/*
* Copyright (c) 2008-2017, Hazelcast, Inc. All Rights Reserved.
*
* Licensed 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 com.hazelcast.mapreduce;
/**
* <p>
* The abstract Reducer class is used to build reducers for the {@link Job}.<br>
* Reducers may be distributed inside of the cluster but there is always only one Reducer
* per key.
* <p/>
* Reducers are always called in a thread-safe way. However, they may be moved from one thread to
* another in the internal thread pool. As of version 3.3.3 the internal state is automatically
* ensured to be visible according to the Java memory model by the framework. The previous
* requirement for making fields volatile is dropped.
* </p>
* <p>
* Due to the fact that there is only one Reducer per key, mapped values needs to be
* transmitted to one of the cluster nodes. To reduce the traffic costs between the
* nodes, a {@link Combiner} implementation can be added to the call which runs alongside
* the mapper to pre-reduce mapped values into intermediate results.
* </p>
* <p>
* A simple Reducer implementation could look like that sum-function implementation:
* <pre>
* public class SumReducer implements Reducer<Integer, Integer>
* {
* private int sum = 0;
*
* public void reduce( Integer value )
* {
* sum += value;
* }
*
* public Integer finalizeReduce()
* {
* return sum;
* }
* }
* </pre>
* </p>
*
* @param <ValueIn> value type of the incoming values
* @param <ValueOut> value type of the reduced values
* @since 3.2
* @deprecated MapReduce is deprecated and will be removed in 4.0.
* For map aggregations, you can use {@link com.hazelcast.aggregation.Aggregator} on IMap.
* For general data processing, it is superseded by <a href="http://jet.hazelcast.org">Hazelcast Jet</a>.
*/
@Deprecated
public abstract class Reducer<ValueIn, ValueOut> {
/**
* This method is called before the first value is submitted to this Reducer instance.
* It can be used to set up any internal needed state before starting to reduce the
* actual values.
*/
public void beginReduce() {
}
/**
* This method is called to supply values to be reduced into a final reduced result.<br/>
* The reduce method might be called multiple times so the eventually reduced value
* needs to be held internally in a member state of the Reducer.
*
* @param value value to be reduced
*/
public abstract void reduce(ValueIn value);
/**
* finalizeReduce is called as last step for a reducing phase per key. It retrieves the
* final reduced result.
*
* @return the final reduced result
*/
public abstract ValueOut finalizeReduce();
}