pipecombiner.java

hadoop:Nutch集群平台

/**
 * Copyright 2006 The Apache Software Foundation
 *
 * 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 org.apache.hadoop.streaming;

import java.io.IOException;
import java.util.Iterator;

import org.apache.hadoop.mapred.JobConf;
import org.apache.hadoop.mapred.Reporter;
import org.apache.hadoop.mapred.OutputCollector;

import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableComparable;

/** A generic Combiner bridge.<br>
 *  To use a Combiner specify -combiner myprogram in hadoopStreaming.
 *  It delegates operations to an external program via stdin and stdout.
 *  In one run of the external program, you can expect all records with
 *  the same key to appear together.
 *  You should not make assumptions about how many times the combiner is
 *  run on your data.
 *  Ideally the combiner and the reducer are the same program, the combiner
 *  partially aggregates the data zero or more times and the reducer
 *  applies the last aggregation pass.
 *  Do not use a Combiner if your reduce logic does not suport
 *  such a multipass aggregation.
 *  @author Michel Tourn
 */
public class PipeCombiner extends PipeReducer {

  String getPipeCommand(JobConf job) {
    return job.get("stream.combine.streamprocessor");
  }

}