcessorContext#forward(Object, Object)} or
* {@link ProcessorContext#forward(Object, Object, To)} is not allowed within any method of
* {@code ValueTransformer} 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 value to a new value.
* Additionally, any {@link StateStore} that is {@link KStream#transformValues(ValueTransformerSupplier, 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}.
*
* @param value the value to be transformed
* @return the new value
*/
VR transform(final V value);
/**
* Close this transformer and clean up any resources. The framework may
* later re-use this transformer by calling {@link #init(ProcessorContext)} on it again.
*
* 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();
}