| /* |
| * 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; |
| } |