com.moz.fiji.mapreduce.lib.examples.TextListReducer.java Source code

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Here is the source code for com.moz.fiji.mapreduce.lib.examples.TextListReducer.java

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/**
 * (c) Copyright 2013 WibiData, Inc.
 *
 * See the NOTICE file distributed with this work for additional
 * information regarding copyright ownership.
 *
 * 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.moz.fiji.mapreduce.lib.examples;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import org.apache.avro.Schema;
import org.apache.avro.mapred.AvroValue;
import org.apache.hadoop.conf.Configurable;
import org.apache.hadoop.conf.Configuration;
import org.apache.hadoop.io.Text;
import org.apache.hadoop.mapred.JobConf;

import com.moz.fiji.mapreduce.FijiReducer;
import com.moz.fiji.mapreduce.avro.AvroValueWriter;

/**
 * A reducer that aggregates its text input values into a list.
 *
 * <p>The MapReduce framework will group all output records from the map phase with the
 * same key.  A call to the <code>reduce()</code> method will be made for each input
 * group.  This reducer will aggregate all the values within a group into a single list to
 * be output as the value.  Avro is used to output the complex list value.</p>
 *
 * @param <K> The input key type, which will also be used as the output key type.
 */
public class TextListReducer<K> extends FijiReducer<K, Text, K, AvroValue<List<CharSequence>>>
        implements Configurable, AvroValueWriter {
    /** The job configuration. */
    private Configuration mConf;

    /** A reusable AvroValue to be used for writing output. */
    private AvroValue<List<CharSequence>> mValue;

    /** {@inheritDoc} */
    @Override
    public void setConf(Configuration conf) {
        mConf = conf;
    }

    /** {@inheritDoc} */
    @Override
    public Configuration getConf() {
        return mConf;
    }

    /** {@inheritDoc} */
    @Override
    public void setup(Context context) throws IOException, InterruptedException {
        // Initialize a reusable AvroValue object to be used for writing the output.
        mValue = new AvroValue<List<CharSequence>>();
    }

    /** {@inheritDoc} */
    @Override
    public void reduce(K key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
        // Construct a list to hold all of the values in the group.
        List<CharSequence> aggregateList = new ArrayList<CharSequence>();

        // Add each input value to our aggregate list.
        for (Text value : values) {
            aggregateList.add(value.toString());
        }

        // Put the aggregate list into the wrapper AvroValue to be written.
        mValue.datum(aggregateList);

        // Write the output key and its aggregate list of values.
        context.write(key, mValue);
    }

    /** {@inheritDoc} */
    @Override
    public Class<?> getOutputKeyClass() {
        // This reducer passes the input key straight on to the output, so we'll set the
        // output key class to match whatever the map's output key class was.
        return new JobConf(getConf()).getMapOutputKeyClass();
    }

    /** {@inheritDoc} */
    @Override
    public Class<?> getOutputValueClass() {
        // We will use Avro to output list of strings in the value record.
        return AvroValue.class;
    }

    /** {@inheritDoc} */
    @Override
    public Schema getAvroValueWriterSchema() {
        // Since we use Avro for the output value record, here we specify its schema.
        return Schema.createArray(Schema.create(Schema.Type.STRING));
    }
}