/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. 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 the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.flink.api.common.functions;
import org.apache.flink.annotation.Public;
import java.io.Serializable;
/**
* Base interface for Reduce functions. Reduce functions combine groups of elements to
* a single value, by taking always two elements and combining them into one. Reduce functions
* may be used on entire data sets, or on grouped data sets. In the latter case, each group is reduced
* individually.
* <p>
* For a reduce functions that work on an entire group at the same time (such as the
* MapReduce/Hadoop-style reduce), see {@link GroupReduceFunction}. In the general case,
* ReduceFunctions are considered faster, because they allow the system to use more efficient
* execution strategies.
* <p>
* The basic syntax for using a grouped ReduceFunction is as follows:
* <pre>{@code
* DataSet<X> input = ...;
*
* DataSet<X> result = input.groupBy(<key-definition>).reduce(new MyReduceFunction());
* }</pre>
* <p>
* Like all functions, the ReduceFunction needs to be serializable, as defined in {@link java.io.Serializable}.
*
* @param <T> Type of the elements that this function processes.
*/
@Public
public interface ReduceFunction<T> extends Function, Serializable {
/**
* The core method of ReduceFunction, combining two values into one value of the same type.
* The reduce function is consecutively applied to all values of a group until only a single value remains.
*
* @param value1 The first value to combine.
* @param value2 The second value to combine.
* @return The combined value of both input values.
*
* @throws Exception This method may throw exceptions. Throwing an exception will cause the operation
* to fail and may trigger recovery.
*/
T reduce(T value1, T value2) throws Exception;
}