org.apache.kafka.streams.kstream.Aggregator Maven / Gradle / Ivy
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* (the "License"); you may not use this file except in compliance with
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*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
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*/
package org.apache.kafka.streams.kstream;
/**
* The {@code Aggregator} interface for aggregating values of the given key.
* This is a generalization of {@link Reducer} and allows to have different types for input value and aggregation
* result.
* {@code Aggregator} is used in combination with {@link Initializer} that provides an initial aggregation value.
*
* {@code Aggregator} can be used to implement aggregation functions like count.
*
* @param key type
* @param input value type
* @param aggregate value type
*
* @see Initializer
* @see KGroupedStream#aggregate(Initializer, Aggregator)
* @see KGroupedStream#aggregate(Initializer, Aggregator, Materialized)
* @see TimeWindowedKStream#aggregate(Initializer, Aggregator)
* @see TimeWindowedKStream#aggregate(Initializer, Aggregator, Materialized)
* @see SessionWindowedKStream#aggregate(Initializer, Aggregator, Merger)
* @see SessionWindowedKStream#aggregate(Initializer, Aggregator, Merger, Materialized)
* @see Reducer
*/
public interface Aggregator {
/**
* Compute a new aggregate from the key and value of a record and the current aggregate of the same key.
*
* @param key
* the key of the record
* @param value
* the value of the record
* @param aggregate
* the current aggregate value
*
* @return the new aggregate value
*/
VAgg apply(final K key, final V value, final VAgg aggregate);
}