1 /* 2 * Licensed to the Apache Software Foundation (ASF) under one 3 * or more contributor license agreements. See the NOTICE file 4 * distributed with this work for additional information 5 * regarding copyright ownership. The ASF licenses this file 6 * to you under the Apache License, Version 2.0 (the 7 * "License"); you may not use this file except in compliance 8 * with the License. You may obtain a copy of the License at 9 * 10 * http://www.apache.org/licenses/LICENSE-2.0 11 * 12 * Unless required by applicable law or agreed to in writing, software 13 * distributed under the License is distributed on an "AS IS" BASIS, 14 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 15 * See the License for the specific language governing permissions and 16 * limitations under the License. 17 */ 18 package org.apache.giraph.reducers; 19 20 import org.apache.hadoop.io.Writable; 21 22 /** 23 * Reduce operations defining how to reduce single values 24 * passed on workers, into partial values on workers, and then 25 * into a single global reduced value. 26 * 27 * Object should be thread safe. Most frequently it should be 28 * immutable object, so that functions can execute concurrently. 29 * Rarely when object is mutable 30 * ({@link org.apache.giraph.master.AggregatorReduceOperation}), 31 * i.e. stores reusable object inside, accesses should be synchronized. 32 * 33 * @param <S> Single value type, objects passed on workers 34 * @param <R> Reduced value type 35 */ 36 public interface ReduceOperation<S, R extends Writable> extends Writable { 37 /** 38 * Return new reduced value which is neutral to reduce operation. 39 * 40 * @return Neutral value 41 */ 42 R createInitialValue(); 43 /** 44 * Add a new value. 45 * Needs to be commutative and associative 46 * 47 * Commonly, returned value should be same as curValue argument. 48 * 49 * @param curValue Partial value into which to reduce and store the result 50 * @param valueToReduce Single value to be reduced 51 * @return reduced value 52 */ 53 R reduce(R curValue, S valueToReduce); 54 /** 55 * Add partially reduced value to current partially reduced value. 56 * 57 * Commonly, returned value should be same as curValue argument. 58 * 59 * @param curValue Partial value into which to reduce and store the result 60 * @param valueToReduce Partial value to be reduced 61 * @return reduced value 62 */ 63 R reduceMerge(R curValue, R valueToReduce); 64 }