我正在使用 MapReduce 框架用 Java 制作一个 Hadoop 应用程序。
我只对输入和输出使用 Text 键和值。在减少到最终输出之前,我使用组合器进行额外的计算步骤。
但我有一个问题,钥匙不去同一个减速器。我在组合器中创建并添加这样的键/值对:
public static class Step4Combiner extends Reducer<Text,Text,Text,Text> {
private static Text key0 = new Text();
private static Text key1 = new Text();
public void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
key0.set("KeyOne");
key1.set("KeyTwo");
context.write(key0, new Text("some value"));
context.write(key1, new Text("some other value"));
}
}
public static class Step4Reducer extends Reducer<Text,Text,Text,Text> {
public void reduce(Text key, Iterable<Text> values, Context context) throws IOException, InterruptedException {
System.out.print("Key:" + key.toString() + " Value: ");
String theOutput = "";
for (Text val : values) {
System.out.print("," + val);
}
System.out.print("\n");
context.write(key, new Text(theOutput));
}
}
我主要创建这样的工作:
Configuration conf = new Configuration();
String[] otherArgs = new GenericOptionsParser(conf, args).getRemainingArgs();
Job job4 = new Job(conf, "Step 4");
job4.setJarByClass(Step4.class);
job4.setMapperClass(Step4.Step4Mapper.class);
job4.setCombinerClass(Step4.Step4Combiner.class);
job4.setReducerClass(Step4.Step4Reducer.class);
job4.setInputFormatClass(TextInputFormat.class);
job4.setOutputKeyClass(Text.class);
job4.setOutputValueClass(Text.class);
FileInputFormat.addInputPath(job4, new Path(outputPath));
FileOutputFormat.setOutputPath(job4, new Path(finalOutputPath));
System.exit(job4.waitForCompletion(true) ? 0 : 1);
从减速器打印的标准输出中的输出是这样的:
Key:KeyOne Value: ,some value
Key:KeyTwo Value: ,some other value
Key:KeyOne Value: ,some value
Key:KeyTwo Value: ,some other value
Key:KeyOne Value: ,some value
Key:KeyTwo Value: ,some other value
这是没有意义的,因为键是相同的,因此它应该是 2 个减速器,它的 Iterable 中有 3 个相同的值
希望你能帮助我深入了解这个:)