MapReduce全局变量之捉虫记

全局变量

写MapReduce程序时候，有时候须要用到全局变量，经常使用的全局变量实现由三种方式：

• 通过作业的Configuration传递全局变量。作业初始化的时候。conf.set()，须要的时候。再用conf.get()读出来。缺点：不能共享较大的数据。

• 通过distributedcache
• 通过HDFS实现：即将全局变量写入一个文件，须要的时候，从该文件读取出来

发现问题

全局变量的代码设置例如以下，在Mapper中通过Configuration无法读出配置"deadline"。

  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
    if (otherArgs.length != 2) {
      System.err.println("Usage: wordcount <in> <out>");
      System.exit(2);
    }
    
    Job job = new Job(conf, "word count");
    //job.getCluster().getClusterStatus().getMapSlotCapacity();
    conf.set("deadline", new Date().toString);
    job.setJarByClass(WordCount.class);
    job.setMapperClass(TokenizerMapper.class);
    job.setCombinerClass(IntSumReducer.class);
    job.setReducerClass(IntSumReducer.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(IntWritable.class);
    FileInputFormat.addInputPath(job, new Path(otherArgs[0]));
    FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }

解决这个问题

但是同事的代码却能够，将代码粘贴出来

 public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
    if (otherArgs.length != 2) {
      System.err.println("Usage: wordcount <in> <out>");
      System.exit(2);
    }  
    Job job = new Job(conf, "word count");
    job.getConfiguration().set("deadline", new Date().toString()); 
    job.setJarByClass(WordCount.class);
    job.setMapperClass(TokenizerMapper.class);
    job.setCombinerClass(IntSumReducer.class);
    job.setReducerClass(IntSumReducer.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(IntWritable.class);
    FileInputFormat.addInputPath(job, new Path(otherArgs[0]));
    FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));
    System.exit(job.waitForCompletion(true) ? 0 : 1);
  }

或者

  public static void main(String[] args) throws Exception {
    Configuration conf = new Configuration();
    String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
    if (otherArgs.length != 2) {
      System.err.println("Usage: wordcount <in> <out>");
      System.exit(2);
    }
    conf.set("deadline", new Date().toString());    
    Job job = new Job(conf, "word count");  
    job.setJarByClass(WordCount.class);
    job.setMapperClass(TokenizerMapper.class);
    job.setCombinerClass(IntSumReducer.class);
    job.setReducerClass(IntSumReducer.class);
    job.setOutputKeyClass(Text.class);
    job.setOutputValueClass(IntWritable.class);
    FileInputFormat.addInputPath(job, new Path(otherArgs[0]));
    FileOutputFormat.setOutputPath(job, new Path(otherArgs[1]));
    System.exit(job.waitForCompletion(true) ?
 0 : 1);
  }

问题分析

跟踪代码：

Job job = new Job(conf, "word count");

到

  @Deprecated
  public Job(Configuration conf, String jobName) throws IOException {
    this(conf);
    setJobName(jobName);
  }

到

  @Deprecated
  public Job(Configuration conf) throws IOException {
    this(new JobConf(conf));
  }

这样，Job里面的conf和main()里面的conf已经不一样了，故导致问题

总结

Configuration全局变量没设置成功的原因：设置參数的Configuration和读取參数的Configuration不一致。