hadoop - mapreduce 作业的链接

Question

我遇到了“mapreduce 作业的链接”。作为 mapreduce 的新手，在什么情况下我们必须链接(我假设链接意味着依次运行 mapreduce 作业)作业？

有什么可以提供帮助的例子吗？

Answer 1

必须链接的作业的经典示例是字数统计，它输出按频率排序的字词。

你需要:

工作 1:

• 输入源映射器(发出单词作为键，一个作为值)
• 聚合缩减器(聚合字数)

工作 2:

• 键/值交换映射器(将频率作为键，词作为值)
• implicit identity reducer(获取按频率排序的词，不必实现)

这是上面映射器/缩减器的例子:

public class HadoopWordCount {


  public static class TokenizerMapper extends Mapper<Object, Text, Text, LongWritable> {

    private final static Text word = new Text();
    private final static LongWritable one = new LongWritable(1);

    public void map(Object key, Text value, Context context) throws IOException, InterruptedException {
      StringTokenizer itr = new StringTokenizer(value.toString());
      while (itr.hasMoreTokens()) {
        word.set(itr.nextToken());
        context.write(word, one);
      }
    }
  }

  public static class KeyValueSwappingMapper extends Mapper<Text, LongWritable, LongWritable, Text> {

    public void map(Text key, LongWritable value, Context context) throws IOException, InterruptedException {
      context.write(value, key);
    }
  }

  public static class SumReducer extends Reducer<Text, LongWritable, Text, LongWritable> {
    private LongWritable result = new LongWritable();

    public void reduce(Text key, Iterable<LongWritable> values, Context context) throws IOException,
        InterruptedException {
      long sum = 0;
      for (LongWritable val : values) {
        sum += val.get();
      }
      result.set(sum);
      context.write(key, result);
    }
}

这里是驱动程序的例子。

它需要两个参数:

1. 一个输入文本文件，用于计算其中的单词数。
2. 输出目录(不应预先存在)- 在{this dir}/out2/part-r-0000 文件中查找输出

public static void main(String[] args) throws Exception {

    Configuration conf = new Configuration();
    Path out = new Path(args[1]);

    Job job1 = Job.getInstance(conf, "word count");
    job1.setJarByClass(HadoopWordCount.class);
    job1.setMapperClass(TokenizerMapper.class);
    job1.setCombinerClass(SumReducer.class);
    job1.setReducerClass(SumReducer.class);
    job1.setOutputKeyClass(Text.class);
    job1.setOutputValueClass(LongWritable.class);
    job1.setOutputFormatClass(SequenceFileOutputFormat.class);
    FileInputFormat.addInputPath(job1, new Path(args[0]));
    FileOutputFormat.setOutputPath(job1, new Path(out, "out1"));
    if (!job1.waitForCompletion(true)) {
      System.exit(1);
    }
    Job job2 = Job.getInstance(conf, "sort by frequency");
    job2.setJarByClass(HadoopWordCount.class);
    job2.setMapperClass(KeyValueSwappingMapper.class);
    job2.setNumReduceTasks(1);
    job2.setSortComparatorClass(LongWritable.DecreasingComparator.class);
    job2.setOutputKeyClass(LongWritable.class);
    job2.setOutputValueClass(Text.class);
    job2.setInputFormatClass(SequenceFileInputFormat.class);
    FileInputFormat.addInputPath(job2, new Path(out, "out1"));
    FileOutputFormat.setOutputPath(job2, new Path(out, "out2"));
    if (!job2.waitForCompletion(true)) {
      System.exit(1);
    }

}